<P> In studies of the ecology of freshwater rivers, habitats are classified as upland and lowland . </P> <P> Upland and lowland are portions of 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 rare occasions, certain lowlands such as Caspian Depression lay below the sea level . </P> <P> Upland habitats are cold, clear, rocky, fast - flowing rivers in mountainous areas; lowland habitats are warm, slow - flowing rivers found in relatively flat lowland areas, with water that is frequently coloured by sediment and organic matter . </P> <P> 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 . </P>

How a river changes its characteristics when it reaches the lowland