Bighorn River drainage basin landform origins, Wyoming and Montana, USA, overview essay

· Bighorn River, Montana, Overview essays, Wyoming
Authors

A geomorphic history based on topographic map evidence

Abstract:

This overview essay uses topographic map evidence to interpret the history of drainage divides within and surrounding the Bighorn River drainage basin. Detailed essays describing a few Bighorn River drainage divide areas near the Yellowstone River in Montana have been written and can be found under Bighorn River on this website’s sidebar category list. Detailed essays describing all other Bighorn River drainage divide areas are planned and will be added as they are finished. The Bighorn River originates at the north end of Wind River Canyon, which is deep canyon eroded across the Owl Creek Mountains in central Wyoming where the Wind River changes its name to become the Bighorn River. From Wind River Canyon the Bighorn River flows in a north direction across the Bighorn Basin before turning to flow in a northeast direction across the Bighorn Mountains north end (in a deep canyon) and then to join the northeast-oriented Yellowstone River. The Wind River originates between the Absaroka Mountains and the Wind River Mountains and flows in a southeast direction across the Wind River Basin to near Riverton, Wyoming where it turns to flow in a northeast and north direction through Wind River Canyon. Bighorn River drainage basin topographic map evidence and in similar Missouri River drainage basin landform origins research project drainage divide areas documents an immense southeast-oriented flood which crossed the entire Bighorn 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 Bighorn 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-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 Yellowstone River-Bighorn River valley and tributary valleys captured the flood waters and diverted flood flow in a northeast 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: Bighorn River drainage basin location map. National Geographic Society map digitally presented using National Geographic Society TOPO software.

Bighorn River drainage basin drainage history

The Bighorn River originates at the north end of Wind River Canyon, a deep canyon eroded across the Owl Creek Mountains of central Wyoming, and flows in a north direction across the Bighorn Basin before turning to flow in a northeast direction in a deep canyon eroded across the Bighorn Mountains north end and then to join the northeast-oriented Yellowstone River in south central Montana. The northeast-oriented Yellowstone River joins the east and southeast-oriented Missouri River with water eventually reaching the Gulf of Mexico. The Bighorn Basin is a large lowland region surrounded on the east by the high Bighorn Mountains, on the south by the high Owl Creek Mountains, and on the west by the high Abasroka Mountains. Major Bighorn River tributaries in the Bighorn Basin area originate in the surrounding mountain ranges. The Wind River originates in northwest Wyoming between the Absaroka Mountains and the Wind River Mountain Range and flows in a southeast direction into and across the Wind River Basin, which is a lowland area between the northwest-southeast oriented Wind River Mountains and west-east oriented Owl Creek Mountains. Near Riverton, Wyoming the Wind River turns to flow in a northeast and then north direction so as to flow through the deep Wind River Canyon to reach the Bighorn Basin, where it becomes the north-oriented Bighorn River.

  • East of the Bighorn River drainage basin in the high Bighorn Mountains and also south of the Bighorn Mountains is the northeast and north-oriented Powder River drainage basin, with the Powder River also draining to the northeast-oriented Yellowstone River. Interestingly the Bighorn River-Powder River drainage divide in the high Bighorn Mountains is often located west of the highest Bighorn Mountain peaks and several Powder River tributaries have eroded deep canyons across what are today high Bighorn Mountains ridges. At the north end of the high Bighorn Mountains the northeast-oriented Tongue River drainage basin is located east of the Bighorn River drainage basin and again the drainage divide is often located along the west edge of the high Bighorn Mountains upland surface. South of the Bighorn Mountains no mountain ranges or high ridges separate what are today west-oriented Wind River tributaries from northeast-oriented Powder River tributaries and east oriented North Platte River tributaries (see figure 2 below), yet the Wind River instead of flowing east and northeast to the Powder River turns and flows north through Wind River Canyon, which has been eroded across the Owl Creek Mountains. South of the Yellowstone National Park area (in the Wyoming northwest corner) is the south and northwest-oriented Snake River drainage basin, with Snake River water eventually reaching the Pacific Ocean. The Snake River-Wind River drainage divide south of Yellowstone National Park is the east-west continental divide. The Yellowstone River flows in a northwest, north, and northwest direction across Yellowstone National Park and the Yellowstone River-Bighorn River drainage divide is located along the Yellowstone National Park east boundary (south half). The Shoshone River is the major east- and northeast-oriented Bighorn River tributary draining from the Yellowstone National Park boundary area.

Figure 2: Northwest Wyoming drainage map illustrating Wind River and Bighorn River drainage basin areas. National Geographic Society map digitally presented using National Geographic Society TOPO software.

Figure 2 provides a somewhat more detailed map of the Wind River and Bighorn River drainage basins in the Wyoming northwest quadrant. The Wind River originates near Togwotee Pass (south of the Yellowstone National Park southeast corner) and flows in a southeast direction between the Wind River Mountains to the southwest and the Owl Creek Mountains to the northeast and north into the Wind River Basin, which is the basin area located between the two mountain ranges. The Wind River continues flowing in a southeast direction to near Riverton and then turns to flow in a northeast direction to Boysen Reservoir at the Wind River Canyon south end. From Boysen Reservoir the Wind River flows in a north direction through the deep Wind River Canyon to enter the Bighorn Basin, where the Wind River name changes so as to become the north-oriented Bighorn River. The Bighorn River then flows in a north direction across the Bighorn Basin to enter Montana where it turns to flow in a northeast direction (north of the figure 2 map area). Surrounding the Bighorn Basin are the Bighorn Mountains to the east, the Owl Creek Mountains to the south, and the Absaroka Mountains to the west and other than Wind River Canyon the Bighorn Basin is only open to the north. However, rather than flowing directly north in the area between the mountain ranges the Bighorn River in southern Montana flows through a deep canyon eroded across the Bighorn Mountains north end before flowing north to the Yellowstone River. The Wind River Basin, south of the Owl Creek Mountains is not enclosed by mountains to the east and southeast and the Wind River elbow of capture (where it turns from flowing in a southeast direction to flowing in a north direction to Wind River Canyon) requires an explanation. The deep canyons eroded across the Owl Creek and Bighorn Mountains also require explanations as do deep mountain passes such as Togwotee Pass, which links the Wind River headwaters with headwaters of west-oriented Snake River tributaries and which is a deep water eroded valley crossing what is today the west-to-east continental divide.

Figure 3: Togwotee Pass linking northwest-oriented Pacific Ocean drainage with southeast-oriented Atlantic Ocean drainage. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

The southeast-oriented Wind River originates near Togwotee Pass, which is illustrated near the center of figure 3. The Wind River as seen in figure 2 after flowing in a southeast direction turns to flow in a north direction through a deep canyon eroded across the Owl Creek Mountains then in a north direction across the Bighorn Basin and the Bighorn Mountains north end to reach the northeast-oriented Yellowstone River. The Yellowstone River eventually flows to the southeast-oriented Missouri River with water eventually reaching the Gulf of Mexico. This round about Wind River-Bighorn River-Yellowstone River-Missouri River route is rather puzzling considering no mountain ranges are located between the Wind River elbow of capture and east-oriented North Platte River tributaries (seen in the figure 2 southeast corner). Wind River water could reach the southeast-oriented Missouri River in eastern Nebraska by a much more direct route without ever having to erode deep canyons to cross high mountain ranges. Yet, today Wind River water takes a circuitous route many hundreds of kilometers longer and passing through deep canyons before it reaches eastern Nebraska. The circuitous route and deep canyons require an explanation. A northwest-oriented tributary of northwest-oriented Split Rock Creek drains the west side of Togwotee Pass. North and west of the figure 3 map area Split Rock Creek joins west-oriented Buffalo Fork (of the Snake River), which flows to the south-oriented Snake River (see figure 2), which eventually turns to flow in a northwest direction with water eventually reaching the Pacific Ocean. The rather round about route taken by water in northwest-oriented Split Rock Creek to reach what eventually becomes the northwest-oriented Snake River is just as puzzling. The Togwotee Pass elevation is 2909 meters and Breccia Peak to the north rises to 3351 meters and Two Ocean Mountain to the south rises to 3269 meters. In other words Togwotee Pass is a deep through valley (at least 360 meters deep) linking a northwest-oriented valley draining to the Pacific Ocean with a southeast-oriented valley draining to the Atlantic Ocean. Togwotee Pass is a water eroded feature and was eroded by large volumes of water moving across what is today the east-west continental divide. A close look at figure 3 reveals many other mountain passes or through valleys crossing present day drainage divides, including the continental divide. Those other through valleys are also water eroded landforms and combined the mountain passes and through valleys provide evidence of multiple water eroded channels, such as might be found in a large-scale anastomosing channel complex, which suggests the entire figure 3 map area was eroded by an immense southeast-oriented flood.

Figure 4: Yellowstone Lake-Shoshone River drainage divide area at Sylvan Pass. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Just as puzzling as deep through valleys eroded across the Snake River-Wind River drainage divide (the west-east continental divide) are deep through valleys eroded across the Yellowstone River-Shoshone River drainage divide, with the Shoshone River flowing to the north and northeast oriented Bighorn River, which flows to the northeast-oriented Yellowstone River. West of the Yellowstone River-Shoshone River drainage divide the Yellowstone River is flowing in a northwest, north, and northwest direction (see figure 2) and then turns to flow in a north, northeast, and southeast direction before turning again to become a northeast-oriented river and to join the Bighorn River (see figure 1). Figure 4 above illustrates the Sylvan Pass area where the Yellowstone River-Shoshone River drainage divide forms the Yellowstone National Park eastern boundary. The lake seen along the figure 4 west edge is Yellowstone Lake, which is a natural lake found along the northwest-oriented Yellowstone River route. West-oriented streams flowing to the lake are Yellowstone River tributaries while east-oriented streams flowing to the figure 4 east edge are Shoshone River tributaries. Sylvan Pass is the deep through valley where  the highway crosses the Yellowstone River-Shoshone drainage divide. There is no spot elevation shown on figure 4 to give the Sylvan Pass elevation, although the elevation appears to be in the 2600 to 2650 meter range. Mountain peaks to the north and south of the pass rise to elevations greater than 3200 meters suggesting Sylvan Pass is a water eroded valley at least 550 meters deep. Study of the Yellowstone River-Shoshone River drainage divide reveals other deep passes eroded across the high drainage divide ridge. Crow Creek Pass and Jonas Pass are two named passes north of Sylvan Pass and other unnamed passes can also be seen. These multiple passes suggest the presence of multiple deep and roughly parallel east-oriented valleys which were eroded across what is now the high Yellowstone River-Shoshone River drainage divide. Such multiple parallel valleys again suggests the valleys were formed as channels in what was a large-scale flood formed anastomosing channel complex moving flood water from what is now the northwest-oriented Yellowstone River drainage basin to what is now the east-oriented Shoshone River drainage basin.

  • Why would immense southeast and east 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? The Bighorn 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. The Bighorn River drainage basin location was probably located south and west of 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 Bighorn 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 today’s Bighorn River drainage basin. 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 this immense southeast-oriented melt water river the deep 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. Bighorn 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-oriented Yellowstone River-Bighorn River valley? 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.
  • In the case of the Yellowstone River-Bighorn River valley (and other major northeast-oriented valleys in Montana, Wyoming, and North and South Dakota) it 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 the 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 the surrounding bedrock surface, creating a situation where new and even deeper northeast-oriented valleys repeatedly eroded headward to capture the immense southeast-oriented ice-marginal melt water floods. The deep 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 east being eroded headward first.

Figure 5a: Map illustrating tributary orientations to the north-oriented Bighorn River in the Bighorn Basin north of Wind River Canyon and the Owl Creek Mountains. Note the multiple southeast-oriented barbed tributaries  from the west and the northwest-oriented tributaries from the east. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 5b: Wind River Canyon where the north-oriented Wind River crosses the Owl Creek Mountains. The Boysen Reservoir (south center edge area) normal pool elevation is 1440 meters and the Wind River elevation drops from that level as it flows north. Owl Creek Mountains peaks on either side of Wind River Canyon rise to elevations greater than 2200 meters meaning Wind River Canyon is approximately 800 meters deep. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 5c: Wind River route south of Wind River Canyon. The Wind River flows in a southeast direction to near Riverton (the town located in the figure 5c southwest quadrant) and then turns to flow in a north-northeast direction to Boysen Reservoir located at the Wind River Canyon south end. No mountain ranges are located between headwaters of west-oriented Badwater Creek (near highway in figure 5c northeast quadrant) with headwaters of northeast-oriented Powder River tributaries and headwaters of east-oriented North Platte River tributaries. Why the Wind River turns north to flow through a high mountain range needs an explanation and is addressed in detailed Bighorn River essays (see sidebar). United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figures 5a, 5b, and 5c above illustrate examples of topographic map evidence supporting the immense southeast-oriented flood interpretation. Note in figure 5a the southeast-oriented barbed tributaries to the present-day north-oriented Bighorn River and also the northwest-oriented tributaries from the east. These northwest-southeast oriented tributary orientations are relics of a southeast-oriented anastomosing channel complex that once moved flood water across the region. At that time the Owl Creek Mountains did not stand high above the Bighorn Basin (to the north) or the Wind River Basin (to the south) and the immense southeast-oriented floods were moving freely to what is now the North Platte River drainage basin. As flood waters were flowing across the region the Owl Creek Mountains began to emerge. Emergence of the Owl Creek Mountains was probably caused by two different factors. First was deep flood erosion of easily eroded sedimentary rocks filling what are today the Bighorn and Wind River Basins. Second was delayed crustal warping triggered by the thick ice sheet tremendous weight (on the region north and east of the Bighorn River drainage basin) and perhaps by the removal of significant overburden from present day mountain areas and perhaps flood deposition of sediments in present day basin areas. As the Owl Creek Mountains emerged flood waters continued to flow south, although flood waters became channeled in fewer and fewer deep valleys, most of which today are through valleys (or mountain passes) eroded across the Owl Creek Mountains. In time the south-oriented flood water was all channeled in what is today the Wind River Canyon valley. At approximately that time headward erosion of the deep northeast oriented Yellowstone River-Bighorn River-Shoshone River valley from the deep “hole” the  rapidly melting ice sheet had formerly occupied beheaded the south-oriented flood flow channel moving south-oriented flood water to Wind River Canyon. Flood waters on the newly beheaded flood flow channel reversed flow direction to create the north-oriented Bighorn River, which then captured the southeast-oriented Wind River to create the Wind River elbow of capture seen in figure 5c. Deep erosion of the Bighorn Basin and Wind River Basin (below of the level of the present-day Wind River-North Platte River drainage divide) was caused by immense southeast-oriented floods still flowing across northwest Wyoming and into the southeast-oriented Wind River drainage basin.

Figure 6a: Bighorn Canyon where the northeast-oriented Bighorn River crosses the Bighorn Mountains north end. Pryor Mountains are located in figure 6a southwest corner. West of the Pryor Mountains is a large through valley linking the Bighorn Basin to the south with Yellowstone River valley to the north (see figure 6b). Why the Bighorn River eroded a deep valley across the Bighorn Mountains north end and did not flow north in valley shown in figure 6b needs an explanation, and is addressed in detailed Bighorn River essays.

Figure 6b: Clarks Fork of the Yellowstone River-Bighorn River drainage divide area south and west of figure 6a map area. North and northeast-oriented Shoshone River is located in the figure 6b southeast corner area and flows to join the north and northeast-oriented Bighorn River east of the figure 6b map area. Southeast and northeast-oriented Clarks Fork Canyon is located in the mountain area in the figure 6b northwest quadrant with Clarks Fork flowing to the figure 6b north center edge area and then north to the Yellowstone River. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

What evidence suggests a deep northeast-oriented valley eroded headward from the deep “hole” the rapidly melting ice sheet had once occupied to behead and reverse south-oriented flood flow in the present day Bighorn Basin? Figure 6a illustrates the Bighorn Canyon area where the Bighorn River crosses the north end of the Bighorn Mountains. The Pryor Mountains are located in the figure 6a southwest corner and west of the Pryor Mountains is a broad through valley linking the Bighorn Basin in the south with the Yellowstone River valley in the north (see figure 6b), yet the Bighorn River today turns away from that through valley to flow in a northeast direction in a deep northeast-oriented canyon across the Bighorn Mountains north end. The deep northeast-oriented Bighorn Canyon valley was eroded headward as the deep northeast-oriented Yellowstone River-Bighorn River-Shoshone River valley eroded headward from the deep “hole” the ice sheet had been occupying across immense southeast-oriented floods to capture the flood waters and to divert the flood flow to space the rapidly melting ice sheet was opening up. At that time flood waters were flowing on a topographic surface at least as high as the highest elevations surrounding Bighorn Canyon today (Bighorn Canyon is approximately 500 meters deep with the Bighorn Mountains ridge rising even higher to the south). South-oriented flood waters west of the Pryor Mountains at that time had not yet been captured by Yellowstone River valley headward erosion and were also flowing on a high level topographic surface. Headward erosion of the deep northeast-oriented Bighorn River-Shoshone River valley captured south-oriented flood waters first, which lowered base level which permitted south-oriented flood flow to initiate erosion of the through alley west of the Pryor Mountains. Yellowstone River valley headward erosion then captured the south-oriented flood flow through what is now the through valley west of the Pryor Mountains. A major reversal of flood flow eroded what are today north-oriented Yellowstone River tributary valleys at the north end of the through valley, including the present day Clarks Fork of the Yellowstone River valley (seen in figure 6b), which captured a major southeast-oriented flood flow route which was then still flowing across what are today high mountains to the west.

Figure 7: North-oriented Bighorn River valley north of Hardin, Montana and near where the Bighorn River joins the northeast-oriented Yellowstone River. Yellowstone River is located in figure 6b northwest corner. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 7 illustrates the Yellowstone River-Bighorn River drainage divide area north and west of Hardin, Montana. The Yellowstone River is located in the figure 7 northwest corner while the Bighorn River flows in a north direction in the figure 7 east half to join the northeast-oriented Yellowstone River north of the figure 7 map area. Note how Bighorn River tributaries from the west are southeast-oriented barbed tributaries and Bighorn River tributaries from the east are northwest-oriented. Also note how most Yellowstone River tributaries from the south are northwest-oriented barbed tributaries. The northwest-southeast oriented alignment of these Yellowstone River and Bighorn River tributaries is evidence of the multiple southeast-oriented flood flow channels beheaded as the deep Bighorn River and Yellowstone River valleys eroded headward across the region. Bighorn River valley headward erosion captured the southeast-oriented flood flow first and flood waters on northwest ends of beheaded flood flow channels reversed flow direction to erode the northwest-oriented Bighorn River tributary valleys. Yellowstone River valley headward erosion next beheaded the southeast-oriented flood flow channels to the newly eroded deep Bighorn River valley. Again flood waters on northwest ends of beheaded flood flow channels reversed flow direction to erode northwest-oriented tributary valleys. Flood waters were moving in anastomosing channels and because flood flow channels were beheaded one at a time and in sequence from north to south (and from east to west) and because the flood flow channels were anastomosing (or interconnected), reversed flood flow on a newly beheaded flood flow channel could capture flood flow from adjacent and yet to be beheaded flood flow channels further to the south or west. Such captures of yet to be beheaded flood flow provided the water volumes to required to erode the northwest-oriented tributary valleys. The north-northwest to south-southeast oriented valley linking the Yellowstone River valley with the Bighorn River valley (used by the railroad and located west of Pine Ridge) was initially eroded by south-oriented flood flow moving into what was then the newly eroded and deep north-oriented Bighorn River valley. Flood flow in that valley was beheaded and reversed by Yellowstone River valley headward erosion to erode the present day north-oriented Yellowstone River tributary valleys located in that valley today.

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.

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