Wallowa Lake fills a depression once occupied by a great river of ice that flowed out of the high Wallowa Mountains to the south. This glacier reached its greatest size in the late Pleistocene age about 12 to 40 thousand years ago. As it spread onto the valley floor, it built great piles of rock debris along its edges called moraines. When the ice melted away, the moraines remained as the high straight ridges visible today. The lake is 283 feet deep, but the glacier was more than 1500 feet thick. These moraines are among the best preserved examples in North America.