Although the quake caused much settling of the earth packed against the downstream side of Hebgen Dam’s concrete core, the relatively slight displacement of the sod cover is interpreted to mean that all three tidal waves passed over the dam before this earth subsided and separated from the core. Thus the water would have begun its race down the valley before the heavy earth-settling shocks hit the dam area.
Those who support the high-water-at-the-moment-of-the-slide theory point to the great volume of water damage way below the slide.
If the slide had come first, it would have dammed off the tidal waves, and prevented such damage. They feel there just wasn’t enough water in the river bed’s normal content to cause the water damage done both upstream and downstream by the slide. And they argue, the mud and dust in the composition of the slide would have taken up most of the water normally found in the reach of the river buried under the slide.
There’s further evidence in the numerous fish found high and dry on the flat along the river bank several feet higher than the streambed. Most of them were small, catfish-like chubs. There were numerous trout, and one 18-inch carp. There is no place in the river below the pool at the toe of the dam where carp would likely be found.
Also, there was further confirmation in the fact that three of the especially made 11-inch squared timbers, eight and a half feet long, with notched ends and two U-bolts used as stop-logs in the Hebgen Dam spillway were found below the slide. Some shadow was cast on this as absolute confirmation by the Montana Power Co.’s explanation that stop-logs have been lost from time to time before the quake.
Those who, in spite of such evidence, oppose the theory that the high water reached the slide area first just don’t feel that the water could have made it all the way down the canyon in so short a time. They feel that it would have taken at least 40 minutes for the big waves to traverse the seven miles. They have some support in L. D. Smith’s testimony that in driving down from Beaver Creek to Rock Creek right after the shocks, he saw no such waves.
At any rate, this is one argument that geologists and hydrologists will be batting around for a long time.
This view of the Madison Canyon slide gives the feeling of the up-canyon and down-canyon flow of the 80-million-ton mass of rock and debris.(Montana Highway Commission)