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38-410: The river is 250–400 m wide in this area, and though in the dry season it may be confined to a shallow channel meandering across sandbanks, at the end of the rainy season any bridge has to be able to withstand a full-width, deep and fast-moving flood. The Great East Road runs for most of its length on watersheds at an elevation of around 1000 m, but the river is at an elevation of 390 m at the bottom of

76-934: A key problem in such canals is ensuring a sufficient water supply. Important examples are the Chicago Portage , connecting the Great Lakes and Mississippi by the Chicago Sanitary and Ship Canal , and the Canal des Deux Mers in France, connecting the Atlantic and the Mediterranean. The name is enshrined at the Height of Land Portage on the route from the Great Lakes in the Atlantic drainage basin to

114-610: A low point on a divide where it is possible to portage a canoe from one river system to another. Drainage divides can be divided into three types: A valley-floor divide occurs on the bottom of a valley and arises as a result of subsequent depositions, such as scree , in a valley through which a river originally flowed continuously. Examples include the Kartitsch Saddle in the Gail valley in East Tyrol , which forms

152-584: A paper in Journal of Natural History in 2012, reporting that: "This microhabitat, as well as interstitial spaces between scree blocks elsewhere on this slope, supports an important assemblage of boreal and arctic bryophytes , pteridophytes , and arthropods that are disjunct from their normal ranges far to the north. This freezing scree slope represents a classic example of a palaeo refugium that significantly contributes to [the] protection and maintenance of regional landscape biodiversity ." Ice Mountain ,

190-576: Is a low drainage divide that runs across a valley , sometimes created by deposition or stream capture . Major divides separating rivers that drain to different seas or oceans are continental divides . The term height of land is used in Canada and the United States to refer to a drainage divide. It is frequently used in border descriptions, which are set according to the "doctrine of natural boundaries ". In glaciated areas it often refers to

228-784: Is difficult to find a meaningful definition of a watershed. A bifurcation is where the watershed is effectively in a river bed, in a wetland, or underground. The largest watershed of this type is the bifurcation of the Orinoco in the north of South America , whose main stream empties into the Caribbean , but which also drains into the South Atlantic via the Casiquiare canal and Amazon River . Since ridgelines are sometimes easy to see and agree about, drainage divides may form natural borders defining political boundaries, as with

266-431: Is elevated terrain that separates neighboring drainage basins . On rugged land, the divide lies along topographical ridges , and may be in the form of a single range of hills or mountains , known as a dividing range . On flat terrain, especially where the ground is marshy , the divide may be difficult to discern. A triple divide is a point, often a summit , where three drainage basins meet. A valley floor divide

304-419: Is formed by rockfall, which distinguishes it from colluvium . Colluvium is rock fragments or soil deposited by rainwash , sheetwash , or slow downhill creep , usually at the base of gentle slopes or hillsides. However, the terms scree , talus , and sometimes colluvium tend to be used interchangeably. The term talus deposit is sometimes used to distinguish the landform from the material of which it

342-455: Is generally good sorting of sediment by size: larger particles accumulate more rapidly at the bottom of the slope. Cementation occurs as fine-grained material fills in gaps between debris. The speed of consolidation depends on the composition of the slope; clayey components will bind debris together faster than sandy ones. Should weathering outpace the supply of sediment, plants may take root. Plant roots diminish cohesive forces between

380-455: Is made. The exact definition of scree in the primary literature is somewhat relaxed, and it often overlaps with both talus and colluvium . The term scree comes from the Old Norse term for landslide , skriða , while the term talus is a French word meaning a slope or embankment. Talus deposits typically have a concave upwards form, where the maximum inclination corresponds to

418-508: Is still a possibility of the deposit slopes themselves failing. If the talus deposit pile shifts and the particles exceed the angle of repose, the scree itself may slide and fail. Phenomena such as acid rain may also contribute to the chemical degradation of rocks and produce more loose sediments. Biotic processes often intersect with both physical and chemical weathering regimes, as the organisms that interact with rocks can mechanically or chemically alter them. Lichen frequently grow on

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456-411: Is the thermal conductivity of the debris material, T s is the ambient temperature above the debris surface, T i is the temperature at the lower surface of the debris, and d is the thickness of the debris layer. Debris with a low thermal conductivity value, or a high thermal resistivity , will not efficiently transfer energy through to the glacier, meaning the amount of heat energy reaching

494-450: Is thought to be most common during the spring and fall, when the daily temperatures fluctuate around the freezing point of water, and snow melt produces ample free water. The efficiency of freeze-thaw processes in scree production is a subject of ongoing debate. Many researchers believe that ice formation in large open fracture systems cannot generate high enough pressures to force the fracturing apart of parent rocks, and instead suggest that

532-518: The Beit Trust . Its proximity to Mozambique (about 2 km) and Rhodesia (60 km), as well as its status as the only access point within the country for Zambia's Eastern Province , makes it strategically vulnerable in any conflict. After Zambia's independence in 1964, President Kenneth Kaunda's support for freedom fighters in neighbouring countries led to the bridge being destroyed in revenge. The Second Luangwa Bridge (opened in 1968)

570-462: The Hudson Bay drainage basin . Scree Scree is a collection of broken rock fragments at the base of a cliff or other steep rocky mass that has accumulated through periodic rockfall . Landforms associated with these materials are often called talus deposits . The term scree is applied both to an unstable steep mountain slope composed of rock fragments and other debris , and to

608-882: The Royal Proclamation of 1763 in British North America which coincided with the ridgeline of the Appalachian Mountains forming the Eastern Continental Divide that separated settled colonial lands in the east from Indian Territory to the west. Another instance of a border matching a watershed in modern times involves the western border between Labrador and Quebec , as arbitrated by the privy council in 1927. Drainage divides hinder waterway navigation . In pre-industrial times, water divides were crossed at portages . Later, canals connected adjoining drainage basins;

646-406: The angle of repose of the mean debris particle size. Scree slopes are often assumed to be close to the angle of repose. This is the slope at which a pile of granular material becomes mechanically unstable. However, careful examination of scree slopes shows that only those that are either rapidly accumulating new material, or are experiencing rapid removal of material from their bases, are close to

684-447: The angle of repose. Most scree slopes are less steep, and they often show a concave shape, so that the foot of the slope is less steep than the top of the slope. Scree with large, boulder-sized rock fragments may form talus caves , or human-sized passages formed in-between boulders. The formation of scree and talus deposits is the result of physical and chemical weathering acting on a rock face, and erosive processes transporting

722-405: The bottom of some scree slopes despite mean annual air temperatures of 6.8–7.5 °C. Scree microclimates maintained by circulating freezing air create micro habitats that support taiga plants and animals that could not otherwise survive regional conditions. A Czech Republic Academy of Sciences research team led by physical chemist Vlastimil Růžička, analyzing 66 scree slopes, published

760-443: The coarse and fine components, degrading the slope. The predominant processes that degrade a rock slope depend largely on the regional climate (see below), but also on the thermal and topographic stresses governing the parent rock material. Example process domains include: Scree formation is commonly attributed to the formation of ice within mountain rock slopes. The presence of joints , fractures , and other heterogeneities in

798-415: The cold winter atmosphere and subnivean spaces in screes. As a result, soil, bedrock, and also subterranean voids in screes do not freeze at high elevations. A scree has many small interstitial voids, while an ice cave has a few large hollows. Due to cold air seepage and air circulation, the bottom of scree slopes have a thermal regime similar to ice caves. Because subsurface ice is separated from

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836-414: The cover layer to the debris-ice interface. If the ice is covered by a relatively thin layer of debris (less than around 2 centimeters thick), the albedo effect is most important. As scree accumulates atop the glacier, the ice's albedo will begin to decrease. Instead, the glacier ice will absorb incoming solar radiation and transfer it to the upper surface of the ice. Then, the glacier ice begins to absorb

874-402: The energy and uses it in the process of melting. However, once the debris cover reaches 2 or more centimeters in thickness, the albedo effect begins to dissipate. Instead, the debris blanket will act to insulate the glacier, preventing incoming radiation from penetrating the scree and reaching the ice surface. In addition to rocky debris, thick snow cover can form an insulating blanket between

912-498: The glacier ice begins melting more rapidly or more slowly is determined by the thickness of the layer of scree on its surface. The amount of energy reaching the surface of the ice below the debris can be estimated via the one-dimensional, homogeneous material assumption of Fourier's law : Q = − k ( T s − T i d ) {\displaystyle Q=-k\left({\frac {T_{s}-T_{i}}{d}}\right)} , where k

950-427: The host rock can be incorporated into the biological structure and weaken the rock. Freeze-thaw action of the entire lichen body due to microclimatic changes in moisture content can alternately cause thermal contraction and expansion, which also stresses the host rock. Lichen also produce a number of organic acids as metabolic byproducts. These often react with the host rock, dissolving minerals, and breaking down

988-437: The ice surface is substantially lessened. This can act to insulate the glacier from incoming radiation. The albedo , or the ability of a material to reflect incoming radiation energy, is also an important quality to consider. Generally, the debris will have a lower albedo than the glacier ice it covers, and will thus reflect less incoming solar radiation. Instead, the debris will absorb radiation energy and transfer it through

1026-428: The material downslope. In high-altitude arctic and subarctic regions, scree slopes and talus deposits are typically adjacent to hills and river valleys. These steep slopes usually originate from late- Pleistocene periglacial processes. There are five main stages of scree slope evolution: Scree slopes form as a result of accumulated loose, coarse-grained material. Within the scree slope itself, however, there

1064-847: The military of Zimbabwe-Rhodesia destroyed the bridge as well as the Chambeshi road and rail bridges in order to stop the planned invasion of their country by the Zimbabwe People's Revolutionary Army (ZIPRA). After another two decades of use, the bridge was rehabilitated under a project of the National Road Fund Agency funded by Danish aid. As of 2022, there are plans to rehabilitate the bridge. 14°58′33.6″S 30°12′45″E  /  14.976000°S 30.21250°E  / -14.976000; 30.21250 Water divide A drainage divide , water divide , ridgeline , watershed , water parting or height of land

1102-470: The mixture of rock fragments and debris itself. It is loosely synonymous with talus , material that accumulates at the base of a projecting mass of rock, or talus slope , a landform composed of talus. The term scree is sometimes used more broadly for any sheet of loose rock fragments mantling a slope, while talus is used more narrowly for material that accumulates at the base of a cliff or other rocky slope from which it has obviously eroded. Scree

1140-543: The rock wall can allow precipitation , groundwater , and surface runoff to flow through the rock. If the temperature drops below the freezing point of the fluid contained within the rock, during particularly cold evenings, for example, this water can freeze. Since water expands by 9% when it freezes, it can generate large forces that either create new cracks or wedge blocks into an unstable position. Special boundary conditions (rapid freezing and water confinement) may be required for this to happen. Freeze-thaw scree production

1178-506: The substrate into unconsolidated sediments. Scree often collects at the base of glaciers , concealing them from their environment. For example, Lech dl Dragon , in the Sella group of the Dolomites , is derived from the melting waters of a glacier and is hidden under a thick layer of scree. Debris cover on a glacier affects the energy balance and, therefore, the melting process. Whether

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1216-475: The surface by thin, permeable sheets of sediment, screes experience cold air seepage from the bottom of the slope where sediment is thinnest. This freezing circulating air maintains internal scree temperatures 6.8-9.0 °C colder than external scree temperatures. These <0 °C thermal anomalies occur up to 1000m below sites with mean annual air temperatures of 0 °C. Patchy permafrost , which forms under conditions <0 °C, probably exists at

1254-421: The surface of, or within, rocks. Particularly during the initial colonization process, the lichen often inserts its hyphae into small fractures or mineral cleavage planes that exist in the host rock. As the lichen grows, the hyphae expand and force the fractures to widen. This increases the potential of fragmentation, possibly leading to rockfalls. During the growth of the lichen thallus , small fragments of

1292-541: The valley. The bridge approaches have to contend with steep rugged slopes and deep ravines covered in forest or thick bush; the area is remote and about 250 km from the nearest city, Lusaka . The First Luangwa Bridge was built in Zambia's colonial era in 1932 as a narrow 300 m long wide steel and reinforced concrete deck on concrete piers and columns, financed, like the Chirundu Bridge and Beit Bridge by

1330-452: The water and ice simply flow out of the fractures as pressure builds. Many argue that frost heaving , like that known to act in soil in permafrost areas, may play an important role in cliff degradation in cold places. Eventually, a rock slope may be completely covered by its own scree, so that production of new material ceases. The slope is then said to be "mantled" with debris. However, since these deposits are still unconsolidated, there

1368-886: The watershed between the Drau and the Gail, and the divides in the Toblacher Feld between Innichen and Toblach in Italy , where the Drau empties into the Black Sea and the Rienz into the Adriatic . Settlements are often built on valley-floor divides in the Alps. Examples are Eben im Pongau , Kirchberg in Tirol and Waidring (In all of these, the village name indicates the pass and

1406-719: The watershed is even explicitly displayed in the coat of arms). Extremely low divides with heights of less than two metres are found on the North German Plain within the Urstromtäler , for example, between Havel and Finow in the Eberswalde Urstromtal . In marsh deltas such as the Okavango , the largest drainage area on earth, or in large lakes areas, such as the Finnish Lakeland , it

1444-499: Was built quickly above the first, with aid from Britain which had been the colonial power in Zambia. It is a cable-stayed bridge designed by Freeman Fox & Partners and built by the Redpath Dorman Long . It has a composite steel-reinforced concrete deck 9.8 m wide, two H-shaped steel pylons 42 m tall on two reinforced concrete piers, and steel cables. The main span is 222 m with approach spans of 40 m each. In 1979

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