<P> Valley or stream erosion occurs with continued water flow along a linear feature . The erosion is both downward, deepening the valley, and headward, extending the valley into the hillside, creating head cuts and steep banks . In the earliest stage of stream erosion, the erosive activity is dominantly vertical, the valleys have a typical V cross-section and the stream gradient is relatively steep . When some base level is reached, the erosive activity switches to lateral erosion, which widens the valley floor and creates a narrow floodplain . The stream gradient becomes nearly flat, and lateral deposition of sediments becomes important as the stream meanders across the valley floor . In all stages of stream erosion, by far the most erosion occurs during times of flood, when more and faster - moving water is available to carry a larger sediment load . In such processes, it is not the water alone that erodes: suspended abrasive particles, pebbles and boulders can also act erosively as they traverse a surface, in a process known as traction . </P> <P> Bank erosion is the wearing away of the banks of a stream or river . This is distinguished from changes on the bed of the watercourse, which is referred to as scour . Erosion and changes in the form of river banks may be measured by inserting metal rods into the bank and marking the position of the bank surface along the rods at different times . </P> <P> Thermal erosion is the result of melting and weakening permafrost due to moving water . It can occur both along rivers and at the coast . Rapid river channel migration observed in the Lena River of Siberia is due to thermal erosion, as these portions of the banks are composed of permafrost - cemented non-cohesive materials . Much of this erosion occurs as the weakened banks fail in large slumps . Thermal erosion also affects the Arctic coast, where wave action and near - shore temperatures combine to undercut permafrost bluffs along the shoreline and cause them to fail . Annual erosion rates along a 100 - kilometre (62 - mile) segment of the Beaufort Sea shoreline averaged 5.6 metres (18 feet) per year from 1955 to 2002 . </P> <P> At extremely high flows, kolks, or vortices are formed by large volumes of rapidly rushing water . Kolks cause extreme local erosion, plucking bedrock and creating pothole - type geographical features called Rock - cut basins . Examples can be seen in the flood regions result from glacial Lake Missoula, which created the channeled scablands in the Columbia Basin region of eastern Washington . </P>

List the ways in which soil is eroded