<Table> <Tr> <Td> </Td> <Td> This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations . Please help to improve this article by introducing more precise citations . (November 2017) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations . Please help to improve this article by introducing more precise citations . (November 2017) (Learn how and when to remove this template message) </Td> </Tr> <P> Downcutting, also called erosional downcutting, downward erosion or vertical erosion is a geological process by hydraulic action that deepens the channel of a stream or valley by removing material from the stream's bed or the valley's floor . The speed of downcutting depends on the stream's base level, the lowest point to which the stream can erode . Sea level is the ultimate base level, but many streams have a higher "temporary" base level because they empty into another body of water that is above sea level or encounter bedrock that resists erosion . A concurrent process called lateral erosion refers to the widening of a stream channel or valley . When a stream is high above its base level, downcutting will take place faster than lateral erosion; but as the level of the stream approaches its base level, the rate of lateral erosion increases . This is why streams in mountainous areas tend to be narrow and swift, forming V - shaped valleys, while streams in lowland areas tend to be wide and slow - moving, with valleys that are correspondingly wide and flat - bottomed . The term gradient refers to the elevation of a stream relative to its base level . The steeper the gradient, the faster the stream flows . Sometimes geological uplift will increase the gradient of a stream even while the stream downcuts toward its base level, a process called "rejuvenation ." This happened in the case of the Colorado River in the western United States, resulting in the process that created the Grand Canyon . </P> <P> Lake bed downcutting is the erosion of cohesive material such as clay or glacial till from a shoreline by wave action . When the sand cover is stripped away and the cohesive layer is exposed, cohesive material is lost to the water column . Unlike sand, cohesive material cannot be replenished by natural events such as bluff erosion . This can result in a process called "bluff recession," in which waves erode and carry away the material at the toe of the bluff and cause it to become steeper . When the slope reaches a certain angle, the bluff becomes unstable and fails, causing it to recede inland . </P>

The lowest point a stream will erode is known as