Continental Divide
Feb 7, 2012
Between these two western Minnesota lakes is a little stretch of land called Traverse Gap or Brown's Valley. Although it's obviously not a mountain range, or even really a hill, it is nonetheless a continental divide: raindrops falling near the lake at the top of this picture eventually drain into Hudson Bay and the Arctic Ocean, while raindrops falling near the lower lake drain into the Gulf of Mexico and the Atlantic. In between, the land rises barely perceptibly; it's still part of the valley, really, almost but not quite as low and flat as the rest of the valley.
We can blame global warming after the Ice Ages for this oddness. As the glaciers began to melt about 14,000 years ago, huge quantities of meltwater pooled hereabouts, forming a vast outlet lake referred to as Glacial Lake Agassiz. The bottom of the glacial lake was extremely flat, mile after mile, silted over with sediment that settled out of the meltwater. The modern-day lake near the top of this picture, Lake Traverse, forms the headwaters of the Red River, which more or less drains this vast flatland as it flows northward, frequently flooding because the land is just too flat to allow for efficient drainage.
The lake near the righthand edge of the picture, Big Stone Lake, forms headwaters for the Minnesota River, which developed late during the post-Ice Age warmup. As the remnants of glacial ice weakened and collapsed, huge boulders that had been trapped within worked loose from the body of the old ice sheet and washed along underneath, scraping through the sediment of the outlet lake bottom and gouging a channel down through bedrock.
This new river channel eventually captured much of the outflow from Glacial Lake Agassiz and drained it southward into the Mississippi and the Gulf of Mexico. It might have claimed all the drainage except for the post-glacial rebound: land that had long been compressed beneath the weight of thousands of feet of ice began to rise a bit once the glaciers were gone, rebounding first around the periphery of the old ice sheets, where melting came soonest. Here in Brown's Valley, in this gap between two headwater lakes, the rebound was almost invisible but not insignificant; the land rose just enough to shed raindrops in the direction of different oceans.