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70-487: Its reservoir is called Big Tujunga Reservoir, and collects runoff from a watershed of 82 square miles (210 km). Although it is located inside the Angeles National Forest , public access to the lake is forbidden. The water is usually kept at a low level, in order to protect against winter floods. The name of the dam is derived from a Tongva village name. The dam is 208 feet (63 m) high from

140-417: A dam constructed across a valley and rely on the natural topography to provide most of the basin of the reservoir. These reservoirs can either be on-stream reservoirs , which are located on the original streambed of the downstream river and are filled by creeks , rivers or rainwater that runs off the surrounding forested catchments, or off-stream reservoirs , which receive diverted water from

210-553: A 24-inch (610 mm) valve was installed at the dam base to pass low flows for habitat conservation . The seismic retrofit project was completed in July 2011 at a cost of $ 100 million. Reservoir A reservoir ( / ˈ r ɛ z ər v w ɑːr / ; from French réservoir [ʁezɛʁvwaʁ] ) is an enlarged lake behind a dam , usually built to store fresh water , often doubling for hydroelectric power generation . Reservoirs are created by controlling

280-537: A block for migrating fish, trapping them in one area, producing food and a habitat for various water-birds. They can also flood various ecosystems on land and may cause extinctions. Creating reservoirs can alter the natural biogeochemical cycle of mercury . After a reservoir's initial formation, there is a large increase in the production of toxic methylmercury (MeHg) via microbial methylation in flooded soils and peat. MeHg levels have also been found to increase in zooplankton and in fish. Dams can severely reduce

350-658: A conventional oil-fired thermal generation plant. For instance, In 1990, the impoundment behind the Balbina Dam in Brazil (inaugurated in 1987) had over 20 times the impact on global warming than would generating the same power from fossil fuels, due to the large area flooded per unit of electricity generated. Another study published in the Global Biogeochemical Cycles also found that newly flooded reservoirs released more carbon dioxide and methane than

420-533: A drop of water seep into the ocean without benefiting mankind." He created the reservoir named Parakrama Samudra ("sea of King Parakrama"). Vast artificial reservoirs were also built by various ancient kingdoms in Bengal, Assam, and Cambodia. Many dammed river reservoirs and most bank-side reservoirs are used to provide the raw water feed to a water treatment plant which delivers drinking water through water mains. The reservoir does not merely hold water until it

490-498: A great help to farmers in helping them overcome dry spells. However, they can promote vector-borne diseases such as malaria or schistosomiasis . Detention basins are designed for temporary capture of flood waters and do not allow for permanent pooling of water and therefore do not make viable or reliable sources of water storage. Retention basins are similar to detention basins for flood control management, but are built for permanent pooling to control sediment and pollutants in

560-455: A high-level reservoir is filled with water using high-performance electric pumps at times when electricity demand is low, and then uses this stored water to generate electricity by releasing the stored water into a low-level reservoir when electricity demand is high. Such systems are called pump-storage schemes. Reservoirs can be used in a number of ways to control how water flows through downstream waterways: Reservoirs can be used to balance

630-570: A loss in both quantity and quality of water necessary for maintaining ecological balance and pose a risk of increasing severity and duration of droughts due to climate change. In summary, they consider it a misadaptation to climate change. Proponents of reservoirs or substitution reserves, on the other hand, see them as a solution for sustainable agriculture while waiting for a truly durable agricultural model. Without such reserves, they fear that unsustainable imported irrigation will be inevitable. They believe that these reservoirs should be accompanied by

700-766: A million people who make their living as fishermen, cattle breeders or farmers, using the annual rise and fall of the river waters and its floodplains. Wetlands are basically sponges that capture and slowly release large amounts of rain, snowmelt, groundwater and floodwater. Trees and other wetlands vegetation slow the speed of flood water and more evenly distribute it across the wetland. The combination of increased water storage and flood water hindrances lower flood heights and reduce erosion. Detention basins and water tanks can be defined as community-built and household water stores, filled by rainwater, groundwater infiltration or surface runoff . They are usually open, and therefore exposed to high levels of evaporation. They can be

770-419: A narrow valley or canyon may cover relatively little vegetation, while one situated on a plain may flood a great deal of vegetation. The site may be cleared of vegetation first or simply flooded. Tropical flooding can produce far more greenhouse gases than in temperate regions. The following table indicates reservoir emissions in milligrams per square meter per day for different bodies of water. Depending upon

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840-470: A nearby stream or aqueduct or pipeline water from other on-stream reservoirs. Dams are typically located at a narrow part of a downstream of a natural basin. The valley sides act as natural walls, with the dam located at the narrowest practical point to provide strength and the lowest cost of construction. In many reservoir construction projects, people have to be moved and re-housed, historical artifacts moved or rare environments relocated. Examples include

910-611: A reservoir is called the "conservation pool". In the United Kingdom, "top water level" describes the reservoir full state, while "fully drawn down" describes the minimum retained volume. There is a wide variety of software for modelling reservoirs, from the specialist Dam Safety Program Management Tools (DSPMT) to the relatively simple WAFLEX , to integrated models like the Water Evaluation And Planning system (WEAP) that place reservoir operations in

980-450: A river of variable quality or size, bank-side reservoirs may be built to store the water. Such reservoirs are usually formed partly by excavation and partly by building a complete encircling bund or embankment , which may exceed 6 km (4 miles) in circumference. Both the floor of the reservoir and the bund must have an impermeable lining or core: initially these were often made of puddled clay , but this has generally been superseded by

1050-740: A river valley, with the potential to wash away towns and villages and cause considerable loss of life, such as the devastation following the failure of containment at Llyn Eigiau which killed 17 people. (see also List of dam failures ) A notable case of reservoirs being used as an instrument of war involved the British Royal Air Force Dambusters raid on Germany in World War II (codenamed " Operation Chastise " ), in which three German reservoir dams were selected to be breached in order to damage German infrastructure and manufacturing and power capabilities deriving from

1120-582: A territorial project that unites all water stakeholders with the goal of preserving and enhancing natural environments. Two main types of reservoirs can be distinguished based on their mode of supply. Circa 3000 BC, the craters of extinct volcanoes in Arabia were used as reservoirs by farmers for their irrigation water. Dry climate and water scarcity in India led to early development of stepwells and other water resource management techniques, including

1190-401: A water table reaches capacity, or all soil is completely saturated, the water table meets the surface of the ground where water discharge in the forms of springs or seeps . It is also possible to artificially recharge aquifers (using wells), for example through the use of Aquifer storage and recovery (ASR). Groundwater is stored in two zones, one being the saturated zone, or Aquifer ,

1260-468: A watercourse that drains an existing body of water, interrupting a watercourse to form an embayment within it, excavating, or building any number of retaining walls or levees to enclose any area to store water. The term is also used technically to refer to certain forms of liquid storage, such the "coolant reservoir" that captures overflow of coolant in an automobile's cooling system. Dammed reservoirs are artificial lakes created and controlled by

1330-476: Is a natural lake whose level was raised by a low dam and into which the River Dee flows or discharges depending upon flow conditions, as part of the River Dee regulation system . This mode of operation is a form of hydraulic capacitance in the river system. Many reservoirs often allow some recreational uses, such as fishing and boating . Special rules may apply for the safety of the public and to protect

1400-461: Is a need to store potable drinking water during the dry season . In agriculture water storage, water is stored for later use in natural water sources, such as groundwater aquifers , soil water , natural wetlands , and small artificial ponds, tanks and reservoirs behind major dams. Storing water invites a host of potential issues regardless of that water's intended purpose, including contamination through organic and inorganic means. Groundwater

1470-484: Is a water reservoir for agricultural use. They are filled using pumped groundwater , pumped river water or water runoff and are typically used during the local dry season. This type of infrastructure has sparked an opposition movement in France, with numerous disputes and, for some projects, protests, especially in the former Poitou-Charentes region where violent demonstrations took place in 2022 and 2023. In Spain, there

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1540-679: Is fraught with substantial land submergence, coastal reservoirs are preferred economically and technically since they do not use scarce land area. Many coastal reservoirs were constructed in Asia and Europe. Saemanguem in South Korea, Marina Barrage in Singapore, Qingcaosha in China, and Plover Cove in Hong Kong are a few such coastal reservoirs. Where water is pumped or siphoned from

1610-501: Is greater acceptance because all beneficiary users are involved in the implementation of the system. The specific debate about substitution reservoirs is part of a broader discussion related to reservoirs used for agricultural irrigation, regardless of their type, and a certain model of intensive agriculture. Opponents view these reservoirs as a monopolization of resources benefiting only a few, representing an outdated model of productive agriculture. They argue that these reservoirs lead to

1680-526: Is likely to make farming more difficult. A range of water stores could help farmers overcome dry spells that would otherwise cause their crops to fail. Field studies have shown the effectiveness of small-scale water storage. For example, using small planting basins to 'harvest' water in Zimbabwe have been shown to boost maize yields, whether rainfall is abundant or scarce. In Niger , they have led to three or fourfold increases in millet yields. As of 2010, it

1750-413: Is located beneath the ground surface in soil pore spaces and in the fractures of rock formations . A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock become completely saturated with water is called the water table . There are two broad types of aquifers: An unconfined aquifer

1820-466: Is needed: it can also be the first part of the water treatment process. The time the water is held before it is released is known as the retention time . This is a design feature that allows particles and silts to settle out, as well as time for natural biological treatment using algae , bacteria and zooplankton that naturally live in the water. However natural limnological processes in temperate climate lakes produce temperature stratification in

1890-536: Is now used by the Aquarius Golf Club. Service reservoirs perform several functions, including ensuring sufficient head of water in the water distribution system and providing water capacity to even-out peak demand from consumers, enabling the treatment plant to run at optimum efficiency. Large service reservoirs can also be managed to reduce the cost of pumping by refilling the reservoir at times of day when energy costs are low. An irrigation reservoir

1960-531: Is poorly suited to forming a single large reservoir, a number of smaller reservoirs may be constructed in a chain, as in the River Taff valley where the Llwyn-on , Cantref and Beacons Reservoirs form a chain up the valley. Coastal reservoirs are fresh water storage reservoirs located on the sea coast near a river mouth to store the flood water of a river. As the land-based reservoir construction

2030-419: Is stored in the reservoir. Any excess water can be spilled via a specifically designed spillway. Stored water may be piped by gravity for use as drinking water , to generate hydro-electricity or to maintain river flows to support downstream uses. Occasionally reservoirs can be managed to retain water during high rainfall events to prevent or reduce downstream flooding. Some reservoirs support several uses, and

2100-473: Is the portion of the reservoir that can be used for flood control, power production, navigation , and downstream releases. In addition, a reservoir's "flood control capacity" is the amount of water it can regulate during flooding. The "surcharge capacity" is the capacity of the reservoir above the spillway crest that cannot be regulated. In the United States, the water below the normal maximum level of

2170-571: Is trapped by the Aswan High Dam, the Nile no longer delivers nutrients in large quantities to the floodplain . This has reduced soil fertility and increased the need for fertilizer. Water stored in dams and reservoirs can be treated for drinking water, but in the past due to poor taxing and high water prices in the US, water supply dams are unable to reach their intended levels of operation. Due to

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2240-409: Is where the surface is not restricted by impervious rocks, so the water table is at atmospheric pressure. In a confined aquifer, the upper surface of water is overlain by a layer of impervious rock, so the groundwater is stored under pressure. Aquifers receive water through two ways, one from precipitation that flows through the unsaturated zone of the soil profile, and two from lakes and rivers. When

2310-452: The Ruhr and Eder rivers. The economic and social impact was derived from the enormous volumes of previously stored water that swept down the valleys, wreaking destruction. This raid later became the basis for several films. All reservoirs will have a monetary cost/benefit assessment made before construction to see if the project is worth proceeding with. However, such analysis can often omit

2380-673: The 1800s, most of which are lined with brick. A good example is the Honor Oak Reservoir in London, constructed between 1901 and 1909. When it was completed it was said to be the largest brick built underground reservoir in the world and it is still one of the largest in Europe. This reservoir now forms part of the southern extension of the Thames Water Ring Main . The top of the reservoir has been grassed over and

2450-514: The Big Tujunga watershed, caused more than 2 million cubic yards (1.5 million m) of sediment to flow into the reservoir, all of which subsequently had to be removed. Aside from the village of Tujunga or Tuxunga (means "old woman's place" in both the languages of the Tongva and Fernandeño ), the former Fernandeño (Native American) village in the area of the dam and Big Tujunga Canyon

2520-731: The Meroitic town of Butana . The Hafirs catch the water during rainy seasons in order to ensure water is available for several months during dry seasons to supply drinking water, irrigate fields and water cattle. The Great Reservoir near the Lion Temple in Musawwarat es-Sufra is a notable hafir in Kush. In Sri Lanka , large reservoirs were created by ancient Sinhalese kings in order to store water for irrigation. The famous Sri Lankan king Parākramabāhu I of Sri Lanka said "Do not let

2590-440: The US. The capacity, volume, or storage of a reservoir is usually divided into distinguishable areas. Dead or inactive storage refers to water in a reservoir that cannot be drained by gravity through a dam's outlet works , spillway, or power plant intake and can only be pumped out. Dead storage allows sediments to settle, which improves water quality and also creates an area for fish during low levels. Active or live storage

2660-534: The amount of water reaching countries downstream of them, causing water stress between the countries, e.g. the Sudan and Egypt , which damages farming businesses in the downstream countries, and reduces drinking water. Water storage Water storage is a broad term referring to storage of both potable water for consumption, and non potable water for use in agriculture. In both developing countries and some developed countries found in tropical climates, there

2730-535: The area flooded versus power produced, a reservoir built for hydro- electricity generation can either reduce or increase the net production of greenhouse gases when compared to other sources of power. A study for the National Institute for Research in the Amazon found that hydroelectric reservoirs release a large pulse of carbon dioxide from decay of trees left standing in the reservoirs, especially during

2800-428: The base of the dam or some distance away. In a flat river valley a reservoir needs to be deep enough to create a head of water at the turbines; and if there are periods of drought the reservoir needs to hold enough water to average out the river's flow throughout the year(s). Run-of-the-river hydro in a steep valley with constant flow needs no reservoir. Some reservoirs generating hydroelectricity use pumped recharge:

2870-465: The build, often through a temporary tunnel or by-pass channel. In hilly regions, reservoirs are often constructed by enlarging existing lakes. Sometimes in such reservoirs, the new top water level exceeds the watershed height on one or more of the feeder streams such as at Llyn Clywedog in Mid Wales . In such cases additional side dams are required to contain the reservoir. Where the topography

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2940-556: The building of a reservoir at Girnar in 3000 BC. Artificial lakes dating to the 5th century BC have been found in ancient Greece. The artificial Bhojsagar lake in present-day Madhya Pradesh state of India, constructed in the 11th century, covered 650 square kilometres (250 sq mi). The Kingdom of Kush invented the Hafir , a type of reservoir, during the Meroitic period . 800 ancient and modern hafirs have been registered in

3010-454: The completion of Hansen Dam downstream, the importance of Big Tujunga Dam for flood control has decreased; however, it is still directly responsible for protecting about 4,600 residents living in the floodplain of Big Tujunga Canyon. In addition, the dam conserves about 4,500 acre⋅ft (5,600,000 m) of runoff per year for drinking water and groundwater recharge purposes. This saves the city of Los Angeles about $ 2,250,000 annually due to

3080-442: The context of system-wide demands and supplies. In many countries large reservoirs are closely regulated to try to prevent or minimize failures of containment. While much of the effort is directed at the dam and its associated structures as the weakest part of the overall structure, the aim of such controls is to prevent an uncontrolled release of water from the reservoir. Reservoir failures can generate huge increases in flow down

3150-509: The cost difference between local and imported water. Because of the small storage capacity of the reservoir relative to the size of its watershed, frequent dredging is required to remove sediment from behind the dam. Most of the sediment is compacted and stored at the Maple Canyon Sediment Placement Site located less than 1 mile (1.6 km) west of the dam. The 2009 Station Fire , which burned some 87% of

3220-460: The developed world Naturally occurring lakes receive organic sediments which decay in an anaerobic environment releasing methane and carbon dioxide . The methane released is approximately 8 times more potent as a greenhouse gas than carbon dioxide. As a human-made reservoir fills, existing plants are submerged and during the years it takes for this matter to decay, will give off considerably more greenhouse gases than lakes do. A reservoir in

3290-507: The downstream river to maintain river quality, support fisheries, to maintain downstream industrial and recreational uses or for a range of other purposes. Such releases are known as compensation water . The units used for measuring reservoir areas and volumes vary from country to country. In most of the world, reservoir areas are expressed in square kilometers; in the United States, acres are commonly used. For volume, either cubic meters or cubic kilometers are widely used, with acre-feet used in

3360-440: The environmental impacts of dams and the reservoirs that they contain. Some impacts, such as the greenhouse gas production associated with concrete manufacture, are relatively easy to estimate. Other impacts on the natural environment and social and cultural effects can be more difficult to assess and to weigh in the balance but identification and quantification of these issues is now commonly required in major construction projects in

3430-499: The first decade after flooding. This elevates the global warming impact of the dams to levels much higher than would occur by generating the same power from fossil fuels . According to the World Commission on Dams report (Dams And Development), when the reservoir is relatively large and no prior clearing of forest in the flooded area was undertaken, greenhouse gas emissions from the reservoir could be higher than those of

3500-431: The flood water. In the past, large dams have often been the focus of water storage efforts. Many large dams and their reservoirs have brought significant social and economic benefits. For example, Egypt's Aswan High Dam , built in the 1960s, has protected the nation from drought and floods and supplies water used to irrigate some 15 million hectares. However, dams can also have great negative impacts. Because sediment

3570-413: The flow in highly managed systems, taking in water during high flows and releasing it again during low flows. In order for this to work without pumping requires careful control of water levels using spillways . When a major storm approaches, the dam operators calculate the volume of water that the storm will add to the reservoir. If forecast storm water will overfill the reservoir, water is slowly let out of

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3640-580: The hydroelectric reservoirs there do emit greenhouse gases, it is on a much smaller scale than thermal power plants of similar capacity. Hydropower typically emits 35 to 70 times less greenhouse gases per TWh of electricity than thermal power plants. A decrease in air pollution occurs when a dam is used in place of thermal power generation, since electricity produced from hydroelectric generation does not give rise to any flue gas emissions from fossil fuel combustion (including sulfur dioxide , nitric oxide and carbon monoxide from coal ). Dams can produce

3710-469: The increased surface area of water that dams create, huge amounts of water is lost to evaporation , much more so than what would have been lost from the river that flowed in its place. Rainfed agriculture constitutes 80% of global agriculture. Many of the 852 million poor people in the world live in parts of Asia and Africa that depend on rainfall to cultivate food crops . As the global population swells, more food will be needed, but climate variability

3780-432: The landscape is relatively flat. Other service reservoirs can be storage pools, water tanks or sometimes entirely underground cisterns , especially in more hilly or mountainous country. Modern reserviors will often use geomembrane liners on their base to limit seepage and/or as floating covers to limit evaporation, particularly in arid climates. In the United Kingdom, Thames Water has many underground reservoirs built in

3850-470: The modern use of rolled clay. The water stored in such reservoirs may stay there for several months, during which time normal biological processes may substantially reduce many contaminants and reduce turbidity . The use of bank-side reservoirs also allows water abstraction to be stopped for some time, for instance when the river is unacceptably polluted or when flow conditions are very low due to drought . The London water supply system exhibits one example of

3920-415: The operating rules may be complex. Most modern reservoirs have a specially designed draw-off tower that can discharge water from the reservoir at different levels, both to access water as the water level falls, and to allow water of a specific quality to be discharged into the downstream river as "compensation water": the operators of many upland or in-river reservoirs have obligations to release water into

3990-522: The other is the pore space of unsaturated soil immediately below the ground surface. Soil moisture is the water held between soil particles in the root zone ( rhizosphere ) of plants, generally in the top 200 cm of soil. Water storage in the soil profile is extremely important for agriculture, especially in locations that rely on rainfall for cultivating plants. For example, in Africa rain-fed agriculture accounts for 95% of farmed land. Wetlands span

4060-481: The pre-flooded landscape, noting that forest lands, wetlands, and preexisting water features all released differing amounts of carbon dioxide and methane both pre- and post-flooding. The Tucuruí Dam in Brazil (completed in 1984) had only 0.4 times the impact on global warming than would generating the same power from fossil fuels. A two-year study of carbon dioxide and methane releases in Canada concluded that while

4130-437: The quality of the water and the ecology of the surrounding area. Many reservoirs now support and encourage less formal and less structured recreation such as natural history , bird watching , landscape painting , walking and hiking , and often provide information boards and interpretation material to encourage responsible use. Water falling as rain upstream of the reservoir, together with any groundwater emerging as springs,

4200-638: The reservoir prior to, and during, the storm. If done with sufficient lead time, the major storm will not fill the reservoir and areas downstream will not experience damaging flows. Accurate weather forecasts are essential so that dam operators can correctly plan drawdowns prior to a high rainfall event. Dam operators blamed a faulty weather forecast on the 2010–2011 Queensland floods . Examples of highly managed reservoirs are Burrendong Dam in Australia and Bala Lake ( Llyn Tegid ) in North Wales . Bala Lake

4270-636: The reservoir's level was temporarily restricted to about 25% of capacity. In 2008 the Los Angeles County Flood Control District began a project to rehabilitate the aging structure. Approximately 75,000 cu yd (57,000 m) of concrete was added to the dam, transforming it from a thin-arch to a thick-arch design. A new spillway was built and the original one was expanded, increasing the floodwater capacity from 23,000 cu ft/s (650 m/s) to more than 90,000 cu ft/s (2,500 m/s). In addition,

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4340-403: The riverbed, 244 feet (74 m) high from the foundations, and 830 feet (250 m) long. It has a width of 138 feet (42 m) at the base, tapering to 8 feet (2.4 m) at the crest. The storage capacity is 5,960 acre⋅ft (7,350,000 m) at the spillway, with a minimum pool of 1,210 acre⋅ft (1,490,000 m) to prevent sediment and debris from entering the dam's outlets. After

4410-562: The surface/sub-surface interface, storing water at various times as groundwater, soil moisture and surface water. They are vital ecosystems that support wildlife and perform valuable ecosystem services , such as flood protection and water cleansing. They also provide livelihoods for millions of people who live within and around them. For example, the Inner Niger River Delta in the Western Sahel zone supports more than

4480-537: The temples of Abu Simbel (which were moved before the construction of the Aswan Dam to create Lake Nasser from the Nile in Egypt ), the relocation of the village of Capel Celyn during the construction of Llyn Celyn , and the relocation of Borgo San Pietro of Petrella Salto during the construction of Lake Salto . Construction of a dammed reservoir will usually require the river to be diverted during part of

4550-499: The use of bank-side storage: here water is taken from the River Thames and River Lee into several large Thames-side reservoirs, such as Queen Mary Reservoir that can be seen along the approach to London Heathrow Airport . Service reservoirs store fully treated potable water close to the point of distribution. Many service reservoirs are constructed as water towers , often as elevated structures on concrete pillars where

4620-412: The water, which tends to partition some elements such as manganese and phosphorus into deep, cold anoxic water during the summer months. In the autumn and winter the lake becomes fully mixed again. During drought conditions, it is sometimes necessary to draw down the cold bottom water, and the elevated levels of manganese in particular can cause problems in water treatment plants. In 2005, about 25% of

4690-476: The world's 33,105 large dams (over 15 metres in height) were used for hydroelectricity. The U.S. produces 3% of its electricity from 80,000 dams of all sizes. An initiative is underway to retrofit more dams as a good use of existing infrastructure to provide many smaller communities with a reliable source of energy. A reservoir generating hydroelectricity includes turbines connected to the retained water body by large-diameter pipes. These generating sets may be at

4760-564: Was called Muxúnga , which means "place of shooting" in the Fernandeño dialect of the Tongva language . The name comes from the verb muxú, which means "shoot him." The dam was completed in 1931 by the Los Angeles County Flood Control District , at a cost of $ 1.2 million ($ 24 million in 2023 dollars). It was originally planned as one of several flood control dams inside Big Tujunga Canyon and

4830-428: Was reported that nearly half of the global population depends on in-home water storage due to a lack of adequate water supply networks. Many of the in-home solutions have improvised from available materials. It has been suggested that the lack of proper tools and equipment for construction, leads to a system more likely to contain breaches, making them more susceptible to contamination from the environment and users. In

4900-591: Was thus referred to as Big Tujunga Dam No. 1 until the larger Hansen Dam was completed in 1940 at the mouth of the canyon, eliminating the need for the other dams. During the Los Angeles flood of 1938 , the dam was able to stop a huge debris flow of boulders and uprooted trees, sparing much of Sunland, Tujunga and Glendale from destruction. In 1976, the dam was recognized as in danger of failure from earthquakes (the San Andreas Fault runs nearby) and

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