If you look at the topography of the area in google maps you will see that the sides of the canyon are somewhat steep and rugged for quite a ways downstream of

If you look at the topography of the area in google maps you will see that the sides of the canyon are somewhat steep and rugged for quite a ways downstream of the dam. Go a bit further and you find Willow Beach, which has a nice flat run north and then west back to the reservoir. I am guessing that being able to run the pipe using trenching equipment rather than tunneling through canyon walls is a big advantage.