Also, this is not the tallest dam, but the tallest earth filled dam. Lou Dobbs made this same error last evening.
One of the reasons they are having problems with the emergency spillway is that the state did not follow design standards and keep the plain underneath the spillway covered in grass. The trees and other growth took root in the ground, and allowed the water to erode the hillside more rapidly than it would have otherwise. (There should have been more rocks and breakwater placed directly underneath the spillway, but under the design standards of the time, those weren't necessarily essential.)
The biggest concern (to me) is water overtopping the dam itself, if both spillways (maximum possible output is about 400,000 cfs) can't cope with the amount of water being brought in by the storm. The entire thing will fail if that happens, a disaster of unimaginable proportions!
All spelled out in plain writing 35 years ago, for those that bothered to read ahead.