Mass Wasting

Rivers in the desert regions of the southwestern states form canyons with fairly steep walls, and the buttes and mesas in those regions also have very steep walls. The topography of the eastern states is much more subdued, dominated by rolling hills and gentle slopes. How does rainfall control this contrast in landscape? Weathering needs water for freezing and thawing, chemical reactions, and growth of roots. Less water means the rate of weathering will be reduced. Also, rainfall affects erosion by water. Less water means the rate at which rivers cut valleys will be reduced. If these were the only factors, however, only the rates would vary; arid regions would weather and erode more slowly than humid ones.

Much of the difference in landscape results from variations in mass wasting. Very steep slopes on dry rock with little or no pore pressure are much more stable than similar slopes on wet rock with high pore pressures. Water adds to the weight trying to break the rock and increases the pore pressure which weakens the rock. Hence the height of a vertical slope which can exist in dry rock is much greater than that in wet rock. Arid climates also have less vegetation to stabilize sediment at the base of a slope, so it is likely to be washed away during the infrequent storms.