sentences:  The valleys of the Red and Minnesota Rivers lay in the beds of Glacial Lake Agassiz and its prehistoric outlet Glacial River Warren; the lands exposed when these bodies of water receded were flat plains between low uplands covered by prairie grasslands.At the Traverse Gap, only a mile (1.6  km) of land separated the Bois des Sioux River, a source stream of the Red (which flowed north to Hudson Bay) and the Little Minnesota River, a source stream of the Minnesota River (tributary to the Mississippi, which flowed south to the Gulf of Mexico).The valley floors and uplands of the watercourses along this gently graded route provided a natural thoroughfare to the south.