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17-517: The lowland region lies to the east of the Caspian Sea and southeast of the Aral Sea in the vast Aral–Caspian Depression but extends to parts above sea level as well. It is one of the largest expanses of sand in the world, covering an area of around 3 million km. On average, the region receives less than 15 inches (380 mm) of rainfall per year. The Karakum desert lies at the southern portion of

34-473: A plain based on elevation above sea level . In studies of the ecology of freshwater rivers , habitats are classified as upland or lowland. Upland and lowland are portions of a plain that are conditionally categorized by their elevation above the sea level . Lowlands are usually no higher than 200 m (660 ft), while uplands are somewhere around 200 m (660 ft) to 500 m (1,600 ft). On unusual occasions, certain lowlands such as

51-1040: A riffle and pool structure and cooler water temperatures. Rivers with a course that drops in elevation very slowly will have slower water flow and lower force. This in turn produces the other characteristics of a lowland river—a meandering course lacking rapids , a river bed dominated by fine sediments and higher water temperatures. Lowland rivers tend to carry more suspended sediment and organic matter as well, but some lowland rivers have periods of high water clarity in seasonal low-flow periods. The generally clear, cool, fast-flowing waters and bedrock and coarse sediment beds of upland rivers encourage fish species with limited temperature tolerances, high oxygen needs, strong swimming ability and specialised reproductive strategies to prevent eggs or larvae being swept away. These characteristics also encourage invertebrate species with limited temperature tolerances, high oxygen needs and ecologies revolving around coarse sediments and interstices or "gaps" between those coarse sediments. The term "upland"

68-414: A waterfall , as softer materials are encountered below the hard layer. Human dams , glaciation , changes in sea level , and many other factors can also change the "normal" or natural gradient pattern. On topographic maps , stream gradient can be easily approximated if the scale of the map and the contour intervals are known. Contour lines form a V-shape on the map, pointing upstream. By counting

85-416: A flattening of the river gradient as approach the terminus at sea level. A stream that flows upon a uniformly erodible substrate will tend to have a steep gradient near its source, and a low gradient nearing zero as it reaches its base level . Of course, a uniform substrate would be rare in nature; hard layers of rock along the way may establish a temporary base level, followed by a high gradient, or even

102-405: A low gradient indicates a more nearly level stream bed and sluggishly moving water, that may be able to carry only small amounts of very fine sediment . High gradient streams tend to have steep, narrow V-shaped valleys , and are referred to as young streams. Low gradient streams have wider and less rugged valleys , with a tendency for the stream to meander . Many rivers involve, to some extent,

119-502: A southeast–north-westerly direction through the lowlands. In the Pliocene and Pleistocene , the territory of the modern Turan depression was the bottom of the vast Turan Sea, which was divided into the modern Caspian and Aral seas about ten thousand years ago. 41°50′00″N 59°58′00″E  /  41.8333°N 59.9667°E  / 41.8333; 59.9667 Lowland Upland and lowland are conditional descriptions of

136-727: Is also used in wetland ecology , where "upland" plants indicate an area that is not a wetland. The generally more turbid , warm, slow-flowing waters and fine sediment beds of lowland rivers encourage fish species with broad temperature tolerances and greater tolerances to low oxygen levels, and life history and breeding strategies adapted to these and other traits of lowland rivers. These characteristics also encourage invertebrate species with broad temperature tolerances and greater tolerances to low oxygen levels and ecologies revolving around fine sediments or alternative habitats such as submerged woody debris ("snags") or submergent macrophytes ("water weed"). Lowland alluvial plains form when there

153-649: Is deposition of sediment over a long period of time by one or more rivers coming from highland regions, and then are deposited in lowland regions for long periods of time. Examples include American Bottom , a flood plain of the Mississippi River in Southern Illinois, Bois Brule Bottom , and Bottomland hardwood forest a deciduous hardwood forest found in broad lowland floodplains of the United States. Stream gradient Stream gradient (or stream slope )

170-453: Is frequently colored by sediment and organic matter. These classifications overlap with the geological definitions of "upland" and "lowland". In geology an "upland" is generally considered to be land that is at a higher elevation than the alluvial plain or stream terrace , which are considered to be "lowlands". The term "bottomland" refers to low-lying alluvial land near a river. Much freshwater fish and invertebrate communities around

187-433: Is not the sole determinant of whether a river is upland or lowland. Arguably the most important determinants are those of stream power and stream gradient . Rivers with a course that drops rapidly in elevation will have faster water flow and higher stream power or "force of water". This in turn produces the other characteristics of an upland river—an incised course , a river bed dominated by bedrock and coarse sediments,

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204-475: Is the grade (or slope) of a stream . It is measured by the ratio of drop in elevation and horizontal distance. It is a dimensionless quantity , usually expressed in units of meters per kilometer (m/km) or feet per mile (ft/mi); it may also be expressed in percent (%). The world average river reach slope is 2.6 m/km or 0.26%; a slope smaller than 1% and greater than 4% is considered gentle and steep, respectively. Stream gradient may change along

221-558: The Caspian Depression lie below sea level. Uplands areas tend to spike into valleys and mountains , forming mountain ranges while lowland areas tend to be uniformly flat, although both can vary such as the Mongolian Plateau . Upland habitats are cold, clear and rocky whose rivers are fast-flowing in mountainous areas; lowland habitats are warm with slow-flowing rivers found in relatively flat lowland areas, with water that

238-845: The Turan Lowlands. The part of the depression adjacent to the Aral Sea is drainless. It includes a network of temporary streams usually ending in sor salt flats , which seasonally become salt lakes . Three of the largest cities in the Turan Depression are Daşoguz in Turkmenistan , Nukus in Uzbekistan , and Urganch , also in Uzbekistan. Vpadina Akchanaya in Turkmenistan is 267 feet (81 meters) below sea level . The Amu Darya River runs in

255-470: The number of lines that cross a certain segment of a stream, multiplying this by the contour interval, and dividing that quantity by the length of the stream segment, one obtains an approximation to the stream gradient. Because stream gradient is customarily given in feet per 1000 feet, one should then measure the amount a stream segment rises and the length of the stream segment in feet, then multiply feet per foot gradient by 1000. For example, if one measures

272-442: The stream course. An average gradient can be defined, known as the relief ratio , which gives the average drop in elevation per unit length of river. The calculation is the difference in elevation between the river's source and the river terminus ( confluence or mouth ) divided by the total length of the river or stream. A high gradient indicates a steep slope and rapid flow of water (i.e. more ability to erode); where as

289-531: The world show a pattern of specialization into upland or lowland river habitats. Classifying rivers and streams as upland or lowland is important in freshwater ecology , as the two types of river habitat are very different, and usually support very different populations of fish and invertebrate species. In freshwater ecology, upland rivers and streams are the fast-flowing rivers and streams that drain elevated or mountainous country, often onto broad alluvial plains (where they become lowland rivers). However, elevation

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