Map interpretation techniques are used to determine the sequence in which western North and South Dakota erosion events occurred. The map interpretation techniques apply the principle of cross cutting relationships by studying asymmetric drainage divides, barbed tributaries, elbows of capture, drainage divide crossings, abandoned headcuts, and similar features on detailed topographic maps to determine the sequence in which drainage basins and valleys within those drainage basins formed. Detailed topographic maps covering western North and South Dakota show numerous closely spaced divide crossings along drainage divides separating the White, Bad, Cheyenne, Moreau, Grand, Cannonball, Heart, Knife, and Little Missouri Rivers. These divide crossings often form links between opposing northwest- and southeast-oriented tributary stream valleys and provide evidence of multiple closely spaced southeast-oriented flow channels that existed prior to formation of the deeper present day east-, northeast-, and north-oriented river valleys. Numerous barbed tributaries in the form of northwest-oriented tributaries to east- and northeast-oriented rivers (and major tributaries to the mentioned rivers) and southeast-oriented tributaries to the northeast- and north-oriented rivers (and tributaries to the mentioned rivers) suggest the deeper river (and tributary) valleys eroded headward across the southeast-oriented flow channels. Asymmetric drainage divides, barbed tributaries, abandoned headcuts, and elbows of capture demonstrate the southeast-oriented flow, which was most likely in the form of floods of ice-marginal melt water moving between the Black Hills uplift and a continental ice sheet’s southwest margin, was captured in sequence by headward erosion of the White, Bad, Cheyenne, Moreau, Grand, Cannonball, Heart, Knife, and Little Missouri River valleys. This erosion event sequence and its probable cause, determined from the map evidence, has major implications related to what is commonly considered to have been a much larger pre-glacial Bell River system, which included segments of each of the studied river valleys, and for all geologic and glacial history interpretations based on a Bell River system pre-glacial age interpretation.
Missouri River basin floods of April-May 1950 in North and South Dakota; Chapter A in Floods of 1950