Glacial Lake Missoula grew behind this ice dam in what is now western Montana, ultimately holding as much water as Lake Erie and Lake Ontario integrated. Geologists approximate that the dam formed and broke dozens of times over a number of thousand years, sending out as much as 600 cubic miles of water tearing across the region with each failure.Detailed view from the image above.Floodwaters flowed to the south and southwest, ultimately funneling into the Columbia River. Finished in 1942, the Grand Coulee Dam was the biggest concrete structure in the world before being gone beyond by South Americas Itaipú Dam in 1984 and Chinas Three Gorges Dam in 2006.