The Minnesota River, rising on the western boundary of the State, flows nearly five hundred miles in a deeply carved valley through the "Big Woods" to the Mississippi. Its source is in the Big Stone Lake, which, with Lake Traverse to the northward, forms part of the Dakota boundary. The Red River of the North, rising in Lake Traverse and gathering together the streams on the western slope of the Itascan plateau, flows northward between Minnesota and North Dakota, and into Manitoba, two hundred and fifty miles to Lake Winnipeg. This river has cut its channel in a nearly level plain, and it is curious that in times of freshet its waters connect, through Lakes Traverse and the Big Stone, with the Minnesota, so that steamboats of light draught can then occasionally pass from the Mississippi waters north to Lake Winnipeg. It was this rich and level plain of the valley of the Red River that in the glacial epoch formed the bed of a vast lake which scientists have named Lake Agassiz. Its area, as indicated by well-marked shore-lines and deltas, was a hundred miles wide and over four hundred miles long, stretching far into Manitoba, and the waters were two to four hundred feet deep. It was held up on the north by the retreating ice-sheet of the great glacier, the outlet being southward, where a channel fifty feet deep, fifty miles long and over a mile wide can now be distinctly traced leading its outflow into the Minnesota River, whose valley its floods then greatly enlarged on the way to the Mississippi. The plain of this lake bed is almost level, descending towards the northward about a foot to the mile, and here the ancient lake deposited the thick, rich, black soils which have made the greatest wheat-growing region of North America.