Marias River-Pondera Coulee drainage divide area landform origins, Pondera County, Montana, USA

· Marias River, Montana
Authors

Abstract:

Topographic map interpretation methods are used to determine landform origins in the Marias River-Pondera Coulee drainage divide area located in Pondera County, Montana. The Marias River flows in an east-southeast and east direction along the north edge of the drainage divide area while Pondera Coulee is an east and east-northeast oriented Marias River tributary located along the drainage divide south edge. Topographic map evidence indicates all present day drainage route valleys eroded headward across the Pondera County region during massive south and southeast oriented floods, which flowed across the entire region. North-south oriented through valleys cross the Marias River-Pondera Coulee drainage divide and provide evidence of multiple south-oriented flood flow routes prior to headward erosion of the Marias River valley. These north-south oriented through valleys provide evidence headward erosion of the Pondera Coulee valley occurred prior to headward erosion of the Marias River valley. Other through valleys cross drainage divides between Marias River and Pondera Coulee tributary valleys located in the Marias River-Pondera Coulee drainage divide area and provide evidence the tributary valleys were eroded in sequence from the south to the north and from the east to the west. Flood waters are interpreted to have been derived from a rapidly melting thick North American ice sheet located in a deep “hole” and were flowing between the rising Rocky Mountain front to the west and the decaying ice sheet’s southwest margin. Flood waters initially flowed on a surface at least as high as the highest elevations in the Marias River-Pondera Coulee drainage divide area and deeply eroded the drainage divide region prior to headward erosion of the deep Marias River valley and subsequently as reversed flow on beheaded flood flow routes moved to the newly eroded and deep Marias River valley.

Preface

The following interpretation of detailed topographic map evidence is one of a series of essays describing similar evidence for all major drainage divides contained within the Missouri River drainage basin and for all major drainage divides with adjacent drainage basins. The research project is interpreting evidence in the context of a previously unexplored deep glacial erosion paradigm, which is fundamentally different from most commonly accepted North American glacial history interpretations. Project essays available at this site may be found by selecting desired Missouri River tributaries and/or states from this essay’s sidebar category list.

Introduction

  • The purpose of this essay is to use topographic map interpretation methods to explore the Marias River-Pondera Coulee drainage divide area landform origins in Pondera County, Montana, USA. Map interpretation methods can be used to unravel many geomorphic events leading up to formation of present-day drainage routes and development of other landform features. While each detailed topographic map feature provides detailed evidence to be explained, the solution must be consistent with explanations for adjacent area map evidence as well as solutions to big picture map evidence puzzles. I invite readers to improve upon my solutions and/or to propose alternate solutions that better explain evidence and are also consistent with adjacent map area and big picture evidence. Readers may do so either by making comments here or by writing and publishing their own essays and then by providing a link to those essays in a comment here.
  • This essay is also exploring a new geomorphology paradigm in which erosional landforms are interpreted as evidence left by immense glacial melt water floods. Implied in that interpretation is the immense floods were derived from a thick North American ice sheet that created a deep “hole” in the North American continent and also melted fast. The previously unexplored paradigm being tested in this and other essays in the Missouri River drainage basin landform origins research project is a thick North American ice sheet, comparable in thickness to the Antarctic ice sheet, occupied the North American region usually recognized to have been glaciated, and through its weight and erosive actions created a deep North American “hole”. The southwestern rim of that deep “hole” is today preserved in the high Rocky Mountains. The ice sheet through its weight and deep erosion (and perhaps deposition along major south-oriented melt water flow routes) caused significant crustal warping and tectonic change, through its action of melting fast produced immense floods that flowed across the continent, and through its action of melting fast systematically opened up space in the ice sheet created “hole” so headward erosion of newly developed north-oriented drainage systems captured immense south-oriented melt water floods and diverted immense melt water floods north into space the ice sheet had once occupied.
  • If this previously unexplored paradigm is correct the geographic region explored by this essay should contain evidence of immense floods that were captured by headward erosion of new valley systems so as to cause the floods to flow in a different direction. Ability of this previously unexplored paradigm to explain Marias River-Pondera Coulee drainage divide area landform evidence in Pondera County, Montana will be regarded as evidence supporting the “thick ice sheet that melted fast” paradigm (see paradigm related essay link in menu at top of page). This essay is included in the Missouri River drainage basin landform origins research project essay collection.

Marias River-Pondera Coulee drainage divide area location map

Figure 1: Marias River-Pondera Coulee drainage divide area location map (select and click on maps to enlarge). National Geographic Society map digitally presented using National Geographic Society TOPO software.

 

  • Figure 1 provides a location map for the Marias River-Pondera Coulee drainage divide area in Pondera County, Montana and illustrates a region of north central Montana with Alberta and the Saskatchewan southwest corner north of the United States-Canada border. The Waterton-Glacier International Peace Park is located along the international border and straddles the figure 1 west edge. North-northwest to south-southeast oriented Rocky Mountain ranges extend from the figure 1 northwest corner area through the International Peace Park to the figure 1 south edge. East of the Rocky Mountain front are the Montana plains, although isolated Rocky Mountain outlier ranges such as the Bears Paw Mountains (straddling the figure 1 east edge) rise above the surrounding plains. Drainage west of the Rocky Mountain front is to the Flathead River with water eventually flowing to the Columbia River and the Pacific Ocean. East of the Rocky Mountain front on the Montana plains drainage is to the Missouri River with water eventually reaching the Gulf of Mexico. The Milk River which flows from the east edge of Glacier National Park (in Montana) into Alberta and then across southern Alberta before turning to flow to Havre, Montana is a Missouri River tributary. Drainage routes in the figure 1 northwest corner and near the figure 1 north edge flow to the Saskatchewan River with water eventually reaching Hudson Bay. The Missouri River flows in a northeast direction from the figure 1 south center edge to Great Falls, Fort Benton, and Loma before turning to flow in a south-southeast and east-northeast direction around the Bears Paw Mountains to the figure 1 east edge (south half). The Marias River is an east-southeast and south oriented Missouri River tributary joining the Missouri River near Loma and is formed at the confluence of Cut Bank Creek and Two Medicine River a short distance south of Cut Bank, Montana. Lake Elwell (also known as Tiber Reservoir) is a large reservoir flooding the Marias River valley and is impounded behind Tiber Dam. Cut Bank Creek, Two Medicine River, and Birch Creek are Marias River tributaries originating along the Rocky Mountain front. Pondera Coulee is a Marias River tributary originating near Pendroy, Montana and joining the Marias River a short distance downstream from Tiber Dam. The Marias River-Pondera Coulee drainage divide area in Pondera County is located between the Marias River and Pondera Coulee and is east of a north-south line from Cut Bank to Valier, Montana. The Marias River-Teton River drainage divide area landform origins, Chouteau County, Montana essay describes landforms east of the study region while Milk River-Marias River drainage divide area landform origins, Toole County, Montana essay describes regions to the north. These and other regional drainage divide area essays can be found using the sidebar category list.
  • The Marias River-Pondera Coulee drainage divide area was eroded by massive south-oriented melt water floods from a rapidly melting thick North American ice sheet, which had been located in a deep “hole.” The deep “hole” had been formed by deep glacial erosion and by crustal warping caused by the ice sheet’s great weight. The ice sheet had been large, probably comparable in extent and thickness to the present day Antarctic Ice Sheet, and at one time had stood high above the surrounding continental surface. At that time the Rocky Mountains and the Rocky Mountain outliers (such as the Bears Paw Mountains) did not stand high, although they were gradually uplifted as the deep “hole” the ice sheet occupied was being formed. Today the upper Missouri River drainage basin in Montana and northern Wyoming and the Saskatchewan River drainage basin in southwest Alberta represent the deep “hole’s” deeply eroded southwest wall while the crests of the Montana and Alberta Rocky Mountain ranges are remnants of the deep “hole’s” southwest rim. Immense ice-marginal melt water floods flowed in south and southeast directions along what are today crests of Rocky Mountain ranges from Alberta across western Montana and southward along routes paralleling the present day east-west continental divide. These giant melt water rivers were systematically dismembered as Rocky Mountain uplift, probably caused by the ice sheet’s great weight and perhaps by deep melt water flood erosion of what are today high mountain regions, proceeded from the south to the north and northwest. Rocky Mountain uplift diverted the massive southeast and south oriented flood waters both to the east and west and the flood waters eroded what are today major southeast-oriented river valleys (e.g. Rio Grande and Arkansas River valleys) and southwest-oriented river valleys (e.g. Colorado River valley). Eventually the combination of Rocky Mountain uplift in northern Wyoming and Montana and of ice sheet melting created a situation where the ice sheet surface no longer stood high above the surrounding bedrock surface and the giant ice-marginal floods flowing on that bedrock surface. About that time large supra glacial melt water rivers began to carve giant ice-walled canyons into the decaying ice sheet surface and these rivers became major regional drainage routes. Huge east- and northeast-oriented valleys eroded headward from these ice-walled canyons to capture the immense ice-marginal melt water floods and to divert the flood waters onto the ice sheet surface.
  • Of particular significance to the upper Missouri River drainage basin in Montana and northern Wyoming was a giant southeast and south oriented ice-walled canyon in Saskatchewan, North Dakota, and South Dakota, which eventually became a giant ice-walled and bedrock-floored canyon that detached the ice sheet’s southwest margin. Today the northeast and east facing Missouri Escarpment in Saskatchewan, North Dakota, and South Dakota is what remains of this giant canyon’s southwest and west wall. The Missouri River valley and its tributary valleys in Montana eroded headward from this giant ice-walled and bedrock-floored canyon (from the North Dakota northwest corner region) to capture both south and southeast oriented  melt water flood waters moving east of the rising Rocky Mountain front and also to capture ice-marginal melt water floods trapped between rising Rocky Mountain ranges and flowing in south-southeast directions from Canada into southwest Montana where further south and southeast flow was blocked by rising Rocky Mountains. These trapped flood waters were then forced to flow in north and northeast directions towards the lower elevations being opened up in the deep “hole” as the decaying ice sheet melted.  East-oriented Missouri River tributaries in the figure 1 map area eroded headward from the Missouri River valley in sequence to capture south and southeast oriented melt water flood waters with the southern tributary valleys being eroded first. For example the east-oriented Teton River valley eroded headward to capture south and southeast oriented flood flow prior to headward erosion of the east-oriented Marias River valley, which captured the south and southeast oriented flood flow to the newly eroded Teton River valley. Headward erosion of the Milk River valley subsequently captured south and southeast oriented flood flow to the newly eroded Marias River valley and headward erosion of Saskatchewan River tributary valleys in Alberta next captured south and southeast oriented flood flow to the newly eroded Milk River valley. Probably these valleys were deep as they eroded headward into the figure 1 map region and the south- and southeast-oriented floods flowing into the newly eroded valleys significantly lowered the landscape to the north. How much erosion took place is difficult to measure, although hundreds of meters of bedrock material may have been stripped from the region.

Detailed location map for Marias River-Pondera Coulee drainage divide area

Figure 2: Detailed location map for Marias River-Pondera Coulee drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

 

  • Figure 2 is a detailed location map for the Pondera County, Montana Marias River-Pondera Coulee drainage divide area. County boundaries are shown and county names are given. Glacier is the county located in the figure 2 northwest region. Much of Glacier County is occupied by the Blackfeet Indian Reservation. Teton County is south of Pondera County and the green area in the figure 2 southwest corner region is the Lewis and Clark National Forest. The western boundaries of Glacier, Pondera, and Teton Counties is the east-west continental divide, which except for mountain pass areas is located along high mountain crests. A short segment of the east-west continental divide can be seen in the figure 2 southwest corner. The northeast oriented South Fork Milk River is seen in the figure 2 northwest corner. Just to the south Cut Bank Creek flows in an east-northeast and the south-southeast direction to the town of Cut Bank and then to join the northeast- and east-oriented Two Medicine River (which joins Cut Bank Creek near the Glacier County southeast corner). Birch Creek is a northeast, east-southeast, and northeast oriented Two Medicine River tributary serving as the Blackfeet Indian Reservation southern boundary. The combined flow of Cut Bank Creek and Two Medicine River forms the Marias River, which flows along or near the Pondera County north border and into southeast Toole County and then to Lake Elwell (or Tiber Reservoir). East of the Cut Bank Creek-Two Medicine River confluence the named Marias River tributaries from the south are east- and north-oriented Bullhead Creek, northeast-oriented Dry Fork Marias River (which makes an east-southeast jog), and Pondera Coulee, which drains in an east direction from west of Conrad to the Pondera County east border and then in a northeast direction to join the Marias River downstream from Tiber Dam. Spring Creek is a northeast-oriented Dry Fork Marias River tributary located between the Dry Fork Marias River and Pondera Coulee in the region west of Conrad. Dupuyer Creek is a northeast-oriented Birch Creek tributary located between northeast-oriented Birch Creek and the northeast-oriented Dry Fork Marias River west of Lake Frances. Several of the unnamed drainage routes north of Lake Frances are irrigation canals and do not reflect natural drainage routes. The hypothesis presented in this essay is the natural valleys eroded headward across the region in sequence from south to north and from southeast to northwest to capture immense south- and southeast-oriented melt water flood flow which was moving across the region. Probably the large melt water floods were flowing between what was then the decaying ice sheet southwest margin to the northeast and the rising Rocky Mountain front which can be seen in the figure 2 southwest corner. In other words the Pondera Coulee valley eroded headward across the region prior to headward erosion of the Marias River-Dry Fork Marias River valley, which eroded headward across the region prior to headward erosion of the Marias River-Two Medicine River-Birch Creek valley.

Dead Indian Coulee drainage divide area

Figure 3: Dead Indian Coulee drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

 

  • Figure 3 illustrates the Dead Indian Coulee drainage divide area south of Lake Elwell. The flooded Marias River valley can be seen along the figure 3 north edge with the Marias River flowing in an east direction (the valley is flooded by Lake Elwell or Tiber Reservoir). Pondera Coulee drains in a northeast direction from the figure 3 south edge (east of center) to the east edge (north of center) and joins the Marias River east of the figure 3 map area. Timber Coulee is an east-southeast, east-northeast, and southeast oriented tributary joining Pondera Coulee near the figure 3 east edge. Note how Pondera Coulee has several unnamed southeast-oriented tributaries in the figure 3 southeast quadrant. Dead Indian Coulee is a north-south oriented through valley the north end of which drains to the Marias River while the south end drains to Pondera Coulee and is located just west of the figure 3 center. The drainage divide between the north- and south-oriented drainage is located between the words “Dead” and “Indian”. The south-oriented Dead Indian Coulee drainage route extends in a south and south-southeast direction almost to the figure 3 south center edge and then turns to drain in a meandering east direction to northeast-oriented Pondera Coulee. Note the southeast-oriented Dead Indian Coulee tributary in the Toole County southeast corner. The north-oriented Dead Indian Coulee drainage has eroded a deep north-oriented, but meandering tributary valley into the Marias River valley south wall. Note northwest-oriented Dead Indian Coulee valley segments and tributaries near the Marias River. The figure 3 map contour interval is 20 meters and the Dead Indian Coulee through valley is defined by two contour lines near the drainage divide. The elevation at the drainage divide is between 1020 and 1040 meters and elevations east of the through valley rise to more than 1060 meters while elevations immediately to the west of the through valley rise to more than 1080 meters. The through valley is evidence of a south-oriented flood flow channel to the newly eroded Pondera Coulee valley at a time when the deep east-oriented Marias River valley did not exist. Headward erosion of the deep east-oriented Marias River valley beheaded the south-oriented flood flow channel and flood waters on the north end of the beheaded flood flow channel reversed flow direction to erode the north-oriented Dead Indian Coulee valley. The multiple southeast-oriented Pondera Coulee tributary valleys were probably eroded by southeast-oriented flood flow routes to the newly eroded Pondera Coulee valley. The northwest-oriented Dead Indian Coulee valley segments and tributary valleys were probably eroded by reversals of flood flow on beheaded southeast-oriented flood flow routes. Other shallow through valleys can be seen, but are better seen on more detailed maps.

Detailed map of Dead Indian Coulee drainage divide area

Figure 4: Detailed map of Dead Indian Coulee drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

 

  • Figure 4 provides a detailed topographic map of the Dead Indian Coulee drainage divide area seen in less detail in figure 3 above. The drainage divide between the north-oriented Dead Indian Coulee and the south-oriented Dead Indian Coulee is located in section 15 just north of the figure 4 center. The figure 4 map contour interval is 20 feet and the through valley floor elevation at the drainage divide is between 3380 and 3400 feet. Elevations in the section 23 northwest corner (east of the through valley) rise to more than 3480 feet while elevations west of the through valley rise to more than 3500 feet. In other words the through valley is between 80 and 120 feet deep based on the adjacent valley walls. There appears to be a low south-facing escarpment extending across sections 20, 21, 22, and 23 and then a much more gentle southward slope extends to Pondera Coulee south of the figure 4 map area (elevations along the Pondera Coulee valley rim are in the 3300 foot range). South of Pondera Coulee there is a gentle north-facing slope with elevations rising to about 3450 feet before another decrease towards the east-oriented Teton River valley. The south-facing escarpment seen in figure 4 may be the north wall of what was a broad east-oriented flood flow channel following the Pondera Coulee alignment. The Dead Indian Coulee through valley was eroded across that south-facing escarpment, which suggests it was eroded by south-oriented flood flow moving to the Pondera Coulee valley at a time when east-oriented flood flow was no longer carving the south-facing escarpment (or north valley wall), but were confined to the narrower Pondera Coulee valley south of the figure 4 map area. What is important to remember is the south-oriented flood flow that eroded the Dead Indian Coulee through valley was coming from north of the present day Marias River valley (north of the figure 4 map area). In other words the deep east-oriented Marias River valley did not exist at the time the south-oriented Dead Indian Coulee through valley was eroded. Probably the deep Marias River valley was eroding headward toward the south-oriented Dead Indian Coulee flood flow channel and captured the south-oriented flood flow almost as soon as the Dead Indian Coulee through valley was eroded. Flood waters on the north end of the beheaded Dead Indian Coulee flood flow channel then reversed flow direction to erode the north-oriented Dead Indian Coulee valley.

Dry Fork Marias River-Pondera Coulee drainage divide area

Figure 5: Dry Fork Marias-Pondera Coulee drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

 

  • Figure 5 illustrates the Dry Fork Marias River-Pondera Coulee drainage divide area west and south of the figure 3 map area. Conrad is the town located in the figure 5 southwest corner and Ledger is the smaller town located west of the figure 5 north center area. Ledger is located in the valley of the northeast-oriented Dry Fork Marias River valley, which extends from the figure 5 west edge (south of center) to the figure 5 north center edge. North of the figure 5 map area the Dry Fork Marias River flows in a north-northeast direction to join the east-southeast oriented Marias River. The south and south-southeast oriented stream joining the Dry Fork Marias River near Ledger is Little Flat Coulee which is linked by a through valley (north of figure 5) to a northwest and north oriented Marias River tributary valley. Pondera Coulee drains in a northeast, east, and east-southeast direction from the figure 5 southwest corner to the figure 5 south edge (near southeast corner) and eventually drains to the Marias River downstream from Tiber Dam. Fowler Coulee is an east-oriented stream draining to the figure 5 east edge (south of center) and east of figure 5 drains to southeast-oriented Powder Coulee, which drains to Pondera Coulee. Note how Favot Coulee is an east-southeast oriented through valley linking the northeast-oriented Dry Fork Marias River valley with the Pondera Coulee valley and also with the Fowler Coulee valley. Also note how in the region north and east of Conrad there are isolated low hills separating broad lowlands or through valleys linking the Dry Fork Marias River valley with the Pondera Coulee valley. The through valleys seen in figure 5 are generally defined by a single 20-meter contour line, but they define an east- and/or southeast-oriented anastomosing channel complex, which was eroded by converging and diverging flood flow channels to what was at that time the actively eroding Pondera Coulee valley. At that time the northeast and north-northeast oriented Dry Fork Marias River valley did not exist nor did the Marias River valley to the north exist and flood waters could freely moving across the region to the actively eroding Pondera Coulee valley. Headward erosion of the east-southeast oriented Marias River valley north of the figure 5 map area beheaded and reversed south-oriented flood flow routes which then eroded the northeast and north-northeast Dry Fork Marias River valley. Continued headward erosion of the east-southeast oriented Marias River valley north of the figure 5 map then beheaded and reversed south and south-southeast oriented flood flow routes to the newly eroded Dry Fork Marias River valley (e.g. Little Flat Coulee flood flow channel) to erode north-oriented Marias River tributary valleys.

Detailed map of Dry Fork Marias River-Pondera Coulee drainage divide area

Figure 6: Detailed map of Dry Fork Marias River-Pondera Coulee drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

 

  • Figure 6 is a detailed topographic map of the Dry Fork Marias River-Pondera Coulee drainage divide area seen in less detail in figure 5 above. The Dry Fork Marias River flows in an east and north direction in the figure 6 northwest corner. Pondera Coulee is located south of the figure 6 map area and drains in an east direction. Favot Coulee is a through valley linking the Dry Fork Marias River valley with the Pondera Coulee valley and extends in an east-southeast direction across the figure 6 center area with a turn to drain in a south direction near the figure 6 southeast corner. Note how the northwest end of Favot Coulee drains in a northwest direction to the Dry Fork Marias River and the southeast end drains in an east-southeast and south direction to Pondera Coulee. The figure 6 map contour interval is ten feet and the through valley floor elevation at the drainage divide is between 3380 and 3390 feet. Elevations north and east of through valley rise to more than 3450 feet while elevations south and west of the through valley rise even higher (to 3521 feet). The through valley is at least 60 feet deep and is a former south-southeast oriented flood flow channel. What is interesting about the through valley is that in section 3 (near figure 6 east edge) Favot Coulee splits into two diverging valleys with one valley draining in a south direction to Pondera Coulee while the other valley continues in an east direction and is linked by another through valley with the east- and southeast-oriented Fowler Coulee (and Powder Coulee) valley, which eventually drains to the Pondera Coulee valley. This Favot Coulee-Fowler Coulee through valley (not seen in figure 6) has a valley floor elevation at the drainage divide of between 3420 and 3430 feet with elevations both north and south rising to more than 3500 feet. This east-oriented through valley is at least 70 feet deep and is evidence of what was once a diverging flood flow channel in an east-southeast or southeast oriented anastomosing channel complex. This anastomosing channel complex was moving large volumes of flood waters to what was then the actively eroding Pondera Coulee valley prior to headward erosion of the Dry Fork Marias River valley. Headward erosion of the northeast-oriented Dry Fork Marias River valley captured the flood flow and diverted the flood waters to the newly eroded Marias River valley, which is located north of the figure 6 map area.

Schultz Coulee-Big Flat Coulee drainage divide area

Figure 7: Schultz Coulee-Big Flat Coulee drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

 

  • Figure 7 illustrates the Schultz Coulee-Big Flat Coulee drainage divide area west of the figure 5 map area and includes overlap areas with figure 5. Conrad is located near the figure 7 south edge in the southeast quadrant. Pondera Coulee is the northeast oriented stream in the figure 7 southeast corner area (south and east of Conrad). Williams is a small town in the figure 7 northwest quadrant. The Dry Fork Marias River flows in a northeast direction from the figure 7 west edge (south of center) and then south of Williams turns to flow in an east-southeast and east direction to the figure 7 east edge (south half). As seen in figure 5 east of the figure 7 map area the Dry Fork Marias River flows in a northeast and north-northeast direction to join the Marias River. Spring Creek is a Dry Fork Marias River tributary flowing in a northeast and east direction from the figure 7 southwest corner to join the Dry Fork Marias River near the McLean State Game Preserve. Note how modern-day irrigation canals link the Spring Creek valley with the Pondera Coulee valley in the figure 7 southeast quadrant. Schultz Coulee originates just south of the figure 7 north center edge and drains in a northeast and north direction to join the Marias River north of the figure 7 map area. Big Flat Coulee is a southeast, northeast, south, and south-southeast oriented Dry Fork Marias River tributary originating just south of the Schultz Coulee headwaters area and is linked to the north-oriented Schultz Coulee valley by a well-defined through valley. Note also how the northeast-oriented Dry Fork Marias River valley south of Williams is also linked by a through valley (now used by irrigation canals) with the northeast- and north-oriented Schultz Coulee valley. The through valleys in the figure 7 map area provide further evidence of a large-scale anastomosing channel complex which first moved large volumes of flood water to the actively eroding Pondera Coulee valley and subsequently was dismembered as headward erosion of the deep Marias River valley north of the figure 7 map area beheaded and reversed south- and southeast-oriented flood flow routes with flood waters on north ends of beheaded flood flow routes reversing flow direction to first erode the northeast and north-northeast oriented Dry Fork Marias River valley (east of figure 7) and then the north-oriented Schultz Coulee valley. Dismemberment of the anastomosing channel complex was much more complicated than described here, but the important thing to remember is the through valleys provide evidence the anastomosing channel complex once existed.

Detailed map of Schultz Coulee-Big Flat Coulee drainage divide area

Figure 8: Detailed map of Schultz Coulee-Big Flat Coulee drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

 

  • Figure 8 illustrates a detailed topographic map of the Schultz Coulee-Big Flat Coulee drainage divide area seen in less detail in figure 7 above. The Dry Fork Marias River flows in an east direction near the south edge of the figure 8 southwest quadrant. Schultz Coulee originates in the figure 8 northeast quadrant and drains in an east and east-northeast direction to the figure 8 northeast corner region. Big Flat Coulee originates in section 8 (south of Schultz Coulee) and drains in a southeast direction to figure 8 east center edge. East and south of figure 8 Big Flat Coulee drains to the Dry Fork Marias River. Note the through valley linking the north-oriented Schultz Coulee valley with the southeast-oriented Big Flat Coulee valley. The figure 8 map contour interval is ten feet and the spot elevation on the through valley floor reads 3561 feet. Elevations east of the through valley rise to more than 3720 feet (3740 feet just east of figure 8) and elevations west of the through valley rise even higher (3777 feet near north edge of section 13). The through valley is at least 160 feet deep and provides evidence of a continuous south-oriented flood flow channel to what was at that time the actively eroding Dry Fork Marias River valley. Note how in section 8 there is a streamlined erosional residual on the through valley floor with a second narrower northwest-southeast oriented through valley paralleling the larger north-south oriented through valley. This erosional residual provides further evidence of the flood origin of the two adjacent through valleys. Perhaps even more interesting than the Schultz Coulee -Big Flat Coulee through valley is the large southeast-oriented escarpment-surrounded basin in section 10 (in the figure 8 northwest quadrant). A northwest-southeast oriented through valley crosses the hill in the figure 8 northwest corner and provides evidence of a southeast-oriented flood flow channel to the southeast-oriented basin. The basin may be an abandoned flood eroded headcut which was retreating across the figure 8 map area. If so flood waters apparently were moving both eastward to the south-oriented Schultz Coulee flood flow channel and also southward to what was probably the actively eroding Dry Fork Marias River valley. Headward erosion of the Bullhead Creek valley and Birch Creek valley (seen in figure 10) from the actively eroding Marias River valley subsequently beheaded the southeast-oriented flood flow to the actively retreating headcut and the headcut has since remained almost unchanged.

Dry Fork Marias River-Spring Creek drainage divide area

Figure 9: Dry Fork Marias River-Spring Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

 

  • Figure 9 illustrates the Dry Fork Marias River-Spring Creek drainage divide area west and slightly south of the figure 7 map area and includes overlap areas with figure 7. Dupuyer Creek flows in a north-northeast direction in the figure 9 northwest corner and north of figure 9 flows to Birch Creek, which then joins the Two Medicine River, which joins Cut Bank Creek to form the Marias River. The Dry Fork Marias River flows from just north of the figure 9 southwest corner to the figure 9 northeast quadrant and then turns to flow in an east-southeast direction to the figure 9 east edge (south of northeast corner). Spring Creek flows in a northeast direction from the figure 9 south edge (west of center) and near the figure 9 east edge turns to flow in an east direction and joins the Dry Fork Marias River east of the figure 9 map area. Pondera Coulee is labeled and drains in a northeast, southeast, and east-northeast direction in the figure 9 southeast quadrant. Note how Pondera Coulee drains in a north-northeast and then south-southeast direction at the south end of a north-south oriented through valley at the west end of Benton Bench. Note also the north-oriented Pondera Coulee tributary south of the word “Pondera”. While not seen in figure 9 that north-oriented tributary drains the north end of a north-south oriented through valley to southeast-oriented Teton River tributaries. The through valley west of Benton Bench was eroded as a south-oriented flood flow channel which diverged into two separate south-oriented at the point where the north-northeast and south-southeast oriented Pondera Coulee segments meet (the elbow of capture). Flood waters at that time were flowing to what were then actively eroding Teton River tributary valleys. Headward erosion of the Pondera Coulee valley captured the eastern south-oriented flood flow channel and the deeper Pondera Coulee valley eroded headward to the point of divergence. At the point of the divergence the deeper Pondera Coulee valley beheaded the western south-oriented flood flow channel and caused a reversal of flood flow on the north end of the beheaded flood flow route. The flood flow reversal eroded the north-northeast oriented Pondera Coulee valley segment and also the north-oriented Pondera Coulee tributary valley. At the same time the northeast-oriented Pondera Coulee valley continued to erode headward as it captured southeast-oriented flood flow routes further to the west. The best evidence for the multiple southeast-oriented flood flow routes is seen along the Dry Fork Marias River-Spring Creek drainage divide. Note how in the area north of Castle Rock and the Black Buttes area to the west there are multiple northwest-southeast oriented through valleys eroded across the drainage divide. The map contour interval is 20 meters and some of the through valleys are defined by three or four contour lines on a side. These through valley were eroded by southeast-oriented flood flow moving to what was then the actively eroding Spring Creek valley at a time prior to headward erosion of the Dry Fork Marias River valley to the northwest. Subsequent headward erosion of the Dry Fork Marias River valley captured the southeast-oriented flood flow and ended flood flow across the drainage divide to the newly eroded Spring Creek valley. .

Birch Creek-Bullhead Creek drainage divide area

Figure 10: Birch Creek-Bullhead Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

 

  • Figure 10 illustrates the Birch Creek-Bullhead Creek drainage divide area north and east of the figure 9 map area and north and slightly west of the figure 7 map area and includes overlap areas with figure 7. Valier, Montana is the town located in the figure 10 southwest quadrant. Birch Creek flows in an east direction from the figure 10 west edge (west of Valier) and then is joined by north-northeast oriented Dupuyer Creek. From its confluence with Dupuyer Creek Birch Creek flows in a north-northwest and north-northeast direction to join the Two Medicine River near the figure 10 north edge. The Two Medicine River after being joined by Birch Creek flows in a northeast direction to the figure 10 north edge and joins Cut Bank Creek north of the figure 10 map area to form the east-southeast oriented Marias River, which flows from the figure 10 north center edge to the figure 10 east edge (north half). The north-oriented tributary joining the Marias River near the Pondera-Toole County line (near figure 10 northeast corner) is Schultz Coulee, which originates near the figure 10 south edge  just north of southeast oriented Big Flat Coulee as seen in figures 7 and 8. Bullhead Creek is an east-northeast, east-southeast, and east oriented Schultz Coulee tributary draining the figure 10 center region. Note how Bullhead Creek has eroded a deep east-oriented valley between Abbott Ridge to the north and Trunk Butte to the south. The figure 10 map contour interval is 20 meters and the through valley floor (between north-northeast oriented Birch Creek and the east-oriented Bullhead Valley) has an elevation of between 1100 and 1120 meters. A spot elevation on Abbott Ridge read 1162 meters while Telephone Hill to the south is shown as reaching an elevation of 1223 meters. The Bullhead Valley is then a large east-oriented through valley, which was eroded by east-oriented flood flow prior to headward erosion of the deep north-oriented Schultz Coulee valley. Flood waters proceeded east of the figure 10 map area to what is now the north-northeast oriented Hilger Coulee valley, which drains to the Marias River. Probably before headward erosion of the Hilger Coulee valley flood waters proceeded further east to an east-oriented Marias River valley segment at the west end of Tiber Reservoir (Lake Elwell). The sequence of events recorded here is related to headward erosion of the deep Marias River valley across the region. As the deep Marias River valley eroded westward it first beheaded and reversed flood flow in the Hilger Coulee area to erode the north-northeast oriented Hilger Coulee valley, which captured the east-oriented flood flow. Next headward erosion of the deep Marias River valley beheaded and reversed flood flow in the Schultz Coulee channel, which resulted in headward erosion of the north-oriented Schultz Coulee valley. Later headward erosion of the deep Marias River valley beheaded and reversed flood flow in the Birch Creek-Two Medicine River valley segments, which resulted in headward erosion of the north-northeast oriented Birch Creek-Two Medicine River valley segments, and which captured the east-oriented flood flow that had been eroding the east-oriented Bullhead Valley.

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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