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Downcutting, also callederosional downcutting,downward erosion orvertical erosion, is ageological process byhydraulic action that deepens thechannel of astream orvalley by removing material from thestream bed or the valley floor. The speed of downcutting depends on the stream'sbase level, the lowest point to which the stream canerode.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 encounterbedrock that resists erosion.
A concurrent process calledlateral erosion is 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 inmountainous areas tend to be narrow and swift, formingV-shaped valleys, while streams inlowland areas tend to be wide and slow-moving, with valleys that are correspondingly wide and flat-bottomed.
Thestream gradient is the ratio between the elevation drop of a stream and the horizontal distance considered. The steeper the gradient, the faster the stream flows. Sometimes geologicaluplift will increase the gradient of a stream even while the stream downcuts toward its base level, a process calledrejuvenation. This happened in the case of theColorado River in the westernUnited States, resulting in the process that created theGrand Canyon.
Lake bed downcutting is the erosion of cohesive material such asclay orglacialtill from a shoreline bywave action. When thesand cover is stripped away and the cohesive layer is exposed, cohesive material is lost to thewater column. Unlike sand, cohesive material cannot be replenished by natural events such asbluff 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.