I'm a 7th grade student. When did the Missoula floods occur?
What a great question - I wish I had a great answer. The Missoula Floods actually happened many times (possibly as many as 30 times!) near the end of the last ice age. This makes it around 15,000 years ago. What probably happened is as follows:
Near the end of the last period of glacial advance, a tongue of ice from a glacier dammed the Clark Fork River and started backing up water into the valley behind the dam. As the water level rose (and the temperature also began to increase), the ice dam weakened and actually began to float - allowing the water in the lake to escape and flood downstream. As the lake emptied, the dam re-built itself and the process started again.
This was first figured out by a man named Harlan Bretz, and the floods are also called "The Bretz Floods" in his honor. Unfortunately for Mr. Bretz, nobody believed him. But he kept looking for more data to support his theory, and in time his ideas have become accepted by most scientists.
Thanks for the question and keep up your interest in science. And be like Mr. Bretz - when you think you're right, don't give up!
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