Origin of Mountain Passes across Continental Divide Segments Surrounding the Southwest Montana Big Hole and Beaverhead River Drainage Basins, USA

The evolution of southwest Montana’s Big Hole and Beaverhead River drainage basins is determined from topographic map evidence related to mountain passes crossing what are today high altitude drainage divides including North America’s east-west Continental Divide. Map evidence, such as orientations of valleys leading away from mountain passes (and saddles) and barbed tributaries found along the downstream drainage routes, is used to reconstruct flow directions of streams and rivers that once crossed the present-day high mountain divides. Large south-oriented anastomosing complexes of diverging and converging channels are interpreted to have eroded what are today closely spaced passes and saddles now notched into high mountain ridges. Water in those south-oriented channels is interpreted to have flowed across emerging mountains and subsiding basins. Headward erosion of deeper southeast-oriented valleys, assisted by crustal warping, concentrated south-oriented water in fewer and deeper valleys as the water flowed from southwest Montana into what are today Idaho and the Snake River drainage basin. Headward erosion of the Big Hole River valley between the emerging Anaconda and Pioneer Mountains, also assisted by crustal warping, reversed all Big Hole Basin drainage so as to create the north-, east-, and south-oriented Big Hole River drainage route. A final and even more major reversal of flow in the present-day north-oriented Montana Missouri River valley, with the assistance of additional crustal warping, next ended all remaining flow to Idaho and the Snake River drainage basin and reversed and captured all drainage in the present-day north-oriented Big Hole, Beaverhead, and Red Rock River drainage basins. The observed map evidence indicates that prior to the final flow reversal events, large volumes of south-oriented water flowed across southwest Montana’s Big Hole and Beaverhead River drainage basins. How to cite this paper: Clausen, E. (2017) Origin of Mountain Passes across Continental Divide Segments Surrounding the Southwest Montana Big Hole and Beaverhead River Drainage Basins, USA. Open Journal of Geology, 7, 1362-1385. https://doi.org/10.4236/ojg.2017.79091 Received: August 15, 2017 Accepted: September 16, 2017 Published: September 19, 2017 Copyright © 2017 by author and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/


Introduction
For more than a century geologists have debated origins of the southwest Montana Big Hole and Beaverhead River drainage systems (see Figure 1 and Understanding Big Hole and Beaverhead River drainage basin histories is critical if the larger Missouri River drainage basin origin is to be understood. While previous researchers agree that water once flowed across high mountain ranges now surrounding the Big Hole and Beaverhead River drainage basins, unfortunately those researchers do not agree on which way the water flowed. Some workers claim that the water flowed in a north direction to what is today a north-oriented Missouri River valley segment and then northward across Canada to the Labrador Sea (considerably north and east of Figure 1). Other researchers are convinced that the water flowed in south directions to the Snake River drainage basin and perhaps even to the Colorado River drainage basin. This paper uses topographic map evidence in an effort to end that controversy.
The Big Hole and Beaverhead River drainage basins are located within the red rectangle outlined in Figure 1, which identifies this paper's study region (shown in more detail in Figure 2). The blue arrows and letters in Figure 1 identify longer drainage routes discussed in this paper with "M'" for the Missouri River, "S" for the Snake River, "F" for Clark Fork, and "C" for the Columbia River. The  what type of drainage system eroded the deep passes and shallower saddles and which direction did the water flow? Figure 2 locates by number the 22 pass areas discussed below. Blue arrows and letters indicate drainage routes, which are identified as follows: "B" Beaverhead River, "BH" Big Hole River, "BI" Bitterroot River, "BO" Boulder River, "F" Clark Fork, "G" Grasshopper Creek, "GA" Gallatin River, "H" Henry's Fork (of the Snake River), "HP" Horse Prairie Creek, "J" Jefferson River, "L" Lemhi River, "M" Missouri River, "MA" Madison River, "R" Red Rock River, "RU" Ruby River, "SA" Salmon River, and "W" Wise River. In addition to passes identified on Figure 2 numerous other passes and saddles near the identified locations are also briefly described to provide a more complete picture of the number and the spacing of the observed divide crossings. In addition to the regional figures this paper provides readers can observe detailed topographic maps of each of the discussed passes using a variety of different map scales at the USGS National Map website [1]. Most mountain passes discussed below have names shown on the detailed topographic maps. However, the detailed maps do not name all of the discussed passes. Unnamed passes have been named in this paper simply to assist with their description.
The north-south highway passing through the study region crosses the east-west Continental Divide at Monida Pass (near study region south edge, number 4 on Figure 2) and also at Deer Lodge Pass (near study region north edge, number 21 on Figure 2). The Monida Pass elevation is 6820 feet (2079 meters) and the Deer Lodge Pass elevation is 5801 feet (1768 meters). Note in Figure 1 and Figure 2 how the Missouri River (M) is formed at Three Forks, Montana where the northeast and east oriented Jefferson River joins the Madison (MA) and Gallatin (GA) Rivers. From Three Forks the Missouri River flows in a north-northwest direction before turning to flow in more of an east direction to North Dakota where it turns again to flow in more of a south direction with water eventually reaching the Gulf of Mexico. Note how the study region is bounded on the north by the north and northwest oriented Clark Fork (F) drainage basin with water eventually reaching the Pacific Ocean and on the south and west by the Snake River (S) drainage basin with water also eventually reaching the Pacific Ocean.

Previous Studies
For more than a century some workers have suggested water once flowed in a south direction across this paper's study region while others have proposed northward flow. Arguments for south oriented flow began when Umpleby [2] concluded the northwest oriented Lemhi River (L in Figure 2) in Idaho (now flow-  [9] working near the study region's southeast corner concluded drainage once flowed in a south direction to where the modern day Colorado River and its south oriented Green River tributary are now located (not shown in figures here, but in Colorado which is seen in Figure 1).
Most recently, without addressing geomorphic or other evidence earlier workers (such as those cited above) presented, Sears [10] [11] proposed a major north-oriented river flowed from the Grand Canyon area (south of Figure 1 could generate the immense volumes of water necessary to erode the valleys in question and further briefly described how melt water from a thick ice sheet that had created and occupied a deep "hole" would eventually reverse its flow direction as the ice sheet melted so as to create all of the north-oriented valleys in the hypothesized pre-ice age Bell River system. Sears [14] in his reply to the Clausen's comment noted the Miocene and Pliocene ages of sediments in Montana north-oriented valleys was not consistent with Pleistocene ages of North American ice sheets and concluded the age inconsistency "falsified" Clausen's hypothesis.
While there has been considerable disagreement concerning which way water flowed across this paper's study region interestingly there has also been remarkable agreement that large rivers did once flow across that Continental Divide and that crustal warping and volcanic activity played important roles as those rivers were dismembered. There also has been agreement that flow in at least some segments of the dismembered river valleys has been reversed so as to create present day drainage systems, although there is considerable disagreement as to which modern day valley segments contain the reversed flow. There also seems to be agreement among researchers who advocate different flow directions that large rivers flowing one way or the other used at least some of the north to south oriented valleys located to the south of this paper's study region.
This paper is intended to continue the discussion by summarizing topographic map evidence, much of which has been described in great detail in Missouri

Red Rock River Drainage Basin
Mountain passes numbered 1 -5 on Figure Figure 2 and Figure  3) location in relation to the west-oriented Red Rock River (R) headwaters, the south-oriented Henry's Fork (of the Snake River) (H) headwaters, the east-oriented West Fork Madison River (WF), and Lone Tree (1a) and Raynolds (1b) Passes. While the previously described Pleistocene lake(s) probably drained in a north direction through Hidden Lake Pass (1) adjacent passes also seen in Figure 3 suggest most pass valley erosion was done by south-oriented water. Evidence for earlier flow across the present-day Red Rock-Madison River drainage divide is found at Lone Tree Pass (number 1a on Figure 3 with an elevation of 7521 feet or 2292 meters), which links a north-oriented Madison River tributary with a southwest-oriented Red Rock River tributary. Lone Tree Pass (1a) does not cross the Continental Divide, but is located adjacent to it. Determining the direction water once moved across Lone Tree Pass requires knowing the flow direction of water that once moved across nearby passes that do now cross the Continental Divide. Three of these passes are south of Lone Tree Pass and link the Red Rock River (R) drainage basin with the Henry's Fork of the Snake River (H) drainage basin. Still another pass, Raynolds Pass (number 1b with an elevation of 6836 feet or 2084 meters) is located east of Hidden Lake Pass (1) (2) and Squaw Pass (2a) are oriented in west-east directions while the unnamed pass (next to Squaw Pass at 2a) to the north is oriented in a northwest-southeast direction. The Henry's Fork (H) valley is oriented in more of a north-south direction and as already mentioned is linked at Raynolds Pass (1b) with the north-oriented Madison River (MA) valley. These valley orientations make it unlikely that water flowed in a north direction in the Henry's Fork (H) valley and then turned in a west direction to enter the Centennial Valley and the Red Rock River (R) drainage basin. The much more probable flow direction for water that eroded these three passes was in an east direction to a south-oriented Henry's Fork (H) valley. If so the present day west-oriented Red Rock River (R) drainage route originated as an east-oriented drainage route and its flow direction has been reversed.
3) Centennial Divide (number 3) with an elevation of 7360 feet or 2500 meters is the name used for the divide between the north-oriented Ruby River (RU in Figure 2 and Beaverhead River (B). Sears [10] suggested that before being deflected to the Monida Pass route the northwest-oriented Snake River (S) flowed between the Beaverhead and Tendoy Mountains. Such an interpretation is extremely unlikely because the initial north-oriented flow route would have required a southeast-oriented jog around the Tendoy Mountains north end (see Figure 4) and also because any deflection process would have been extremely complex.

Beaverhead Mountain Passes across the Horse Prairie
Creek-Lemhi River Drainage Divide  The barbed tributaries provide evidence that flow across the Deadman-Bannock Pass ridge (and also in the Red Rock (R)-Beaverhead River (B) and Lemhi River (L) valleys) originally was in a south direction. The similarity of the Bannack, Deadman, and Bannock Pass elevations suggests diverging and converging south-oriented river channels may have eroded the three different passes. If so the south-oriented diverging river channels must also have included flow channels that eroded Monida Pass and passes at the Centennial Valley east end such as Red Rock Pass (2). Huge volumes of water would have been required to initiate and maintain multiple south-and southeast-oriented river channels as the Beaverhead, Tendoy, and Centennial Mountains were being uplifted.
2) Lemhi Pass (number 8 on Figure 2 and Figure 4) with an elevation of 7373 feet or 2247 meters represents a saddle approximately 1000 feet (305 meters) lower than the mountain crest to the north. Lemhi Pass is located on a narrow ridge between the southeast-oriented Trail Creek headwaters valley and the deep northwest-oriented Agency Creek headwaters valley. Further downstream Trail Creek turns in a northeast direction to join east-and southeast-oriented Horse Prairie Creek (HP) with water eventually reaching the Missouri River (M). Downstream from its northwest-oriented headwaters Agency Creek turns in a southwest and northwest direction before flowing in a southwest and west direction to join the north-and northwest-oriented Lemhi River (L) with water eventually reaching the Salmon River (SA). The northwest and southeast orientation of valley segments closest to Lemhi Pass suggests either northwest-or southeast-oriented flowing water once moved through the pass. Based on evidence for previously described passes southeast-oriented water provides a much simpler explanation for the eight passes discussed so far than northwest-oriented flow.

1) Big Hole River-Bloody Dick
Creek Pass (number 10 on Figure 2 and    Figure 1).

Passes across the Big Hole-Salmon River Drainage Divide
2) Bradley Gulch Pass (number 14 on Figure 2 and Figure 5) is named here after a valley leading away from it and has an elevation of 7140 feet (2176 meters). The Pass is an 800-foot (244-meter) deep north-to-south oriented notch linking north-and northeast-oriented Pioneer Creek with south-oriented Bradley Gulch. Pioneer Creek flows to northeast-oriented Ruby Creek and the North Fork Big Hole River (NBH in Figure 5) while Bradley Gulch drains to southwest-oriented North Fork Sheep Creek and as described above eventually to the south-oriented North Fork Salmon River (NSA in Figure 5). Drainage divides at both Moosehorn Pass and Bradley Gulch Pass are asymmetric with steep slopes leading down to North Fork Sheep Creek, which flows in a 1000-foot (305-meter) deep valley below. Gradients to the Big Moosehorn Creek and Pioneer Creek headwaters are much lower and those two creeks then flow along much gentler gradients into the Big Hole Basin. The asymmetric divides and pass orientations strongly suggest both of the passes were eroded by southwest-oriented water moving from the Big Hole Basin to the south-oriented North Fork Salmon River valley.
3) Big Hole Pass (number 15 on Figure 2 and Figure 5) with an elevation of 7055 feet (2150 meters) links an east-oriented Pioneer Creek segment and the northeast-oriented Ruby Creek-North Fork Big Hole River (NBH) with the much deeper valley of west-and southwest-oriented Dahlonega Creek, which flows to the south-oriented North Fork Salmon River (NSA on Figure 5). The drainage divide is asymmetric with steep slopes leading approximately 2000 feet (610 meters) from the Pass into the deep Dahlonega Creek valley. Big Hole Pass is the lowest point on a 700-foot (213 meters) deep saddle notched into a low elevation north to south oriented Continental Divide segment. The saddle orientation, its width, and the asymmetric divide suggest large volumes of water once moved in a west direction from the Big Hole Basin to the south-oriented North Fork Salmon River valley. Further, the alignment of the North Fork Salmon River valley with the north-oriented Salmon River (SA in Figure 2 and Figure 5 Figure 6), which joins Silver Bow Creek (SB in Figure 6) to form north-and northwest-oriented Clark Fork

Discussion
Unusually large volumes of south-oriented water over prolonged periods of time North-and northwest-oriented Clark Fork (F) also flows toward a documented ice sheet margin and is joined by the south-oriented Flathead River, which at one time must have flowed from that ice sheet margin. Ice sheet melt water probably from the Flathead River drainage basin filled Lake Missoula, which is interpreted in Baker [22] to have catastrophically drained during late Pleistocene time so as to deeply erode the Washington State Channeled Scablands. The north-and northwest-oriented Clark Fork (F) drainage basin is usually assumed to have already been in existence when giant floods eroded the Washington State Channeled Scablands. However it is possible much of the Washington State flooding occurred earlier than commonly thought and large volumes of ice sheet melt water also moved in a south direction to erode this paper's southwest Montana study region. An ice sheet melt water source is more consistent with the mountain pass and other drainage history evidence than the previously described Miocene rainfall hypothesis, but such an ice sheet to explain all of the evidence described here must have created and occupied a deep "hole" in the North American continent. The opening up of space in that deep "hole" as the large ice sheet melted is necessary to explain headward erosion of deep north-and northeast-oriented valleys across massive south -and southeast-oriented ice marginal melt water floods so as to create the previously mentioned Bell River system of north-oriented valleys and to behead and reverse flow in what is today Montana's north-oriented Missouri River (M) valley segment.

Conclusions
Detailed topographic map evidence documents large volumes of south-oriented water eroded what are today the numerous deep passes and shallower saddles that are notched into North America's east-west Continental Divide and the Big Hole-Beaverhead River divide. All observed passes and saddles, with the possible exceptions of Deer Lodge and Hidden Lake Passes, through which south-oriented water almost certainly flowed, appear to be erosional features carved by numerous and closely spaced south-oriented rivers or streams that once flowed The earliest evidence for south-oriented water erosion is found in Anaconda, Beaverhead, and Pioneer Mountain passes and saddles. South-oriented drainage across what must have been a subsiding Big Hole Basin was diverted by Beaverhead Mountain uplift so as to flow in a southeast direction along the southeast-oriented Bloody Dick Creek (BD) alignment to Bannock, Deadman, Bannack, and Monida Passes and also to passes at the east end of what must have been a subsiding Centennial Valley. Crustal warping eventually ended southeast-oriented flow from the Big Hole Basin to Bannock, Deadman, and Bannack Passes although south-oriented flow on the Beaverhead River (B) alignment continued to reach Monida Pass and passes at the Centennial Valley eastern end.
At the same time as the above described events were taking place headward erosion of the deep south-oriented North Fork Salmon River valley was capturing south-oriented flow that was moving into the Big Hole Basin. Headward erosion of the deep east-oriented Big Hole River (BH) valley segment (between the Anaconda Range and Pioneer Mountains) next beheaded and reversed southwest-and south-oriented flow moving to the North Fork Salmon River (NSA in Figure 5) valley so as to create the north-, east-, and south-oriented Big Hole River (BH) drainage system. Crustal warping by this time had reversed south-oriented flow across the present day north-oriented Bitterroot River (BI) drainage basin due to Anaconda Range emergence. Continued crustal warping and a major flow reversal in what is today Montana's north-oriented Missouri River (M) valley finally reversed Jefferson River (J) flow, which reversed flow in the Beaverhead (B) and Red Rock River (R) valleys so as to create the north-oriented drainage systems seen today. Glaciation in newly uplifted Ana-E. Clausen