During the last glacial maximum, for a period of about 7000 years ending roughly 10,000 years ago, glaciers formed ice dams across large rivers in North America. Water blocked by dams naturally spreads out to form lakes, and some of the lakes in question were of staggering size, the largest bigger in area than all of today’s Great Lakes combined.
Ice dams aren’t stable, in part because ice floats, so eventually the dams that created those lakes collapsed, resulting in truly catastrophic floods, including a whole series of them over 70 centuries in the U.S. Pacifid Northwest. The traces of those lakes and floods remain today, with beaches high above any modern shorelines in Minnesota and North Dakota and unusual terrain such as the so-called channeled scablands in eastern Washington State. An even bigger flood farther east may have dumped so much water into the Arctic Ocean that it eventually disrupted important ocean currents in the Atlantic, which may explain the Younger Dryas, an abrupt cooling event that lasted about a thousand years.
These two videos (which reference each other in passing) explain this remarkable geological history, including some of how it came to be discovered. Human settlers may have been around to witness some of these events and suffer from them.