A geomorphic history based on topographic map evidence

This overview essay provides highlights from more detailed essays which use topographic map evidence to interpret the history of drainage divides within and surrounding the Montana Missouri River drainage basin (upstream from the Yellowstone River confluence near the Montana-North Dakota border). The detailed essays can be found under MT Missouri River on this website’s sidebar category list and also under names of Montana Missouri River tributaries. The Missouri River originates at Three Forks Montana where the Jefferson, Madison, and Gallatin Rivers meet to form the Missouri River. From Three Forks the Missouri River flows in a northwest direction before turning to flow in a northeast direction past Fort Benton to turn again to flow in a southeast, east, and east-southeast direction to join the north-oriented Musselshell River. From the Musselshell River confluence the Missouri River then flows in a northeast and east direction to the North Dakota border. Topographic map evidence illustrated in the detailed essays and in similar Missouri River drainage basin landform origins research project essays for other states documents immense southeast-oriented floods which crossed the entire Montana Missouri River drainage basin, including the present day high mountain ranges. Flood waters were derived from a rapidly melting thick North American ice sheet, which was located in a deep “hole” created by deep glacial erosion and crustal warping caused by the thick ice sheet’s tremendous weight. Prior to Montana Missouri River drainage basin development ice-marginal southeast-oriented melt water floods flowed on a topographic surface now represented by the highest level Rocky Mountain erosion surfaces. Ice sheet melting opened up space in the deep “hole” the decaying ice sheet had been occupying and deep northeast and east-oriented valleys eroded headward from the ice sheet’s southwest margin to capture the high-level southeast-oriented floods and to divert flood waters into the deep “hole”. Headward erosion of the deep Montana Missouri River valley and tributary valleys captured the flood waters and diverted flood flow in a northeast and east direction so as to flow into space the rapidly melting ice sheet had once occupied. Mountain ranges emerged as flood waters deeply eroded surrounding regions and/or as ice sheet-triggered crustal warping uplifted present day mountain ranges.

Figure 1: Montana Missouri River drainage basin location map (select and click on map to enlarge). National Geographic Society map digitally presented using National Geographic Society TOPO software.

Montana Missouri River drainage basin  drainage history

The Montana Missouri River begins at Three Forks, Montana where the Jefferson, Madison, and Gallatin Rivers meet to form the Missouri River. From Three Forks the Missouri River flows in a north and northwest direction before turning to flow in a northeast direction. After passing Fort Benton the Missouri River turns to flow in a southeast, east, and east-southeast direction to join the north-oriented Musselshell River. From the Musselshell River the Missouri River flows in and an east direction to the North Dakota and the Yellowstone River confluence. Once in North Dakota and east of the figure 1 map area the Missouri River turns to flow in a southeast direction and eventually joins the south-oriented Mississippi River with Missouri River water finally reaching the Gulf of Mexico. Major Montana Missouri River tributaries from the west and north are the Teton and Marias Rivers, which join the northeast oriented Missouri River near Fort Benton and the east-southeast oriented Milk River which joins the Missouri River downstream from Fort Peck Lake (a large reservoir). Major tributaries from the south are the northeast and north-oriented Judith River, southeast, northeast, and north-oriented Musselshell River, and northwest, north, southeast, and northeast-oriented Yellowstone River. Major Yellowstone River tributaries include the north-oriented Bighorn, Tongue, and Powder Rivers.

• Study of figure 1 suggests the Montana Missouri River drainage basin is primarily a north, northeast, and/or east-oriented drainage basin, which is puzzling because east of Montana the Missouri River is a south-oriented river. Just looking at a general regional drainage map like figure 1 one is tempted to suggest the Montana Missouri River drainage basin and its north-oriented tributary drainage basins originated as north or northeast oriented drainage basins and then were blocked with water diverted east and then southeast along a continental ice sheet margin. However, topographic map evidence along drainage divides between these north and northeast oriented Missouri River headwaters and tributaries provides evidence the Montana Missouri River drainage basin was formed by headward erosion of deep northeast and east-oriented valleys across immense southeast and south-oriented floods. The best explanation for this topographic map evidence is massive south and southeast oriented melt water floods were derived from a rapidly melting thick North American ice sheet, which had been located in a deep ice sheet created  “hole.” The deep northeast and east-oriented valleys were eroded headward from space in the deep “hole” that was opening up as the thick ice sheet rapidly melted. A brief overview of this interpretation is given below.

Figure 2: Jefferson River, Madison River, and Gallatin River drainage basins in southwest Montana and northwest Wyoming (the Wyoming northwest corner is located in the figure 2 southeast corner, Idaho is south of Montana further to the west). National Geographic Society map digitally presented using National Geographic Society TOPO software.

To fully understand Montana Missouri River drainage history it is necessary to look at the Jefferson, Madison, and Gallatin River drainage basins, which form the Missouri River headwaters. These three rivers all have headwaters in the same general region, which can be seen in the figure 2 southeast quadrant. The Madison and Gallatin Rivers originate in the Yellowstone National Park northwest corner (located in the Wyoming northwest corner) with the Madison River flowing in a northwest and then north direction to reach Three Forks where the Missouri River begins. The Gallatin River flows in a north and north-northwest direction to reach Three Forks. The Red Rock River originates near Upper Red Rock Lake (located west in Montana a short distance west of Yellowstone National Park) and flows in a west, northwest, and northeast direction to join the northeast oriented Beaverhead River, which then joins the Big Hole River to form the northeast and east-oriented Jefferson River, which joins the Madison and Gallatin Rivers at Three Forks. Further complicating the picture is the Yellowstone River also originates in the Yellowstone National Park region of northwest Wyoming and flows in a northwest and north direction to just east of the Gallatin River and Madison River headwaters. Still further complicating the picture is the Snake River originates in the same general region and flows in a south direction before turning to flow in a northwest direction to eventually join the Columbia River and to reach the Pacific Ocean. In other words, these rivers are all originating in the same general region, which is located along the east-west continental divide.

Figure 3: Reduced size topographic map of drainage divides between northwest-oriented Madison River (flowing to figure 3 north center area), west-oriented Red Rock River (flowing to figure 3 southwest corner), and south-oriented Henrys Fork of the Snake River (flowing south from Henrys Lake in the figure 3 southeast corner area). Note how these diverging rivers are linked by through valleys. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 3 illustrates an example of evidence in the Jefferson, Madison, Gallatin, and Yellowstone River headwaters areas. Note how the north-northwest oriented Madison River valley is linked by a well-defined through valley at Raynolds Pass with south-oriented drainage to Henrys Lake, which eventually reaches the Pacific Ocean. There is no elevation given for Raynolds Pass, although from the contour lines the elevation appears to be between 2050 and 2100 meters. Black Mountain to east rises to more than 3000 meters. Mountains to the west are lower, but several rise to elevations greater than 2500 meters and in the figure 3 northwest quadrant mountains rise to approximately 2900 meters. In other words Raynolds Pass is large through valley 500-900 or more meters deep which crosses the east-west continental divide. The valley is a water eroded valley, which means at one time large volumes of water flowed across the continental divide. Note also the northeast-oriented through valley in which Elk Lake, Hidden Lake, and Cliff Lake are located, which links the west-oriented Red Rock River drainage basin with the north-northwest oriented Madison River valley. The floor of that through valley also appears to have an elevation of between 2050 and 2100 meters, with nearby mountains on either side rising to elevations of 2500 meters or more and by going further to the northwest or east or southeast mountains rising to elevations of 2900 meters or more. While some evidence suggests the valley may be located along a fault line or other structural feature, the valley also is a water eroded feature. Further note how the southeast and northeast-oriented West Fork Madison River is linked by several through valleys with the west-oriented Red Rock River valley. These through valleys are also water eroded features and provide further evidence that large volumes of water once flowed across what are today drainage divides separating diverging river drainage basins. How were these through valleys eroded and where did the large volumes of water required to erode these large and deep through valleys come from? These through valleys and numerous other through valleys like them were eroded by immense southeast-oriented floods which flowed across the Yellowstone Plateau area and which were systematically captured and diverted in different directions by headward erosion of deep valleys into the region.

• Why would immense southeast-oriented floods be flowing across what are now high mountain ranges and what is now the east-west continental divide and how could those southeast-oriented flood waters flow across what are today high mountain elevations and the east-west continental divide? The Missouri River drainage basin history began with development of a North American ice sheet comparable in size to the present day Antarctic Ice Sheet, if not larger. The ice sheet was thick, probably several kilometers thick, and was located in a deep “hole”, which the ice sheet had formed by a combination of deep glacial erosion and crustal warping caused by the ice sheet weight. When at its maximum size the ice sheet stood high above the pre-glacial surface, but also had roots that extended well below the pre-glacial surface, which no longer exists. Most if not all of the present day Missouri River drainage basin location was probably located south and west of the thick ice sheet’s southwest margin, although evidence for the ice sheet’s southwest margin has probably been removed by deep melt water flood erosion. The pre-glacial surface under the ice sheet was completely destroyed by deep glacial erosion and the pre-glacial surface adjacent to the ice sheet and elsewhere on the North American continent was deeply eroded by deep melt water flood erosion and was also probably significantly altered by crustal warping caused by the thick North American ice sheet presence.

• Events important to Missouri River drainage basin history began as the ice sheet was rapidly melting and had melted to the point that in the south it no longer stood high above the surrounding non-glaciated surface, which had probably already been significantly lowered by deep melt water erosion. Immense melt water floods were flowing in a southeast direction along the ice sheet’s southwest margin and were just beginning to deeply erode the region between the Rocky Mountains and the ice sheet’s southwest margin, which at that time was located north and east of the Montana and Wyoming Rocky Mountains. At that time the Rocky Mountains did not stand high above the ice sheet surface, or for that matter above the surface located between the Rocky Mountains and the ice sheet southwest margin. Initially immense floods of melt water flowing from the rapidly melting ice sheet flowed across and along what is now the east-west continental divide. Flood waters flowing into northern Wyoming were probably derived from immense southeast and south-oriented supra-glacial melt water rivers which had carved giant ice-walled and ice-floored (later bedrock-floored) canyons into the decaying ice sheet surface and which flowed to the ice sheet’s southwest margin in present day Alberta and then across the east-west continental divide into a gigantic southeast-oriented river which was flowing along the alignment of the present day northwest-southeast oriented Rocky Mountain Trench. Until headward erosion of deep valleys from the Pacific Ocean systematically captured and eroded the Rocky Mountain Trench this immense southeast-oriented melt water river the Rocky Mountain Trench valley did not exist and flood waters flowed in a southeast direction across western Montana to the present day Yellowstone Plateau area and then in a southeast direction across Wyoming. Missouri River drainage basin drainage history began as deep northeast-oriented valleys eroded headward from the deep “hole” the decaying ice sheet had once occupied to capture this gigantic southeast-oriented melt water river and to divert the flood waters into ice sheet’s deep “hole”.

• How did the deep “hole” the ice sheet had been occupying open up so as to permit headward erosion of the deep northeast and east-oriented Missouri River valley and its various deep northeast and north-oriented tributary valleys? Remember, the ice sheet was thick and had deep roots. The ice sheet roots may have extended more than a kilometer below the pre-glacial surface on which the ice sheet had formed. The deep “hole” had probably been formed by a combination of deep glacial erosion of the pre-glacial surface underlying the ice sheet and of crustal warping caused by the weight of an ice sheet several kilometers thick. The crustal warping, which almost certainly did not occur instantaneously, probably also affected regions elsewhere on the continent and may have contributed to uplift of the Rocky Mountains and other North American mountain ranges and high plateau areas as flood waters flowed across them. In other words, not only was the rapidly decaying ice sheet located in a deep “hole” that was opening up as the ice sheet melted, but delayed crustal warping caused by the ice weight was raising mountain ranges and high plateaus regions south and west of the ice sheet margin. The combination of these two events created a situation where the gigantic southeast-oriented melt water river flowing across and along what is now the east-west continental divide was systematically captured by headward erosion of deep northeast-oriented valleys, which were eroding headward from the decaying ice sheet location.

Figure 4: Missouri River in western and central Montana. Northeast-oriented Yellowstone River is located in figure 4 southeast corner and east-southeast oriented Milk River is north of figure 4 map area. National Geographic Society map digitally presented using National Geographic Society TOPO software.

Figure 4 illustrates a region in western and central Montana, all of which is included in the Missouri River drainage basin. The Missouri River flows in a northwest direction from Three Forks to near Wolf Creek and then turns to flow in a northeast direction almost to Big Sandy, before turning to flow in a southeast, east and east-southeast direction to join the north-oriented Musselshell River and then to flow in a northeast direction to the figure 4 east edge. Note how the Musselshell River begins in the Little Belt Mountains and flows in a southeast and then northeast direction before turning north to join the Missouri River. Also note the northeast oriented Yellowstone River located in the figure 4 southeast corner. This figure 4 map area was probably south and west of the decaying ice sheet southwest margin at the time most figure 4 river valleys were eroded and prior to headward erosion of those deep river valleys immense southeast-oriented floods moved across the entire figure 4 map area on a topographic surface now preserved (if it is preserved at all) on the highest level erosion surfaces in the present day mountain ranges. Today those mountains rise as much two kilometers above the deeper river valleys, and it is difficult to imagine how the landscape has been altered since that time. However, north and east of the figure 4 map area at that time were the decaying roots of the thick ice sheet, with perhaps much thicker ice located still further to the north and east. Crustal warping caused by the thick ice sheet weight had already begun to raise mountain ranges further to the south and west and was starting to raise mountain ranges in the figure 4 map area. As these mountain ranges were being uplifted deep northeast and east-oriented valleys eroded headward from space in the deep “hole” being opened up. The deep valleys eroded headward in sequence from the south and east to the north and west and captured the immense southeast-oriented floods and diverted the flood waters into the deep “hole” the ice sheet had occupied.

• In the case of the northeast and east oriented Missouri River valley, the northeast-oriented Yellowstone River-Powder River, Yellowstone River-Tongue River, and Yellowstone River-Bighorn River valleys (and other major northeast-oriented valleys in Montana, Wyoming, and North and South Dakota) they eroded headward from a huge southeast and south-oriented ice-walled and ice-floored (later bedrock-floored) canyon which was carved by an immense southeast and south-oriented supra-glacial melt water river north and east of the ice sheet southwest margin. The Missouri Escarpment in North and South Dakota and in Saskatchewan is today what remains of that giant canyon’s west and southwest wall. The ice floor of that giant southeast and south-oriented ice-walled canyon was significantly lower in elevation than the bedrock surface south and west of the decaying ice sheet margin and the huge melt water river flowing in that southeast and south-oriented ice-walled canyon represented the region’s major drainage route, which captured the immense southeast-oriented ice-marginal floods by eroding deep northeast-oriented tributary valleys headward across the ice sheet’s southwest margin and then headward into the adjacent bedrock surface. These deep northeast-oriented valleys diverted immense southeast-oriented ice-marginal floods into space the ice sheet had once occupied. Melting of what had been the thick ice sheet roots progressively lowered both the ice sheet surface, the ice-walled canyon floor, and surrounding bedrock surface, creating a situation where new and even deeper northeast-oriented valleys repeatedly eroded headward to capture immense southeast-oriented ice-marginal melt water floods. The deep east and northeast-oriented Missouri River and northeast-oriented Yellowstone River-Powder River, Yellowstone River-Tongue River, Yellowstone River-Bighorn River valleys we see today were probably eroded very late in the ice sheet melt down history and were probably preceded by several earlier, but similar northeast-oriented valleys which also diverted massive southeast-oriented ice-marginal floods onto the decaying ice sheet surface. Valleys were eroded headward in sequence, with valleys located in the south and east being eroded headward first.

Figure 5: Missouri River route from east of Helena, Montana to Great Falls, Montana. The Missouri River flows in a northwest direction from Canyon Ferry Dam (east of Helena) to Holter Dam (near Wolf Creek) and then turns to flow in a northeast direction to Great Falls. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

The Missouri River in Montana has several northeast-oriented segments. Proceeding downstream from Three Forks the first significant northeast-oriented segment is located between the location of the present day Holter Dam and the Missouri River elbow of capture south of Big Sandy. From Three Forks the Missouri River flows in a north and northwest direction before turning near Holter Dam to flow in a northeast direction. Figure 5 illustrates the elbow of capture where the northwest-oriented Missouri River turns to flow in a northeast direction. Note on figure 5 how the northwest-oriented Missouri River has southwest-oriented barbed tributaries from the Big Belt Mountains to east and also has several southeast-oriented barbed tributaries from the west. These barbed tributaries provide evidence the northwest-oriented Missouri River valley segment probably originated as a southeast-oriented drainage route. Also note the northeast-oriented Missouri River segment has southeast-oriented barbed tributaries including the southeast-oriented Dearborn River and also has northwest-oriented barbed tributaries including the Smith River. The northwest-southeast orientation of at least some of the northeast-oriented Missouri River segment tributaries provides evidence the northeast-oriented Missouri River valley eroded headward across what was probably an immense southeast-oriented anastomosing channel complex. If so at the time the northeast-oriented Missouri River valley eroded headward into the figure 5 map area the mountains did not stand high above surrounding valleys and basins as they do today. Instead flood waters were able to flow freely in multiple channels in a southeast direction across the entire figure 5 map area. Headward erosion of what had to be a very deep northeast-oriented Missouri River valley captured the southeast-oriented flood flow one channel at a time from the northeast to the southwest. Flood waters from the northwest eroded the northwest valley wall and the southeast-oriented tributary valleys seen today. Flood waters on the northwest ends of the beheaded southeast-oriented flood flow channels reversed flow direction to erode the northwest-oriented tributary valleys. Because channels were beheaded one channel at a time and also because flood flow channels were anastomosing (interconnected) reversed flood flow in a newly beheaded flood flow channel could capture flood waters from channels further to the south and west. The northwest-oriented Missouri River valley segment was eroded headward along the alignment of a beheaded southeast-oriented flood flow channel and was especially successful in capturing flood waters from flood flow channels further to the south and west (headward erosion of the Gallatin, Madison, and Jefferson River valleys in sequence captured southeast-oriented flood flow and diverted the water north into the newly formed Missouri River valley which today is located in the figure 5 map area.

Figure 6: Missouri River elbow of capture south of Big Sandy, Montana. Bear Paw Mountains are located in figure 6 northeast quadrant. Note northeast-oriented through valley linking northeast-oriented Missouri River segment with northeast-oriented Big Sandy Creek segment. North of the figure 6 map area Big Sandy Creek joins the east-southeast oriented Milk River, which flows north of the Bear Paw Mountains to eventually join the Missouri River in eastern Montana. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

The northeast oriented Missouri River valley segment in figures 5 and 6 was eroded after headward erosion of the deep southeast, northeast and north-oriented Musselshell River valley, which was eroded after headward erosion of the deep northeast oriented Yellowstone River valley (see figure 1 to see how valleys were eroded headward from the deep “hole” in sequence from the southeast to the northwest). Figure 6 uses reduced size topographic maps (with no contour lines shown in the Bear Paw Mountain area in the figure 6 northeast quadrant) to illustrate how headward erosion of these deep northeast and east-oriented valleys from the deep “hole” the melting ice sheet was opening up probably resulted in the development of a massive northeast and east-oriented anastomosing channel complex, which captured flood waters flowing in a gigantic southeast-oriented anastomosing channel complex. Note in figure 6 how a northeast-oriented through valley extends from the Missouri River elbow of capture (where the northeast-oriented Missouri River turns to flow in a south-southeast direction) to Big Sandy and the northeast-oriented Big Sandy Creek valley. Big Sandy Creek originates in the high Bear Paw Mountains and flows in a southwest direction before making a U-turn (seen south and east of Big Sandy) to flow in a northeast direction along the west Bear Paw Mountains flank to join the east-southeast-oriented Milk River, which flows in a large valley along the Bear Mountains north flank. For a time flood waters probably flowed in both valleys, with the Bear Paw Mountains being a highland area between two immense diverging and converging flood flow channels. Prior to erosion of the deep east-oriented Milk River valley and the northeast-oriented Big Sandy Creek-Missouri River valley flood waters flowed across what are today the high Bear Paw Mountains. The Bear Paw Mountains emerged as flood waters flowed across the region. Bear Paw Mountains emergence as a high mountain range was probably caused by a combination of deep flood water erosion of the surrounding regions and by crustal warping triggered by the tremendous ice sheet weight further to the north and east. Detailed essays documenting topographic map evidence of flood flow routes across the Bear Paw Mountains can be found under Milk River on the sidebar category list.

Figure 7: Missouri River drainage system in northeast Montana. National Geographic Society map digitally presented using National Geographic Society TOPO software.

Figure 7 illustrates the Missouri River route and tributaries for northeast Montana and illustrates the region north and east of the figure 4 map area. Note how the Missouri River joins the north-oriented Musselshell River near the figure 4 west center edge and then flows in a northeast direction to Fort Peck Dam. From Fort Peck Dam the Missouri River flows in an east direction to Culbertson before turning to flow in an east and southeast direction to join the northeast-oriented Yellowstone River just east of the North Dakota border. From the Yellowstone River the Missouri River flows in a northeast direction to Williston, North Dakota. The east-southeast, northeast, and southeast oriented Milk River joins the Missouri River downstream from Fort Peck Dam, which illustrates where the diverging anastomosing channels described in the figure 6 discussion converge again. Note how the northeast-oriented Yellowstone River has multiple southeast-oriented tributaries from the northwest and multiple northwest-oriented tributaries from the southeast. This northwest-southeast tributary orientation is better seen for the Missouri and other major tributary rivers on detailed topographic maps is evidence the river valleys were systematically eroded headward across an immense southeast-oriented anastomosing channel complex. Figure 8 below uses reduced size topographic maps to illustrate northwest-southeast oriented drainage alignment of Missouri River and Big Dry Creek tributaries in the Fort Peck Reservoir area (Dry Arm is the Big Dry Creek flooded valley). As previously described northwest-oriented tributary valleys were eroded by reversals of flood flow on northwest ends of beheaded flood flow routes.

• While not visible on figure 7 the deep Missouri River valley did not initially erode headward along its present route from Williston, North Dakota to Poplar, Montana, but instead eroded headward from Crosby, North Dakota area (north of Williston) across the Medicine Lake area to the Poplar, Montana area. Evidence illustrating initial deep Missouri River valley is provided in detailed essays describing drainage divide between the Poplar River and Big Muddy Creek and between Big Muddy Creek and Little Muddy River. Figure 9 below illustrates a segment of the large abandoned valley, which was eroded headward from the ice sheet’s southwest margin prior to headward erosion of the present day Missouri River valley (from the Yellowstone River confluence area to Poplar Montana).

Figure 8: Reduced sized topographic of flooded northeast-oriented Missouri River valley and north-northwest oriented Big Dry Creek valley at Fort Peck Reservoir illustrating northwest-southeast orientation of Missouri River and Big Dry Creek tributaries. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 8 illustrates evidence the deep northeast-oriented Missouri River valley eroded headward across multiple southeast-oriented channels such as might be found in a southeast-oriented anastomosing channel. The north-northwest oriented Dry Arm in figure 8 is the flooded Big Dry Creek valley, which was also eroded headward across the southeast-oriented flood flow slightly in advance of headward erosion of the deep northeast-oriented Missouri River valley. Southeast-oriented oriented Big Dry Creek tributary valleys were eroded by southeast-oriented flood flow moving into the newly eroded and deep Big Dry Creek valley. Northwest-oriented Big Dry Creek tributary valleys were eroded by reversals of flood flow on northwest ends of beheaded flood flow channels. Because flood flow channels were beheaded in sequence from north to south and because flood flow channels were anastomosing (or interconnected) reversed flow in newly beheaded flood flow channels captured flood flow from channels further south. Such captures of flood water provided water volumes required to erode the northwest-oriented tributary valleys. Headward erosion of the deep northeast-oriented Missouri River valley beheaded the southeast-oriented flood flow channels to the newly eroded and deep Big Dry Creek valley. Again southeast-oriented tributary valleys were eroded by southeast-oriented flood flow moving into the newly eroded and deep Missouri River valley. And again, northwest-oriented tributary valleys were eroded by reversals of flood flow on northwest ends of beheaded flood flow routes.

Figure 9: Reduced size topographic map of large abandoned valley extending in a northeast direction from the present day east-oriented Missouri River valley near Poplar, Montana (located in figure 9 southwest corner). United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 9 illustrates a large abandoned valley extending in a northeast direction from the present day east-oriented Missouri River valley toward the North Dakota northwest corner. West of the Poplar, Montana area (in southwest corner of figure 9) the Missouri River valley is much wider than it is to the east, suggesting at one time the Missouri River flowed in a northeast direction from the Poplar area into the North Dakota northwest corner. The Popular River-Big Muddy Creek and the Big Muddy Creek and Little Muddy River essays and essays found under ND Missouri River on the sidebar category list illustrate how this abandoned valley once extended to the present day northeast-facing Missouri Escarpment, which is what remains of the southwest wall of what was once a gigantic southeast and south-oriented ice-walled and bedrock-floored canyon carved in the decaying ice sheet surface. This abandoned northeast-oriented valley eroded headward from the immense southeast and south-oriented river which flowed in that giant ice-walled canyon and then eroded headward along the present day Missouri River-Musselshell River and Milk River valley alignments (in sequence from south to north) to capture southeast oriented ice-marginal flood flow. The valley seen here was eroded very late in the ice sheet melt down history at a time when deep ice-walled and bedrock-floored canyons were eroding headward into the ice sheet surface. These deep ice-walled canyons were for all practical purposes chopping the decaying thick ice sheet up into a large number of isolated and small ice sheet masses. At the same time as the ice-walled canyons eroded headward they intersected with each other, which opened up new flood flow routes across what had been the ice sheet floor. These new flood flow routes systematically captured the immense southeast and south-oriented melt water river into which this northeast-oriented abandoned Missouri River valley had been draining and the flood waters were first diverted east to what is now the Saint Lawrence River drainage basin and later north to what is now the Hudson Bay area. This diversion of what had been huge south-oriented floods to the north had a profound impact on the North American climate, which halted the decaying ice sheet’s rapid melt down.

• The south-oriented flood waters had been flowing to the Gulf of Mexico where they displaced warm water which then moved north in the Atlantic Ocean to produce warm climates responsible for the ice sheet’s rapid melt down. However, when the giant floods were diverted north the warm Gulf of Mexico water was no longer displaced and instead cold North Atlantic water was displaced and pushed south. The result was a general cooling of North American climates which in turn caused the north-oriented flood waters to freeze on the former ice sheet floor. The frozen flood waters surrounded the former thick ice sheet remnants to create what was for all practical purposes a new thin ice sheet. Unlike the previous the thick ice sheet the new thin ice sheet did not deeply erode the region, however the thin ice sheet did block the northeast-oriented flood flow routes such as this northeast-oriented valley seen in figure 9. The blockage forced the remaining ice marginal drainage to flow in an east and southeast direction to the deep Yellowstone River valley and then to flow for a distance in that valley before spilling in a southeast and east direction along the ice sheet margin. The thin ice sheet also did move some debris and some that debris ended up in the former northeast-oriented valley. The Popular River-Big Muddy Creek and the Big Muddy Creek and Little Muddy River drainage divide areas essays and other essays under MT Missouri River and ND Missouri River on the sidebar category list provide much more detailed evidence and descriptions supporting this climate change interpretation.

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.