Actually, they took out that section of the mountain so they could grout the caverns below that mountain to try and stop the leaks in the dam. They looked at alternatives but they were too expensive so that was the way they had to do it. There is a full report of the construction recommendations on the corps web site, over 270 pages of it and it is booooring!!
The runoff is a different story. One reply I got form the Corps said that they were in compliance with the water quality of the runoff and they were doing everything they could to completly elimnate it but it was nearly impossible to do so.