How did river erosion erode the yosemite valley?

The effectiveness of erosion by flowing water depends both on processes of weathering - the breakdown of larger rocks into smaller individual rock and mineral fragments that can be transported - and on stream volume and velocity, which determines the size and amount of material that can be transported. With the increasing late Cenozoic elevation of the Yosemite region, the major streams coursing down its western slope were rejuvenated and made more vigorous by their increased slopes. Under these conditions the major streams cut canyons whose channels became progressively deepened relative to the upland areas between them, areas which even today retain comparatively moderate relief. The upper basins and middle reaches of the Merced and Tuolumne Rivers, for example, were later modified by glacial erosion, but initial canyon cutting was accomplished solely by the action of streams.