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116-547: The river first formed as the Sierra Nevada rose about 10 million years ago, and sediment eroded from its canyon helped form the flat floor of the San Joaquin Valley. Glaciation during the ice ages carved the high elevation parts of the watershed, including Yosemite Valley, into their present shape. Historically, there was an extensive riparian zone which provided habitat for millions of migrating birds, and

232-627: A fish hatchery , the Merced River Hatchery, was built beside the Merced River just downstream of the Crocker-Huffman Diversion Dam, the lowermost Merced River dam. Fall chinook salmon travel up a fish ladder into the hatchery's pools, which are supplied with water diverted from the Merced River. Yosemite Valley saw significant amounts of damage when the river flooded the valley in 1997 . In 2024

348-454: A proglacial lake above the valley created by an ice dam as a result of the 1815 eruption of Mount Tambora , which threatened to cause a catastrophic flood when the dam broke. Perraudin attempted unsuccessfully to convert his companions to his theory, but when the dam finally broke, there were only minor erratics and no striations, and Venetz concluded that Perraudin was right and that only ice could have caused such major results. In 1821 he read

464-522: A broad and flat valley floor. Sediments of glacial origin continued to travel down the Merced River following then, helping to form the flat floor of the Central Valley. Of the many Native American tribes that have lived on the Merced River the most prominent were the Miwok (consisting of Plains Miwok and Sierra Miwok ), Paiute , and Ahwahnechee . Many Plains Miwok settled in the lowlands along

580-526: A fertilizer that causes massive algal blooms that pulls large amounts of CO 2 out of the atmosphere. This in turn makes it even colder and causes the glaciers to grow more. In 1956, Ewing and Donn hypothesized that an ice-free Arctic Ocean leads to increased snowfall at high latitudes. When low-temperature ice covers the Arctic Ocean there is little evaporation or sublimation and the polar regions are quite dry in terms of precipitation, comparable to

696-570: A geologist and professor of forestry at an academy in Dreissigacker (since incorporated in the southern Thuringian city of Meiningen ), adopted Esmark's theory. In a paper published in 1832, Bernhardi speculated about the polar ice caps once reaching as far as the temperate zones of the globe. In Val de Bagnes , a valley in the Swiss Alps , there was a long-held local belief that the valley had once been covered deep in ice, and in 1815

812-402: A large decline in riparian habitat. Diversion of water for irrigation often reduces the river to a small stream by the time it reaches its mouth. Efforts to mitigate environmental damage include habitat conservation work, re-establishment of historic streamflow patterns, and the construction of a salmon hatchery. The headwaters of the Merced River are at 8,017 feet (2,444 m) at the foot of

928-408: A local chamois hunter called Jean-Pierre Perraudin attempted to convert the geologist Jean de Charpentier to the idea, pointing to deep striations in the rocks and giant erratic boulders as evidence. Charpentier held the general view that these signs were caused by vast floods, and he rejected Perraudin's theory as absurd. In 1818 the engineer Ignatz Venetz joined Perraudin and Charpentier to examine

1044-467: A molten globe. In order to persuade the skeptics, Agassiz embarked on geological fieldwork. He published his book Study on Glaciers ("Études sur les glaciers") in 1840. Charpentier was put out by this, as he had also been preparing a book about the glaciation of the Alps. Charpentier felt that Agassiz should have given him precedence as it was he who had introduced Agassiz to in-depth glacial research. As

1160-486: A new culture, the Ahwahnechee , derived from Ahwahnee , meaning "the valley shaped like a big mouth" (referring to U-shaped Yosemite Valley). In the early 19th century, several military expeditions sent by Spanish colonists from coastal California traveled into the Central Valley. One of these trips, headed by lieutenant Gabriel Moraga , arrived on the south bank of the Merced River on September 29, 1806. They named

1276-419: A popular recreational area in part because of Yosemite National Park. There are many activities within the watershed, including boating, fishing, camping and hiking. Whitewater rafting is permitted throughout Merced River Canyon from the downstream half of Yosemite Valley to the entrance of Lake McClure . The most difficult rapids in this segment rate Class III and Class IV, mostly upstream of El Portal . There

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1392-544: A prize-winning paper on the theory to the Swiss Society, but it was not published until Charpentier, who had also become converted, published it with his own more widely read paper in 1834. In the meantime, the German botanist Karl Friedrich Schimper (1803–1867) was studying mosses which were growing on erratic boulders in the alpine upland of Bavaria. He began to wonder where such masses of stone had come from. During

1508-441: A record low of 500 fish during several years in the 1950s, to a high of 30,000 in 1984. Since the 1970s, the annual chinook run has averaged about 5,300. Of the 127 bird species found along the Merced River, only 35 occur along the entire length of the river. Many of these birds are migratory and only pass the area a few times every year, while 109 species of birds are found only in the breeding season . Birds are more abundant along

1624-499: A result of personal quarrels, Agassiz had also omitted any mention of Schimper in his book. It took several decades before the ice age theory was fully accepted by scientists. This happened on an international scale in the second half of the 1870s, following the work of James Croll , including the publication of Climate and Time, in Their Geological Relations in 1875, which provided a credible explanation for

1740-411: A significant causal factor of the 40 million year Cenozoic Cooling trend. They further claim that approximately half of their uplift (and CO 2 "scrubbing" capacity) occurred in the past 10 million years. There is evidence that greenhouse gas levels fell at the start of ice ages and rose during the retreat of the ice sheets, but it is difficult to establish cause and effect (see the notes above on

1856-567: A spectacular narrow gorge between massive, glacially resistant granite cliffs. The gorge opens up after Bunnell Point and Sugarloaf Dome confine the river to form Bunnell Cascade, before turning southward through the Lost Valley of the Merced, and then spills over a granite cliff into Little Yosemite Valley , named for its resemblance to Yosemite Valley downstream. The Merced River drops over Nevada Fall and Vernal Fall , together known as

1972-575: A steep walled canyon for 2.5 miles (4.0 km) where the river receives the Red Peak Fork and then collects into Washburn Lake , 7,612 feet (2,320 m) above sea level. The Merced continues to the northwest for 3 miles (4.8 km) where collects into Merced Lake. Leaving Merced Lake, the river continues to the west northwest for 2.3 miles (3.7 km) where the canyons open up into Echo Valley . The river then turns generally westward for another 3 miles (4.8 km), where it snakes through

2088-440: Is Merced Falls Dam, an irrigation diversion dam . The lowermost, the Crocker-Huffman Diversion Dam, was built in 1906 and completely blocks the passage of anadromous fish up the Merced River. Together, these diversions remove almost half the natural flow of the Merced River, and an even greater proportion during dry years. For several months in 2022, all of the water in the river was diverted to agriculture. Cascades Diversion Dam

2204-662: Is a long period of reduction in the temperature of Earth 's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers . Earth's climate alternates between ice ages, and greenhouse periods during which there are no glaciers on the planet. Earth is currently in the ice age called Quaternary glaciation . Individual pulses of cold climate within an ice age are termed glacial periods ( glacials, glaciations, glacial stages, stadials, stades , or colloquially, ice ages ), and intermittent warm periods within an ice age are called interglacials or interstadials . In glaciology ,

2320-429: Is also boating on Lake McClure. Camping throughout the upper Merced watershed is generally only permitted in designated campgrounds. Campgrounds along the Merced River and its tributaries include ones at Railroad Flat, McCabe Flat, Willow Placer, Merced Lake, Vogelsang Lake, Sunrise Creek, May Lake, Bridalveil Creek, and a ski hut at Ostrander Lake , the source of Bridalveil Creek. The name "Railroad Flat" originates from

2436-578: Is estimated to potentially outweigh the orbital forcing of the Milankovitch cycles for hundreds of thousands of years. Each glacial period is subject to positive feedback which makes it more severe, and negative feedback which mitigates and (in all cases so far) eventually ends it. An important form of feedback is provided by Earth's albedo , which is how much of the sun's energy is reflected rather than absorbed by Earth. Ice and snow increase Earth's albedo, while forests reduce its albedo. When

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2552-533: Is impacted by invasive Asiatic clam , Chinese mitten crab , and New Zealand mud snail . Many species of plants are found throughout the middle and upper basin, including California poppy , white alder , Oregon ash , oak , poison oak , bigleaf maple , Indian rhubarb , buttonbush , willow , whiteleaf manzanita , and historically, sugar pine , before heavy logging in the late 19th century. Squirrels , raccoon , jackrabbits , bats , skunks , beavers , mule deer , coyote , bobcat and black bear are among

2668-649: Is that several factors are important: atmospheric composition , such as the concentrations of carbon dioxide and methane (the specific levels of the previously mentioned gases are now able to be seen with the new ice core samples from the European Project for Ice Coring in Antarctica (EPICA) Dome C in Antarctica over the past 800,000 years); changes in Earth's orbit around the Sun known as Milankovitch cycles ;

2784-516: Is the increased aridity occurring with glacial maxima, which reduces the precipitation available to maintain glaciation. The glacial retreat induced by this or any other process can be amplified by similar inverse positive feedbacks as for glacial advances. According to research published in Nature Geoscience , human emissions of carbon dioxide (CO 2 ) will defer the next glacial period. Researchers used data on Earth's orbit to find

2900-566: Is the third largest tributary of the San Joaquin River. Before irrigation started in the Central Valley and dams were constructed, the river's natural flow at the mouth was much higher than the current average of 661 cubic feet per second (18.7 m/s), or about 479,000 acre-feet (591,000,000 m) per year. Upstream on the river, at Happy Isles, the average flow is 355 cubic feet per second (10.1 m/s). The United States Geological Survey has river gauges at three locations along

3016-473: The Alps of Savoy . Two years later he published an account of his journey. He reported that the inhabitants of that valley attributed the dispersal of erratic boulders to the glaciers, saying that they had once extended much farther. Later similar explanations were reported from other regions of the Alps. In 1815 the carpenter and chamois hunter Jean-Pierre Perraudin (1767–1858) explained erratic boulders in

3132-655: The Carboniferous and early Permian periods. Correlatives are known from Argentina, also in the center of the ancient supercontinent Gondwanaland . Although the Mesozoic Era retained a greenhouse climate over its timespan and was previously assumed to have been entirely glaciation-free, more recent studies suggest that brief periods of glaciation occurred in both hemispheres during the Early Cretaceous . Geologic and palaeoclimatological records suggest

3248-606: The Clark Range subrange of the Sierra Nevada, rising at the confluence of the Triple Peak Fork and Merced Peak Fork after they cascade down glacially polished slopes from the high country in the southeastern corner of Yosemite National Park. From its headwaters, the river flows north for a short distance and collects the Lyell Peak Fork. The course of the Merced then turns to the north west and flows through

3364-683: The Himalayas are a major factor in the current ice age, because these mountains have increased Earth's total rainfall and therefore the rate at which carbon dioxide is washed out of the atmosphere, decreasing the greenhouse effect. The Himalayas' formation started about 70 million years ago when the Indo-Australian Plate collided with the Eurasian Plate , and the Himalayas are still rising by about 5 mm per year because

3480-765: The Late Ordovician and the Silurian period. The evolution of land plants at the onset of the Devonian period caused a long term increase in planetary oxygen levels and reduction of CO 2 levels, which resulted in the late Paleozoic icehouse . Its former name, the Karoo glaciation, was named after the glacial tills found in the Karoo region of South Africa. There were extensive polar ice caps at intervals from 360 to 260 million years ago in South Africa during

3596-584: The National Register of Historic Places but was deemed too dangerous, and was subsequently removed. Today, the Merced River above Lake McClure is completely free-flowing and unobstructed by any dams. The Merced Irrigation District (MID) operates most of the irrigation infrastructure on the lower river, which supplies water to 154,394 acres (624.81 km) of farmland. As a whole, the system includes about 4,000 control gates and 793 miles (1,276 km) of canals and laterals. Irrigation has taken most of

Merced River - Misplaced Pages Continue

3712-553: The Pacific Crest Trail near Tuolumne Meadows . Trails also follow the river through Little Yosemite Valley to the headwaters area, and along Illilouette, Bridalveil, Yosemite, Alder and Chilnualna Creeks, and the lower South Fork of the Merced River. There are no trails along some segments, including the lower Bridalveil Creek, upper South Fork, and specifically Tenaya Canyon , which is extremely dangerous. Download coordinates as: Ice age An ice age

3828-482: The Pleistocene Ice Age. Because this highland is at a subtropical latitude, with four to five times the insolation of high-latitude areas, what would be Earth's strongest heating surface has turned into a cooling surface. Kuhle explains the interglacial periods by the 100,000-year cycle of radiation changes due to variations in Earth's orbit. This comparatively insignificant warming, when combined with

3944-715: The Turonian , otherwise the warmest period of the Phanerozoic, are disputed), ice sheets and associated sea ice appear to have briefly returned to Antarctica near the very end of the Maastrichtian just prior to the Cretaceous-Paleogene extinction event . The Quaternary Glaciation / Quaternary Ice Age started about 2.58 million years ago at the beginning of the Quaternary Period when

4060-633: The Yosemite Valley Railroad , which once travelled up Merced River Canyon into Yosemite Valley. The old railroad grade still exists, and is now the site of a public trail. Many other trails lead throughout the Merced River watershed, notably the John Muir Trail , which starts near Happy Isles and climbs the Giant Staircase, past Vernal and Nevada Falls, into Little Yosemite Valley and north along Sunrise Creek to join

4176-454: The granite batholith that makes up much of the Sierra Nevada. Extensive layers of marine sedimentary rock that originally made up the ancient Pacific seabed were also pushed up by the rising granite, and the ancestral Merced River formed on this layer of rock. Over millions of years, the Merced cut a deep canyon through the softer sedimentary rock, eventually hitting the hard granite beneath. The encounter with this resilient rock layer caused

4292-523: The "Giant Staircase", then receives Illilouette Creek and flows into Yosemite Valley, where it meanders between pine forests and meadows that fill the valley floor. Tenaya , Yosemite , Bridalveil and Pigeon Creeks join the Merced River in Yosemite Valley. Beyond the glacial moraine at the western end of the valley the river flows through the steep Merced River Canyon, picks up Cascade Creek and turns south near El Portal . State Route 140 follows

4408-482: The 1870s. The first was Coulterville Road, followed by Big Oak Flat Road, a trading route from Stockton to Merced Canyon. Environmental movements led by John Muir and Robert Underwood Johnson convinced the U.S. Congress to establish Yosemite National Park in 1890. With the creation of the national park tourism to the Valley and the Merced River increased significantly, leading to many other roads being built throughout

4524-618: The Atlantic, increasing heat transport into the Arctic, which melted the polar ice accumulation and reduced other continental ice sheets. The release of water raised sea levels again, restoring the ingress of colder water from the Pacific with an accompanying shift to northern hemisphere ice accumulation. According to a study published in Nature in 2021, all glacial periods of ice ages over

4640-441: The Central Valley and the Sierra Nevada foothills. In 1808 and 1810, Moraga led further expeditions along the lower Merced River below Merced Canyon, each time coming to nothing. Eventually, plans to establish a mission chain in the Valley were abandoned. In 1855, Merced County was created, named after the Merced River. Following the establishment of Merced County and the independence of California from Mexico, many settlers came to

4756-467: The Indo-Australian plate is still moving at 67 mm/year. The history of the Himalayas broadly fits the long-term decrease in Earth's average temperature since the mid-Eocene , 40 million years ago. Another important contribution to ancient climate regimes is the variation of ocean currents, which are modified by continent position, sea levels and salinity, as well as other factors. They have

Merced River - Misplaced Pages Continue

4872-553: The Merced Irrigation District . The District proposed the Exchequer Dam , completed in the mid-1920s and raised in the 1960s, as a water storage facility on the Merced River. Irrigation with water from the Merced River continued to grow substantially until most of the arable land around the river, some 120,000 acres (490 km), was under cultivation. By the late 1950s and early 1960s, irrigation in

4988-471: The Merced River area and established small towns on the Merced River. One of the first was Dover , established in 1844 at the confluence of the Merced River with the San Joaquin River. Dover functioned as an "inland seaport " where boats delivered supplies from the San Francisco Bay area to settlers in the San Joaquin Valley. Some towns that followed were Hopeton , Snelling and Merced Falls ,

5104-515: The Merced River to mostly stop its downcutting, although tributary streams continued to widen the ancient canyon. Over about 80 million years, erosion caused the transportation of massive amounts of alluvial sediment to the floor of the Central Valley, where it was trapped between the California Coast Range on the west and the Sierra Nevada on the east, forming an incredibly flat and fertile land surface. The present-day form of

5220-493: The Merced River watershed. Due to differences in climate, there is a large disparity between species found in the upper watershed (above Lake McClure) and along the lower river. There are 26 species of native and introduced fish in the lower Central Valley portion of the river, including Sacramento sucker , smallmouth bass , largemouth bass and carp . Anadromous fish species found in the lower river are chinook salmon , Pacific lamprey and striped bass . The upper section of

5336-399: The Merced River. El Portal Road , constructed through Merced Canyon in 1926, put an end to passenger service on the railway, but operations continued until the mid-1940s, when major flooding occurred, destroying sections of the railroad. In the early 20th century, when the upper Merced River basin lay mostly protected, the lower river was the subject of dam-building and irrigation diversions by

5452-541: The Merced River. Following the construction of the Central Pacific and Southern Pacific Railroads , many of the river towns on the Merced River were deserted. Several cities that did achieve prominence, however, include Merced and Turlock , both located on the railroad. The California Gold Rush in the 1850s saw gradually increasing mining in Merced Canyon and Yosemite Valley. Many Native Americans in

5568-562: The Merced River: at Happy Isles , above Lake McClure , and at the mouth. The first two record flows unaffected by dams and human intervention, but discharge at the mouth is chiefly controlled by New Exchequer Dam. The Lake McClure gauge, located at the former mining town of Bagby, is probably the most accurate gauge for flows overall. The average annual flow recorded there was 1,185 cubic feet per second (33.6 m/s) from 1923 to 1966. A peak of 92,500 cubic feet per second (2,620 m/s)

5684-588: The New York Times reported that farmers and growers had drained significant volumes of water during the 2022 drought leaving part of the Merced dry without authorities being aware. Despite its partial status as a National Wild and Scenic River, the Merced River has many dams and irrigation diversions. New Exchequer Dam is the largest dam on the river and forms Lake McClure , which holds 1,032,000 acre-feet (1.273 × 10 m) of water for irrigation, flood control and hydropower generation. This modern structure

5800-579: The North Atlantic Ocean far enough to block the Gulf Stream. Ice sheets that form during glaciations erode the land beneath them. This can reduce the land area above sea level and thus diminish the amount of space on which ice sheets can form. This mitigates the albedo feedback, as does the rise in sea level that accompanies the reduced area of ice sheets, since open ocean has a lower albedo than land. Another negative feedback mechanism

5916-603: The North Atlantic during a warming cycle may also reduce the global ocean water circulation . Such a reduction (by reducing the effects of the Gulf Stream ) would have a cooling effect on northern Europe, which in turn would lead to increased low-latitude snow retention during the summer. It has also been suggested that during an extensive glacial, glaciers may move through the Gulf of Saint Lawrence , extending into

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6032-469: The San Joaquin Valley was to such an extent that many of the rivers ran dry in sections. Upriver of the Merced River confluence with the San Joaquin, the latter river was usually dry, only regaining flow where the Merced River enters. In the mid-20th century, the flow in the Merced River diminished to such a degree that very few salmon returned to spawn in the lower section of the Merced River. In 1991,

6148-827: The Scandinavian peninsula. He regarded glaciation as a regional phenomenon. Only a few years later, the Danish-Norwegian geologist Jens Esmark (1762–1839) argued for a sequence of worldwide ice ages. In a paper published in 1824, Esmark proposed changes in climate as the cause of those glaciations. He attempted to show that they originated from changes in Earth's orbit. Esmark discovered the similarity between moraines near Haukalivatnet lake near sea level in Rogaland and moraines at branches of Jostedalsbreen . Esmark's discovery were later attributed to or appropriated by Theodor Kjerulf and Louis Agassiz . During

6264-544: The Swiss Alps with his former university friend Louis Agassiz (1801–1873) and Jean de Charpentier. Schimper, Charpentier and possibly Venetz convinced Agassiz that there had been a time of glaciation. During the winter of 1836–37, Agassiz and Schimper developed the theory of a sequence of glaciations. They mainly drew upon the preceding works of Venetz, Charpentier and on their own fieldwork. Agassiz appears to have been already familiar with Bernhardi's paper at that time. At

6380-606: The Tahoe, Tenaya and Tioga stages, of which the Tioga was the smallest. The Tioga glacier left at the mouth of Yosemite Valley a rocky moraine . This moraine was actually one of several moraines deposited by the four glaciations, which include Medial Moraine and Bridalveil Moraine. After the Tioga Glacier retreated this moraine formed a lake that flooded nearly the entire valley. Gradual sedimentation filled Lake Yosemite, creating

6496-880: The Val de Bagnes in the Swiss canton of Valais as being due to glaciers previously extending further. An unknown woodcutter from Meiringen in the Bernese Oberland advocated a similar idea in a discussion with the Swiss-German geologist Jean de Charpentier (1786–1855) in 1834. Comparable explanations are also known from the Val de Ferret in the Valais and the Seeland in western Switzerland and in Goethe 's scientific work . Such explanations could also be found in other parts of

6612-621: The ability to cool (e.g. aiding the creation of Antarctic ice) and the ability to warm (e.g. giving the British Isles a temperate as opposed to a boreal climate). The closing of the Isthmus of Panama about 3 million years ago may have ushered in the present period of strong glaciation over North America by ending the exchange of water between the tropical Atlantic and Pacific Oceans. Analyses suggest that ocean current fluctuations can adequately account for recent glacial oscillations. During

6728-402: The air temperature decreases, ice and snow fields grow, and they reduce forest cover. This continues until competition with a negative feedback mechanism forces the system to an equilibrium. One theory is that when glaciers form, two things happen: the ice grinds rocks into dust, and the land becomes dry and arid. This allows winds to transport iron rich dust into the open ocean, where it acts as

6844-424: The amount found in mid-latitude deserts . This low precipitation allows high-latitude snowfalls to melt during the summer. An ice-free Arctic Ocean absorbs solar radiation during the long summer days, and evaporates more water into the Arctic atmosphere. With higher precipitation, portions of this snow may not melt during the summer and so glacial ice can form at lower altitudes and more southerly latitudes, reducing

6960-674: The area revolted, leading to conflicts between miners and the Ahwahnechee. In 1851 the Mariposa Battalion was formed to drive the remaining Ahwahnechee out of the valley into reservations. The Battalion fought an Ahwahnechee group led by Chief Tenaya over the South Fork of the Merced River. Eventually, they succeeded in driving most of the Indians out of the Yosemite Valley, first into a reservation near Fresno . Following

7076-474: The beginning of 1837, Schimper coined the term "ice age" ( "Eiszeit" ) for the period of the glaciers. In July 1837 Agassiz presented their synthesis before the annual meeting of the Swiss Society for Natural Research at Neuchâtel. The audience was very critical, and some were opposed to the new theory because it contradicted the established opinions on climatic history. Most contemporary scientists thought that Earth had been gradually cooling down since its birth as

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7192-495: The causes of ice ages. There are three main types of evidence for ice ages: geological, chemical, and paleontological. Geological evidence for ice ages comes in various forms, including rock scouring and scratching, glacial moraines , drumlins , valley cutting, and the deposition of till or tillites and glacial erratics . Successive glaciations tend to distort and erase the geological evidence for earlier glaciations, making it difficult to interpret. Furthermore, this evidence

7308-473: The cities of Hopeton , Delhi and Livingston . It joins the San Joaquin River at Hills Ferry , a few miles south of Turlock . The Merced River watershed encompasses 1,726 square miles (4,470 km) in the central Sierra Nevada. To the north it is bordered by the watershed of the Tuolumne River , the other major river draining Yosemite National Park. On the south, the Merced watershed borders on

7424-463: The concentrations of greenhouse gases) may alter the climate, while climate change itself can change the atmospheric composition (for example by changing the rate at which weathering removes CO 2 ). Maureen Raymo , William Ruddiman and others propose that the Tibetan and Colorado Plateaus are immense CO 2 "scrubbers" with a capacity to remove enough CO 2 from the global atmosphere to be

7540-710: The continental ice sheets are the Greenland and Antarctic ice sheets and smaller glaciers such as on Baffin Island . The definition of the Quaternary as beginning 2.58 Ma is based on the formation of the Arctic ice cap . The Antarctic ice sheet began to form earlier, at about 34 Ma, in the mid- Cenozoic ( Eocene-Oligocene Boundary ). The term Late Cenozoic Ice Age is used to include this early phase. Ice ages can be further divided by location and time; for example,

7656-405: The continents and pack ice on the oceans would inhibit both silicate weathering and photosynthesis , which are the two major sinks for CO 2 at present." It has been suggested that the end of this ice age was responsible for the subsequent Ediacaran and Cambrian explosion , though this model is recent and controversial. The Andean-Saharan occurred from 460 to 420 million years ago, during

7772-431: The continents are in positions which block or reduce the flow of warm water from the equator to the poles and thus allow ice sheets to form. The ice sheets increase Earth's reflectivity and thus reduce the absorption of solar radiation. With less radiation absorbed the atmosphere cools; the cooling allows the ice sheets to grow, which further increases reflectivity in a positive feedback loop. The ice age continues until

7888-540: The current glaciation, more temperate and more severe periods have occurred. The colder periods are called glacial periods , the warmer periods interglacials , such as the Eemian Stage . There is evidence that similar glacial cycles occurred in previous glaciations, including the Andean-Saharan and the late Paleozoic ice house. The glacial cycles of the late Paleozoic ice house are likely responsible for

8004-617: The deposition of cyclothems . Glacials are characterized by cooler and drier climates over most of Earth and large land and sea ice masses extending outward from the poles. Mountain glaciers in otherwise unglaciated areas extend to lower elevations due to a lower snow line . Sea levels drop due to the removal of large volumes of water above sea level in the icecaps. There is evidence that ocean circulation patterns are disrupted by glaciations. The glacials and interglacials coincide with changes in orbital forcing of climate due to Milankovitch cycles , which are periodic changes in Earth's orbit and

8120-552: The early Proterozoic Eon. Several hundreds of kilometers of the Huronian Supergroup are exposed 10 to 100 kilometers (6 to 62 mi) north of the north shore of Lake Huron, extending from near Sault Ste. Marie to Sudbury, northeast of Lake Huron, with giant layers of now-lithified till beds, dropstones , varves , outwash , and scoured basement rocks. Correlative Huronian deposits have been found near Marquette, Michigan , and correlation has been made with Paleoproterozoic glacial deposits from Western Australia. The Huronian ice age

8236-405: The existence of glacial periods during the Valanginian , Hauterivian , and Aptian stages of the Early Cretaceous. Ice-rafted glacial dropstones indicate that in the Northern Hemisphere , ice sheets may have extended as far south as the Iberian Peninsula during the Hauterivian and Aptian. Although ice sheets largely disappeared from Earth for the rest of the period (potential reports from

8352-519: The first person to suggest drifting sea ice was a cause of the presence of erratic boulders in the Scandinavian and Baltic regions. In 1795, the Scottish philosopher and gentleman naturalist, James Hutton (1726–1797), explained erratic boulders in the Alps by the action of glaciers. Two decades later, in 1818, the Swedish botanist Göran Wahlenberg (1780–1851) published his theory of a glaciation of

8468-512: The flow above Happy Isles comes from melting snow. In the dry season, groundwater provides the only base flow to the river. In addition, the river is fed by dozens of high Sierra lakes, the largest of which include Merced Lake, Tenaya Lake , Ostrander Lake, the Chain Lakes, and May Lake. The foothills experience a drier Mediterranean climate , while the San Joaquin Valley floor is dry enough to be considered semi-desert . The Merced River

8584-565: The following years, Esmark's ideas were discussed and taken over in parts by Swedish, Scottish and German scientists. At the University of Edinburgh Robert Jameson (1774–1854) seemed to be relatively open to Esmark's ideas, as reviewed by Norwegian professor of glaciology Bjørn G. Andersen (1992). Jameson's remarks about ancient glaciers in Scotland were most probably prompted by Esmark. In Germany, Albrecht Reinhard Bernhardi (1797–1849),

8700-417: The geographical distribution of fossils. During a glacial period, cold-adapted organisms spread into lower latitudes, and organisms that prefer warmer conditions become extinct or retreat into lower latitudes. This evidence is also difficult to interpret because it requires: Despite the difficulties, analysis of ice core and ocean sediment cores has provided a credible record of glacials and interglacials over

8816-581: The gold rush, the Ahwahnechee were allowed back into Yosemite Valley, but further incidents prompted a second battalion to drive them out, this time to the Mono Lake area. Many place names in the valley have their origin from the Mariposa Battalion. Even before the establishment of Yosemite National Park , tourists began to travel into the Merced Canyon and Yosemite Valley as early as 1855. The first roads were constructed into Yosemite Valley in

8932-546: The headwaters of the San Joaquin River. To the east, it is bordered by the Sierra crest which divides it from the endorheic Great Basin watershed of Mono Lake . Much of the Merced River basin is at high elevation, where an alpine climate prevails. The Sierra receives heavy snowfall in the winter, which melts in the spring and early summer causing annual flooding. By late autumn the river level has dropped considerably, and some smaller tributaries dry up altogether. Up to 85% of

9048-434: The historical warm interglacial period that looks most like the current one and from this have predicted that the next glacial period would usually begin within 1,500 years. They go on to predict that emissions have been so high that it will not. The causes of ice ages are not fully understood for either the large-scale ice age periods or the smaller ebb and flow of glacial–interglacial periods within an ice age. The consensus

9164-503: The last 1.5 million years were associated with northward shifts of melting Antarctic icebergs which changed ocean circulation patterns, leading to more CO 2 being pulled out of the atmosphere . The authors suggest that this process may be disrupted in the future as the Southern Ocean will become too warm for the icebergs to travel far enough to trigger these changes. Matthias Kuhle 's geological theory of Ice Age development

9280-554: The last glacial period the sea-level fluctuated 20–30 m as water was sequestered, primarily in the Northern Hemisphere ice sheets. When ice collected and the sea level dropped sufficiently, flow through the Bering Strait (the narrow strait between Siberia and Alaska is about 50 m deep today) was reduced, resulting in increased flow from the North Atlantic. This realigned the thermohaline circulation in

9396-512: The latest Quaternary Ice Age ). Outside these ages, Earth was previously thought to have been ice-free even in high latitudes; such periods are known as greenhouse periods . However, other studies dispute this, finding evidence of occasional glaciations at high latitudes even during apparent greenhouse periods. Rocks from the earliest well-established ice age, called the Huronian , have been dated to around 2.4 to 2.1 billion years ago during

9512-453: The latter named for a set of rapids on the Merced River near the present-day site of McSwain Dam. In the late 1880s a flour mill , woolen mill and a few lumber mills were constructed at Merced Falls. The Sugar Pine Lumber Company and Yosemite Lumber Company operated lumber mills at Merced Falls for over thirty years, relying on narrow-gauge railroads to ship lumber from the Sierra Nevada along

9628-466: The lower Merced River. The Sierra (or Mountain) Miwok lived in the upper Merced Canyon and in Yosemite Valley, and at the time the first white explorers came to the area, there were about 450 Sierra Miwok split among ten permanent villages. Paiute, of origin from the eastern Sierra near the Mono Lake area, also lived in the upper watershed of the Merced River. The Sierra Miwok and Mono Lake Paiute eventually, through cultural interaction over time, formed

9744-401: The mammal species found in the middle and upper watershed. One species of interest is the limestone salamander , an extremely rare amphibian whose only habitat is the Merced Canyon downstream of Yosemite Valley. The salamander depends on the limestone walls of the Merced Canyon to survive. To protect the salamander, a 20-mile (32 km) segment of the canyon covering 1,600 acres (6.5 km)

9860-412: The middle and upper sections of the Merced River include mourning dove , Cassin's finch , California quail , dark-eyed junco , woodpecker , dipper , great blue heron , scrub jay , red-winged blackbird , red-tailed hawk , turkey vulture , cliff swallow , canyon wren , merganser , and bald eagles . Common insects found along the river include mayflies , stoneflies and caddisflies . The river

9976-484: The most recent glacial periods, ice cores provide climate proxies , both from the ice itself and from atmospheric samples provided by included bubbles of air. Because water containing lighter isotopes has a lower heat of evaporation , its proportion decreases with warmer conditions. This allows a temperature record to be constructed. This evidence can be confounded, however, by other factors recorded by isotope ratios. The paleontological evidence consists of changes in

10092-526: The motion of tectonic plates resulting in changes in the relative location and amount of continental and oceanic crust on Earth's surface, which affect wind and ocean currents ; variations in solar output ; the orbital dynamics of the Earth–Moon system; the impact of relatively large meteorites and volcanism including eruptions of supervolcanoes . Some of these factors influence each other. For example, changes in Earth's atmospheric composition (especially

10208-516: The mouth of Yosemite Valley, well into Merced Canyon. These glaciers formed the granite cliffs that now constitute landmarks such as Half Dome , El Capitán , and Cloud's Rest . The first and largest glacier was the Sherwin or Pre-Tahoe glacier, which eroded the upper Merced watershed to an extent close to its present form. Three stages followed during the Wisconsinian glaciation ; these were

10324-459: The names Riss (180,000–130,000 years bp ) and Würm (70,000–10,000 years bp) refer specifically to glaciation in the Alpine region . The maximum extent of the ice is not maintained for the full interval. The scouring action of each glaciation tends to remove most of the evidence of prior ice sheets almost completely, except in regions where the later sheet does not achieve full coverage. Within

10440-521: The past few million years. These also confirm the linkage between ice ages and continental crust phenomena such as glacial moraines, drumlins, and glacial erratics. Hence the continental crust phenomena are accepted as good evidence of earlier ice ages when they are found in layers created much earlier than the time range for which ice cores and ocean sediment cores are available. There have been at least five major ice ages in Earth's history (the Huronian , Cryogenian , Andean-Saharan , late Paleozoic , and

10556-567: The reduction in weathering causes an increase in the greenhouse effect . There are three main contributors from the layout of the continents that obstruct the movement of warm water to the poles: Since today's Earth has a continent over the South Pole and an almost land-locked ocean over the North Pole, geologists believe that Earth will continue to experience glacial periods in the geologically near future. Some scientists believe that

10672-628: The river Rio de Nuestra Señora de la Merced (River of Our Lady of Mercy), who is the patron saint of the diocese of Barcelona and is celebrated on September 24. Another expedition to the Central Valley in 1805 also named the Kings River upon reaching it on January 6, 1805, which is the feast of the Magi or Epiphany. Moraga's expedition was part of a series of exploratory ventures, funded by the Spanish government, to find suitable sites for missions in

10788-506: The river had one of the southernmost runs of chinook salmon in North America. Miwok and Paiute people lived along the river for thousands of years before Spanish and Mexican military expeditions passed through in the early 19th century. The California Gold Rush brought many people into California and some settled in towns along the lower Merced River. A railroad was built along the Merced canyon, enabling mining and logging in

10904-571: The river out of the west entrance to the national park, a few miles before the South Fork Merced River , the largest tributary, joins from the left. The river arcs northwest to receive the North Fork , and a few miles after it enters Lake McClure , formed by New Exchequer Dam . Below New Exchequer, the river flows west through a heavily irrigated region of the Central Valley, passing through McSwain and Crocker-Huffman Dams and

11020-481: The river, defined as the stretch from Lake McClure to the headwaters, has 11 species of fish. Historically, the range of anadromous fish extended to the waterfalls at the head of Yosemite Valley, but dams have blocked their migration since the early 1900s, and diversions frequently dewater their remaining spawning habitat in the lower part of the river. Environmental measures enacted in the late 20th century have had some success in boosting chinook salmon populations, from

11136-469: The role of weathering). Greenhouse gas levels may also have been affected by other factors which have been proposed as causes of ice ages, such as the movement of continents and volcanism. The Snowball Earth hypothesis maintains that the severe freezing in the late Proterozoic was ended by an increase in CO 2 levels in the atmosphere, mainly from volcanoes, and some supporters of Snowball Earth argue that it

11252-403: The slow-moving lower river, which has more suitable riparian habitat compared to the rocky, swift upper river. Common species of bird throughout the basin include ruby-crowned kinglet , cedar waxwing , American robin , yellow-rumped warbler , tree swallow and European starling , and several endangered species, including white-tailed kite and Swainson's hawk . Birds that occur commonly in

11368-500: The spread of ice sheets in the Northern Hemisphere began. Since then, the world has seen cycles of glaciation with ice sheets advancing and retreating on 40,000- and 100,000-year time scales called glacial periods , glacials or glacial advances, and interglacial periods, interglacials or glacial retreats. Earth is currently in an interglacial, and the last glacial period ended about 11,700 years ago. All that remains of

11484-557: The summer of 1835 he made some excursions to the Bavarian Alps. Schimper came to the conclusion that ice must have been the means of transport for the boulders in the alpine upland. In the winter of 1835–36 he held some lectures in Munich. Schimper then assumed that there must have been global times of obliteration ("Verödungszeiten") with a cold climate and frozen water. Schimper spent the summer months of 1836 at Devens, near Bex, in

11600-481: The temperatures over land by increased albedo as noted above. Furthermore, under this hypothesis the lack of oceanic pack ice allows increased exchange of waters between the Arctic and the North Atlantic Oceans, warming the Arctic and cooling the North Atlantic. (Current projected consequences of global warming include a brief ice-free Arctic Ocean period by 2050 .) Additional fresh water flowing into

11716-477: The term ice age is defined by the presence of extensive ice sheets in the northern and southern hemispheres. By this definition, the current Holocene period is an interglacial period of an ice age. The accumulation of anthropogenic greenhouse gases is projected to delay the next glacial period. In 1742, Pierre Martel (1706–1767), an engineer and geographer living in Geneva , visited the valley of Chamonix in

11832-547: The tilt of Earth's rotational axis. Earth has been in an interglacial period known as the Holocene for around 11,700 years, and an article in Nature in 2004 argues that it might be most analogous to a previous interglacial that lasted 28,000 years. Predicted changes in orbital forcing suggest that the next glacial period would begin at least 50,000 years from now. Moreover, anthropogenic forcing from increased greenhouse gases

11948-435: The upper Merced River watershed, however, was formed by glaciers , and the lower watershed was indirectly but significantly affected. When the last glacial period or Ice Age arrived, a series of four tremendous valley glaciers filled the upper basin of the Merced River. These glaciers rose in branches upstream of Yosemite Valley, descending from the Merced River headwaters, Tenaya Canyon and Illilouette Creek. Tenaya Canyon

12064-413: The upper Merced River watershed. Other national forests protecting more of the Merced River upper basin followed, including Sierra National Forest and Stanislaus National Forest . The Yosemite Valley Railroad , originally established with the discovery of mineral deposits in Yosemite Valley and Merced Canyon, continued functioning through the early 20th century carrying tourists to Yosemite Valley along

12180-420: The upper watershed, and later carrying tourists to Yosemite National Park. Conflicts between settlers and Native Americans resulted in wars, including the expulsion of the Ahwahnechee from Yosemite. Large-scale irrigation was introduced to the San Joaquin Valley in the late 19th century, and led to the construction of numerous state, federal and privately owned dams, which blocked migrating salmon and caused

12296-417: The water out of the lower river, which is often reduced to a small stream where it joins the San Joaquin. Irrigation return flows carry pesticides , fertilizer and other pollutants into the river. The MID is federally required to allow at least 15,000 acre-feet (19,000,000 m) of water annually to flow continually down the river, not including floodwater releases. The Merced River and its tributaries are

12412-552: The world. When the Bavarian naturalist Ernst von Bibra (1806–1878) visited the Chilean Andes in 1849–1850, the natives attributed fossil moraines to the former action of glaciers. Meanwhile, European scholars had begun to wonder what had caused the dispersal of erratic material. From the middle of the 18th century, some discussed ice as a means of transport. The Swedish mining expert Daniel Tilas (1712–1772) was, in 1742,

12528-429: Was a timber crib dam built in 1917 near where the Merced River flows out of Yosemite Valley. Originally built to generate hydropower, the dam was decommissioned in 1985 but remained standing for a number of years afterward. After the great flood of 1997, the U.S. Bureau of Reclamation surveyed the dam and found it in danger of failure. Classified as a "high hazard" structure, it was originally considered for inclusion on

12644-708: Was actually eroded even deeper by an arm of the Tuolumne Glacier, which formed the Grand Canyon of the Tuolumne and Hetch Hetchy Valley on the Tuolumne River in the north. Little Yosemite Valley formed as a result of the underlying rock being harder than that below the Giant Staircase, the cliff wall containing Vernal Fall and Nevada Fall . These three branches of each glacier combined to form one large glacier about 7,000 feet (2,100 m) thick at maximum, stretching 25 miles (40 km) downstream past

12760-625: Was caused by the elimination of atmospheric methane , a greenhouse gas , during the Great Oxygenation Event . The next well-documented ice age, and probably the most severe of the last billion years, occurred from 720 to 630 million years ago (the Cryogenian period) and may have produced a Snowball Earth in which glacial ice sheets reached the equator, possibly being ended by the accumulation of greenhouse gases such as CO 2 produced by volcanoes. "The presence of ice on

12876-429: Was caused in the first place by a reduction in atmospheric CO 2 . The hypothesis also warns of future Snowball Earths. In 2009, further evidence was provided that changes in solar insolation provide the initial trigger for Earth to warm after an Ice Age, with secondary factors like increases in greenhouse gases accounting for the magnitude of the change. The geological record appears to show that ice ages start when

12992-740: Was designated an "Area of Critical Environmental Concern" in 1986. When the North American Plate on its slow journey westwards encountered the Pacific Plate approximately 250 million years ago during the Paleozoic , the latter began to subduct under the North American continent. Intense pressure underground caused some of the Pacific Plate to melt, and the resulting upwelling magma pushed up and hardened into

13108-455: Was difficult to date exactly; early theories assumed that the glacials were short compared to the long interglacials. The advent of sediment and ice cores revealed the true situation: glacials are long, interglacials short. It took some time for the current theory to be worked out. The chemical evidence mainly consists of variations in the ratios of isotopes in fossils present in sediments and sedimentary rocks and ocean sediment cores. For

13224-435: Was preceded by the old Exchequer Dam forming 281,000-acre-foot (347,000,000 m) Exchequer Reservoir. The old concrete arch dam, completed in 1926, was flooded out when the new concrete-faced rockfill dam was built in 1967, but occasionally reappears during periods of low water. Downstream of New Exchequer Dam is McSwain Dam, which serves as a regulating forebay for New Exchequer Dam and also generates power. Below this dam

13340-489: Was reported there on December 23, 1955. For the mouth gauge, the highest flow was only 13,600 cubic feet per second (390 m/s) in 1950. Finally, for the gauge at Happy Isles, the largest flow ever recorded was 10,100 cubic feet per second (290 m/s) in the 1997 Yosemite floods, which destroyed many campgrounds, roads, paths, and bridges in the valley. According to a study in 2006, there were 37 fish species, 127 bird species, and 140 insect and invertebrate species found in

13456-526: Was suggested by the existence of an ice sheet covering the Tibetan Plateau during the Ice Ages ( Last Glacial Maximum ?). According to Kuhle, the plate-tectonic uplift of Tibet past the snow-line has led to a surface of c. 2,400,000 square kilometres (930,000 sq mi) changing from bare land to ice with a 70% greater albedo . The reflection of energy into space resulted in a global cooling, triggering

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