Catastrophic Flood Origin, Little Missouri River Valley, Montana, South Dakota, and North Dakota

Eric N. Clausen

Mosaics of photographically reduced topographic maps demonstrate the Little Missouri River valley was developed by gigantic floods. Catastrophic flood landforms, oriented in a northwest-southeast direction, cross the entire Little Missouri drainage basin. Field evidence, consisting of abundant flood-deposited alluvium, supports map evidence. Flood-produced landforms, cut in easily eroded claystone bedrock, appear fresh, suggesting that floods occurred late during the last glacial cycle.

Sheets of water, several hundred kilometers wide, flowed in a southeast direction, parallel with a continental ice margin. Erosion lowered the regional surface from a level corresponding to the tops of the highest present-day buttes to the surface now crossed by the headwaters of the Moreau, Grand, Cannonball, Heart, and Green Rivers. Spillway trenches served to channel flow and rapidly cut headward into easily eroded claystone. These trenches include the Moreau, Grand, Cannonball, Heart, and Missouri valleys. The Missouri valley in western North Dakota became the dominant spillway as tributary trenches systematically cut off flow feeding competing spillways.

Formation of the Little Missouri spillway, first as a north-trending valley, progressively cut off floodwaters flowing into the Heart, Cannonball, Grand, and Moreau spillways. The north end of the Little Missouri spillway also was cut off by a deeper east-trending spillway. Huge sheets of floodwater continued to pour across the divide west of the Little Missouri continuing to lower that surface. These floodwaters were cut off by development of the Yellowstone spillway in eastern Montana.

AAPG Search and Discovery Article #91033©1988 AAPG Rocky Mountain Section, Bismarck, North Dakota, 21-24 August 1988