This is one of several landslide complexes on the Owyhee that have resulted from foundering margins of intra-canyon lava flows. This particular slide (actually 2 of them...you can see the other near the upper right corner) resulted from relocation of the river following an infusion of copious amounts of basalt into the canyon. The river set up on the contact between the basalt and the less durable valley wall and incised a new channel. Progressive incision of the channel unloads the margin of the lava flow and it fails. This sets off a chain of intriguing processes in some instances that result in catastrophic flooding and more landsliding. You may also note that there are some decent landslides on the west side of the river as well (view in image is to north).
Go check out this landslide and its brethren in Flash Earth [better imagery than GE] here: http://www.flashearth.com/?lat=43.081911&lon=-117.70153&z=14.7&r=0&src=msa.
This place is the landslide capital of Oregon. If time allows, I will post some more images of the spectacular landslides of the Owyhee River before the month runs out.