This is the Middle Fork of the Tuolumne River where it meets the South Fork (see http://maps.google.com/?ie=UTF8&ll=37.827785,-120.020013&spn=0.017287,0.054932&t=p&z=15):
http://yosemitephotos.net/main.php/v/yosemite/DSCN2519.JPG.html
It's a nice little hike and roaring right now. Later in the year it's a great place to swim.