Figure 1: The Little Missouri River originates near Devils Tower in northeast Wyoming and flows across the southeast corner of Montana and the northwest corner of South Dakota before flowing into North Dakota where it turns east to flow to an east-oriented Missouri River valley segment (select and click on maps to enlarge). National Geographic Society map digitally presented using National Geographic Society TOPO software.

• The Little Missouri River is the easternmost of several major north-oriented Missouri River tributaries, which flow in a north direction to reach what is today a south-oriented river. Directly west of the Little Missouri River drainage basin is the northeast oriented Yellowstone River drainage basin and its north-oriented tributary Powder River drainage basin. The Yellowstone River joins the Missouri River near the Montana-North Dakota border. West of the Yellowstone River other north-oriented Missouri River tributaries seen on figure 1 include the northeast and north-oriented Redwater River and the north-oriented Musselshell River. West of the figure 1 map area the Missouri River itself is a north-oriented river. These and other north-oriented Missouri River tributaries have led many previous geomorphologists to suggest the Missouri River valley was eroded headward along the margin of a North American ice sheet that blocked an extensive pre-glacial north-oriented drainage system. Abandoned north-oriented valleys north of the present day Missouri River valley are often claimed to be routes pre-glacial rivers used. While such a hypothesis is tempting at first, careful study of Little Missouri River drainage divides, tributary valley orientations, and drainage divides with adjacent drainage basins reveals the Little Missouri River valley is not a pre-glacial feature. In fact, the evidence demonstrates the Little Missouri River valley eroded headward across an immense southeast-oriented flood, which flowed across the entire Little Missouri River drainage basin. Such a flood requires a very different glacial history and glacial model than the glacial history described by the incorrect pre-glacial drainage system hypothesis.

Figure 2: Little Missouri River and adjacent rivers in western North Dakota and eastern Montana. National Geographic Society map digitally presented using National Geographic Society TOPO software.

If the pre-glacial origin hypothesis is incorrect, then how and when was the Little Missouri River drainage basin formed? Evidence documenting Little Missouri River valley (and Yellowstone River valley and Redwater River valley) headward erosion across an immense southeast-oriented flood can be seen on large region maps such as figures 1, 2, and 3 in this essay. For example, in figure 2 note how east of the north-oriented Little Missouri River in southwest North Dakota are southeast-oriented headwaters of the Knife, Green, Heart, and Cannonball Rivers and of southeast-oriented Cedar Creek (which is a Cannonball River tributary, the Green River is a Heart River tributary). Note also in figure 2 how tributaries to the northeast-oriented Yellowstone River are northwest-oriented from the east and southeast-oriented from the west. These northwest-southeast oriented tributaries provide evidence the north-oriented Little Missouri River valley and the northeast-oriented Yellowstone River valley eroded headward across multiple southeast-oriented flood flow channels. Why were these flood flow channels, why was flood flow oriented in a southeast direction, and where were the immense volumes of water required to erode the Little Missouri, Yellowstone, and other river valleys coming from?

• Headward erosion of the Little Missouri River valley first captured and beheaded in sequence from north to south multiple southeast-oriented drainage channels eroding the southeast-oriented Knife River, Green River, Heart River, Cannonball River, and Cedar Creek valleys. The most logical reason for multiple southeast-oriented drainage routes is they were channels in what was then a southeast-oriented flood formed anastomosing channel complex. Flood waters on northwest ends of beheaded flood flow routes reversed flow direction to erode short northwest-oriented Little Missouri River tributary valleys (best seen on more detailed maps). Next, Yellowstone River valley headward erosion captured and beheaded the southeast-oriented flood flow channels to the newly eroded Little Missouri River valley and flood waters on northwest ends of beheaded flood flow channels reversed flow direction to erode northwest-oriented Yellowstone River tributary valleys. This process of headward erosion of northeast and north-oriented valleys across multiple southeast-oriented flood flow channels was repeated over and over again and each additional example demonstrating the process was repeated further reinforces the anastomosing channel complex interpretation.

Figure 3: Little Missouri River in northeast Wyoming, southeast Montana, and northwest South Dakota. The unnamed river flowing from Broadus to Powderville, Montana is the north-oriented Powder River. National Geographic Society map digitally presented using National Geographic Society TOPO software.

Why was the southeast-oriented flood so large it flowed across the entire present day Little Missouri River drainage basin? Figure 3 illustrates how Little Missouri River valley headward erosion south of North Dakota continued to capture and behead southeast-oriented flood flow channels which were eroding the North Fork Grand River valley, valleys of southeast-oriented South Fork Grand River tributaries, the North and South Fork Moreau River valleys, and the southeast-oriented Belle Fourche River valley and valleys of southeast-oriented Belle Fourche River tributaries. While figure 3 does not show many Little Missouri River, Boxelder Creek, or Powder River tributaries, more detailed maps illustrated in the detailed essays (and also in figure 4 in this overview essay) show many northwest-oriented tributaries from the east and many southeast-oriented tributaries from the west. The Little Missouri River headwaters area in northeast Wyoming, which located on the western margin of the Black Hills, is somewhat more complex, but is described in the Little Missouri River-Belle Fourche River drainage divide in northeast Wyoming essay. Evidence presented in that essay documents the Little Missouri River headwaters valley in northeast Wyoming was also eroded by southeast-oriented flood flow. In other words, the Little Missouri River valley eroded headward across a gigantic southeast-oriented anastomosing channel complex and diverted an immense flood north and northeast into North Dakota.

Figure 4: Boxelder Creek-Little Missouri River drainage divide in southeast Montana shoeing northwest-southeast oriented through valleys crossing the drainage divide. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Do more detailed topographic maps provide evidence supporting the southeast-oriented anastomosing channel complex interpretation? Figure 4 uses reduced size topographic maps to illustrate the Boxelder Creek-Little Missouri River drainage divide area in southeast Montana. The northeast and north-oriented Little Missouri River is located in the figure 4 east half. Northeast-oriented Boxelder Creek is located in the figure 4 west half and north of figure 4 joins the Little Missouri River. Note how on figure 3 no Little Missouri River or Boxelder Creek tributaries are shown, yet on figure 4 there are numerous southeast-oriented Little Missouri River tributaries from the west and numerous northwest oriented Boxelder Creek tributaries from the east. Also note how the southeast-oriented Little Missouri River tributary valleys are linked by through valleys crossing the drainage divide with northwest oriented Boxelder Creek tributary valleys. Cottonwood Creek is the southeast-oriented Little Missouri River tributary located in the deepest of the northwest-southeast oriented through valleys. Prairie Dog Creek is the northwest-oriented Boxelder Creek tributary at the northwest end of that deep through valley, although between Prairie Dog Creek and Cottonwood Creek a segment of the through valley is drained by an unnamed northwest- and north-oriented Boxelder Creek tributary. Topographic maps illustrated in the detailed Little Missouri River drainage basin essays document the presence of similar aligned tributary valleys and through valleys crossing drainage divides for all drainage divides surrounding the Little Missouri River drainage basin and also for drainage divides between major Little Missouri River tributary drainage basins.

Figure 5: Topographic map evidence illustrating how Little Missouri River valley headward erosion captured southeast-oriented flood flow eroding “The Jumpoff”, a large southeast-oriented headcut. United States Geological Survey map digitally presented using National Geographic Society TOPO software.   

Maps illustrated in the Little Missouri River drainage basin landform origins overview essay and related detailed essays (and also in essays for adjacent river drainage basins) illustrate many more landform features which can best be explained in the context of immense southeast oriented and flood formed anastomosing channel complexes, which were captured by headward erosion of deep north and northeast-oriented valleys. Figure 5 illustrates a large escarpment surrounded basin, where the escarpment surrounding the basin is named “The Jumpoff”. The north-oriented Little Missouri River is located west of “The Jumpoff” and “The Jumpoff” represents the drainage divide between the north-oriented Little Missouri River drainage basin and the east-oriented South Fork Grand River drainage basin. “The Jumpoff” is a large abandoned headcut, which was eroded by southeast and/or east-oriented sheets of flood water which were literally stripping away the northwest South Dakota surface. Headward erosion of the Little Missouri River valley captured the southeast-oriented flood water and diverted the flood water north and flood flow to what had been the actively eroding South Fork Grand River drainage basin suddenly ceased. Flood waters on northwest ends of beheaded southeast-oriented flood flow channels reversed flow direction to erode the short and shallow northwest-oriented Little Missouri River tributary valleys. Since that time the figure 5 landscape has not been significantly altered.

• Why was the north-oriented Little Missouri River valley able to erode headward and capture an immense southeast-oriented flood? The flood water was flowing in a southeast direction along the southwest margin of a rapidly melting North American ice sheet. The ice sheet had been thick and had been located in a deep hole, which had been formed by a combination of deep glacial erosion and crustal warping caused by the ice sheet weight. At one time this thick ice sheet stood high above the surrounding surface while the ice sheet roots extended far below the surrounding surface. As this thick ice sheet melted immense volumes of melt water poured off its surface and flowed in any direction the landscape at that time permitted and the landscape at that time did not look like the landscape today. Flood water erosion deeply eroded the pre-existing landscape while crustal warping caused by ice sheet weight continued to uplift regions elsewhere on the continent.

• In time the ice sheet melted to the point that its surface, at least in the North and South Dakota region was not significantly higher than the surrounding landscape and southeast-oriented ice-marginal floods still flowing along the ice sheet’s southwest margin. At the same time immense melt water rivers flowing across the ice sheet surface had carved deep ice-walled and ice-floored canyons into the ice sheet surface and in time these giant ice-walled and ice-floored canyons became giant ice-walled and bedrock-floored canyons. One of the largest of these huge ice-walled and bedrock-floored canyons was located north and east of the present day Missouri Escarpment, which is located north and east of the present day Missouri River in North and South Dakota (the Missouri Escarpment is what remains of that ice-walled and bedrock-floored canyon’s west and southwest wall). The floor of that giant southeast and south-oriented ice-walled and bedrock-floored canyon was lower in elevation than the landscape south and west of the ice sheet margin and in time flood waters moving along the ice sheet’s southwest margin were able to breach the northwest-southeast and north-south oriented ice barrier (the ice sheet’s detached southwest margin) to reach the deeper Ice-walled and bedrock floored canyon floor. The deep Little Missouri River valley eroded headward from one the deep breaches.

Additional information and sources of maps studied

This essay has provided only a sample of the detailed topographic map evidence supporting the flood erosion interpretation. Many additional illustrations could be provided. Readers are encouraged to look at mosaics of detailed topographic maps to see the abundance of available data. Maps used in this study were created and published by the United States Geologic Survey and can be obtained directly from the United States Geological Survey and/or from dealers offering United States Geological Survey maps. Hard copy maps can also be observed at United States Geological Survey map depositories which are located throughout the United States and elsewhere. Illustrations used here were created using National Geographic Society TOPO software and digital map data. TOPO software and map data can be obtained from the National Geographic Society and/or dealers offering National Geographic Society digital map data.