blackfoot-valley

If at just the right time you were standing there in sight of that upper- most arm of the lake, you might have seen a surge or a shudder of ripples as the head of the lake suddenly began to recede at the rate of about one mile per hour - the speed of a slow walk. Just as suddenly, the lake would have disap- peared from your view in a matter of minutes, the only water left visable in the form of a flowing river - the Blackfoot. You certainly would have been left won- dering what in the world had happened! Maybe you would have run home to tell of the event, or maybe you would have resumed your hunting expedition. In any case, that astonishing, but not personally threatening event, would have been your only tip-off to a truly cataclysmic happening farther west. As a dam, the trouble with ice is that it floats. Since ice weighs only 90% that of an equal volume of water, when the filling of a lake behind an ice dam reaches nine-tenths the height of the dam, it begins to exert buoyancy on the dam, lifting it. As the ice was lifted, its tongue broke off suddenly, mak- ing sounds like a stupendous barrage of massed artillery. It broke into a mass of pieces and - swoosh - unimaginably enormous volumes of water, released under hydraulic pressure (or head) produced by the lake’s 2,000-foot depth, rushed forward carrying granite bolders the size of houses and even greater larger chunks of ice. The water raced towards the hapless valley below at speeds in excess of 60 miles per hour in waves that reached hundreds of feet high. The lake, estimated to have contained over 500 cubic miles of water, would have drained completely in a few days, possibly a week. The greatest volume of flow would have been in the first day or two, since the “head” pres- sure naturally diminished as the lake level lowered. The water, with its load of 20