Glaciers reshaped North Dakota by altering river courses and molding the state's gently rolling, hilly terrain. Before about three million years ago, the rivers in what is now North Dakota flowed northeast in broad valleys and did not include a Missouri River. During the most recent Ice Age, a one-to-two-mile-thick ice sheet advanced and retreated repeatedly across North America, scouring the land, moving sediment, and releasing meltwater into new and existing valleys. About 25,000 years ago, south- and west-flowing glaciers dammed rivers that had once flowed northeast. East of present-day Washburn, a glacier blocked the Knife River, creating a reservoir lake that eventually spilled over at its southeast corner and rapidly cut a trench southward to today's Bismarck, forming the valley segment seen here. Similar overflows on other preglacial rivers in western North Dakota created diversion trenches that sent meltwater south, eroded deep valleys, and linked older and newer valleys into the river system now known as the Missouri River. Eastward-flowing parts of that river occupy older, broader valleys inherited from preglacial drainage into Canada, while southward-flowing parts tend to run through narrower, steeper trenches, and in some places ancient broad valleys remain without streams.