A geomorphic history based on topographic map evidence

Abstract:

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 Powder River drainage basin. Detailed essays can be found under Powder River on this website’s sidebar category list. Detailed essays describing the Powder River drainage divide areas in southeast Montana and Wyoming’s Powder River basin have been published and detailed essays describing remaining Powder River drainage divide areas in the Bighorn Mountains and south of the Bighorn Mountains will be added as published. The Powder River originates in central Wyoming and flows in a northeast direction through northeast Wyoming and southeast Montana, and then turns to flow in a northwest direction to join the northeast-oriented Yellowstone River as a barbed tributary. Major Powder River tributaries originate in the high Bighorn Mountains and flow in a southeast direction before turning to flow in a northeast direction to join the northeast-oriented Powder River. Topographic map evidence illustrated in the detailed and in similar Missouri River drainage basin landform origins research project essays published on this website documents an immense southeast-oriented flood which crossed the entire Powder River drainage basin, including the present day high Bighorn Mountains. 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 ice sheet’s tremendous weight. Prior to Powder River drainage basin development ice-marginal southeast-oriented melt water floods flowed on a topographic surface now represented by some of 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-Powder River valley and tributary valleys captured the flood waters and diverted the flood flow in a northeast direction so as to flow into space the rapidly melting ice sheet had once occupied. The Bighorn Mountains emerged as flood waters eroded the Powder River drainage basin. Either deep flood erosion of surrounding regions and/or uplift of the Bighorn Mountains was responsible for emergence of the present day Bighorn Mountains.

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

Montana and Wyoming Powder River drainage basin location map

The South Fork Powder River originates in central Wyoming, west of Casper and south of the Bighorn Mountains, and flows in a northeast direction to join the southeast-oriented North Fork Powder River, which originates on the west side of the high Bighorn Mountains before flowing through a deep valley or water gap to cross the mountain core and to join the northeast-oriented South Fork in the Powder River Basin east of the Bighorn Mountains. The northeast-oriented Middle Fork Powder River is a North Fork Powder River tributary and also originates on the Bighorn Mountains west slope and flows in a deep valley eroded across the mountain core to reach the Powder River Basin east of the mountains. Buffalo Creek is an east-southeast and northeast oriented Middle Fork Powder River tributary, which also originates on the Bighorn Mountains west slope before flowing across the mountain core to reach the Middle Fork. Once the South and North Forks are combined the Powder River flows in a northeast, north, and northeast direction to reach the Montana border. Once in Montana the Powder River flows in a northeast direction before turning to flow in a north-northwest direction to join the northeast-oriented Yellowstone River as a barbed tributary (see figures 1 and 3). The northeast-oriented Yellowstone River joins the east- and southeast-oriented Missouri River with water eventually reaching the Gulf of Mexico. Other major Powder River tributaries shown in figure 2 below originate in the high Bighorn Mountains and include southeast and north-northeast oriented Crazy Woman Creek and east and northeast oriented Clear Creek. West of the Powder River drainage basin in Wyoming is the north-oriented Bighorn River drainage basin in the south and the north-northeast oriented Tongue River drainage basin in the north. Except at the Bighorn Mountains south end the Bighorn River-Powder River drainage divide is located in the high Bighorn Mountains, although for much its distance the drainage divide is west of highest Bighorn Mountains ridges and peaks. South of the Powder River drainage basin is the east-oriented North Platte River drainage basin, with the North Platte River flowing in a southeast direction from Casper, Wyoming. East of the Powder River drainage basin in Wyoming is the Cheyenne River drainage basin and the Belle Fourche River drainage basin, which is a Cheyenne River drainage basin component. In the Wyoming northeast corner the north-oriented Little Missouri River drainage basin is located east of Powder River drainage basin. The Platte River, Cheyenne River, and Little Missouri River all flow to the Missouri River. Powder River tributaries from the east are northwest oriented and are shorter than tributaries from the west.

Figure 2: Powder River drainage basin in northeast Wyoming. National Geographic Society map digitally presented using National Geographic Society TOPO software.

Figure 3 below illustrates the Powder River drainage basin in southeast Montana. The Powder River flows in a northeast direction from the Wyoming border to Broadus and Powderville, Montana before turning to join the northeast-oriented Yellowstone River upstream from Terry, Montana. North of the figure 3 map area the Yellowstone River joins the southeast-oriented Missouri River, with water eventually reaching the Gulf of Mexico. Essays describing Yellowstone River drainage basin drainage divides are found under Yellowstone River on this website’s sidebar category list. East of the Powder River drainage basin in southeast Montana is the north-northeast and north oriented Little Missouri River drainage basin. Note on figure 3 how Little Missouri River tributaries from the west are north- or northeast-oriented while there are almost no Little Missouri River tributaries shown from the east. North and east of the figure 3 map area the Little Missouri River joins the southeast-oriented Missouri River. Essays describing Little Missouri River drainage basin drainage divides are found under Little Missouri River drainage on the sidebar category list. West of the Montana Powder River drainage basin is the north-northeast oriented Tongue River drainage basin. Note how the Tongue River flows in a north-northeast direction from Decker, Montana (on the Montana-Wyoming border) almost to Miles City, Montana and then, like the Powder River, turns to flow in a northwest direction to join the northeast-oriented Yellowstone River as a barbed tributary. Essays describing Tongue River drainage basin drainage divides are found under Tongue River drainage basin on the sidebar category list.

• Note how Yellowstone River tributaries from the north and west are generally southeast-oriented and how in addition to the Powder River and Tongue River many streams south and east of the Yellowstone River have northwest-oriented segments. This northwest-southeast drainage alignment is much more obvious on more detailed maps and is illustrated in much greater detail in the Powder River drainage basin detailed essays. Each of the detailed essays includes nine or more detailed maps including at least eight topographic maps of specific Powder River drainage basin drainage divide areas. Detailed essays describe all southeast Montana Powder River drainage divide areas and all northeast Wyoming Powder River drainage divide areas located east of the Bighorn Mountain base. Detailed essays for Powder River drainage basin drainage divides in the Bighorn Mountains and south of the Bighorn Mountains have yet to be published, but will be added as they are ready. Topographic map evidence presented in the Missouri River drainage basin landform origins research project essays already published on this website strongly suggests immense southeast-oriented floods crossed the entire Powder River drainage basin, including areas located in what are today the high Bighorn Mountains

Figure 3: Powder River drainage basin in southeast Montana. National Geographic Society map digitally presented using National Geographic Society TOPO software.

Why would immense southeast-oriented floods be flowing across what are now the high Bighorn Mountains and how could those southeast-oriented flood waters flow across the high mountain elevations? The Powder 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 Powder 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 Powder 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 Powder 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. Powder 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-Powder 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-Powder 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 4: North Fork Powder River headwaters in high Bighorn Mountains. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

What evidence suggests immense southeast-oriented floods crossed what are today the high Bighorn Mountains? Detailed essays describing Bighorn Mountain drainage divides have not yet been written, but study of Bighorn Mountains topographic map evidence, such as that seen in figures 4 and 5, reveals Powder River tributaries originate on the Bighorn Mountains west slope and are linked by through valleys with west-oriented Bighorn River tributary valleys. Figure 4 above uses reduced size topographic maps to illustrate the North Fork Powder headwaters in the high Bighorn Mountains. The North Fork Powder River originates near Powder River Pass located near the highway in the figure 4 north center area. Note how Powder River Pass is a deep through valley linking the south-oriented North Fork Powder River valley with the east and northeast oriented North Fork Crazy Woman Creek valley. East of the figure 4 map area the North Fork Crazy Woman Creek turns to flow in a southeast direction to reach the Bighorn Mountains base where it joins the northeast-oriented South Fork Crazy Woman Creek to form northeast-oriented Crazy Woman Creek, which then joins the north-oriented Powder River. As can be seen in figures 4 and 2 the North Fork Powder River flows in a south and southeast direction and east of the Bighorn Mountains joins the northeast-oriented South Fork Powder River to form the north-oriented Powder River. Powder River Pass is major through valley linking what are today two diverging drainage routes. The through valley is more than 300 meters deep and is a water eroded feature and was eroded by large volumes of east-oriented water. Close study of the figure 4 map area reveals many other similar through valley linkages between present day Powder River tributary valleys and also between Powder River tributary valleys and west-oriented Bighorn River tributary valleys. Obviously at the time the through valleys were eroded the figure 4 topography did not look like it looks today, because today there is no source for large volumes of east-oriented water. In other words, at the time Powder River Pass and other figure 4 through valleys were eroded the Bighorn Mountains did not stand high above what is today the deep Bighorn Basin to the west, but instead large volumes of east-oriented water were able to flow from what is now the Bighorn Basin area across what are now the high Bighorn Mountains to what were then actively eroding Powder River tributary valleys. While it may be difficult to imagine based on the present day topography, the through valleys provide convincing evidence Bighorn Mountains were crossed by an immense southeast-oriented flood. And at the time flood waters crossed the Bighorn Mountains the deep Bighorn Basin to west did not exist, but was subsequently eroded by the immense southeast-oriented flood and/or lowered in elevation by crustal warping related to ice sheet weight and/or flood water erosion and deposition.

Through valleys linking west-oriented Bighorn River tributary valleys with east-oriented Powder River tributary valleys are located along the entire present day Bighorn River-Powder River drainage divide can be at different elevations. Figure 5 is located at the Bighorn Mountains south end and illustrates the drainage divide between north and northwest-oriented Deep Creek (and it tributaries) with the northeast-oriented Middle Fork Powder River and its tributaries. North and west of the figure 5 map area Deep Creek flows to north and northwest-oriented Nowood Creek, which is a Bighorn River tributary (see figure 2). North and east of figure 5 the Middle Fork Powder River joins the southeast oriented North Fork Powder River, which then joins northeast-oriented South Fork Powder River (see figure 2). Note how multiple west to east oriented through valleys link the northeast-oriented Middle Fork Powder River valley with the much deeper north-oriented Deep Creek valley. The through valleys are more than 100 meters deep and are water eroded features. The water was coming from the west, which means at the time the through valleys were eroded the Deep Creek valley did not exist and regions to west were at least as high as the present day Deep Creek-Middle Fork Powder River drainage divide. The multiple through valleys are evidence the water was moving in multiple channels, such as might be found in a flood formed anastomosing channel complex. In other words, the figure 5 map area alone provides evidence for a large east-oriented flood moving across the Bighorn Mountain south end. But, similar through valleys are located along the entire Bighorn River-Powder River drainage divide and the floors of these through valleys range in elevation from less than 1900 meters south of the Bighorn Mountains to more than 3500 meters in the highest Bighorn Mountains regions. The multiple through valleys provide evidence of multiple southeast and east-oriented flood flow channels. The differences in the through valley floor elevations provides evidence which can be used to determine how the Bighorn Mountains emerged as a high mountain range while flood waters were deeply eroding the surrounding region. Emergence of the present day high Bighorn Mountains was probably caused by a combination of factors including deep flood water erosion of easily eroded sedimentary rocks in which the Bighorn Mountains had been buried and also delayed crustal warping related to the immense ice weight. The ice sheet weight caused crustal down warping under the ice sheet, but probably also caused crustal up warping else on the continent. Aiding the crustal up warping process was the removal of large quantities of overburden by the deep flood water erosion and perhaps the deposition of sediments in adjacent basin areas.

Figure 6: Reduced size topographic maps illustrating northwest-southeast alignment of tributaries near confluence of northeast-oriented Clear Creek with north-northeast oriented Powder River. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 6 uses reduced size topographic maps to illustrate evidence for immense southeast-oriented floods in the Powder River Basin area east of the Bighorn Mountains. The north-northeast oriented river flowing across the figure 6 center area is the Powder River and the major northeast-oriented tributary is Clear Creek, which begins in the high Bighorn Mountains (see figure 2 to see where figure 6 is located). Northwest-oriented streams in the figure 6 northwest corner are Tongue River tributaries. Note how with the exception of Clear Creek and one northeast-oriented Clear Creek tributary all Powder River tributaries from the west and north are southeast-oriented and enter the north-northeast oriented Powder River as barbed tributaries. Also note how all Powder River tributaries from the south and east are northwest-oriented. This northwest-southeast orientation of tributary valleys is evidence the present day Powder River valley eroded headward across a massive southeast-oriented flood. Flood waters were flowing on topographic surface at least as high as the highest figure 6 elevations today and were probably flowing in what was then a southeast-oriented anastomosing channel complex. Headward erosion of the deep north-northeast oriented Powder River valley across this southeast-oriented anastomosing channel complex captured the flood waters and diverted the flood flow in a north-northeast direction to the deep “hole” the melting ice sheet had once occupied. Powder River valley headward erosion beheaded the flood flow channels one at a time in sequence from the northeast to the southwest. Flood waters on northwest ends of newly beheaded flood flow channels reversed flow direction to erode what are today northwest-oriented tributary valleys. Because the flood flow channels were beheaded one at a time and also because the flood flow channels were anastomosing (or interconnected) reversed flood flow in a newly beheaded channel could capture southeast-oriented flood flow from channels further to the southwest. Such captures of flood water from yet to be beheaded flood flow channels provided the water volumes needed to erode significant northwest-oriented tributary valleys. Flood flow to the newly eroded Powder River valley was subsequently beheaded and reversed by headward erosion of the deep north-northeast oriented Tongue River valley, which is located northwest of the figure 6 map area. Further evidence of former southeast-oriented flood flow channels is found in shallow through valleys, which today link valleys of northwest-oriented Tongue River tributaries with southeast-oriented Powder River tributary valleys.

• Detailed essays presently available for the Powder River, Yellowstone River, Tongue River, and Bighorn River drainage basins use several hundred topographic maps to illustrate and describe evidence the deep Yellowstone River-Powder River valley (and Powder River tributary valleys) eroded headward from the Yellowstone River-Powder River confluence region across the Montana and Wyoming Powder River Basin and systematically captured southeast-oriented flood waters and diverted the flood flow into the deep “hole” the rapidly melting ice sheet had previously occupied.

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.