Big Hole River-Salmon River drainage divide area landform origins along the continental divide, Beaverhead County, Montana and Lemhi County, Idaho, USA

Authors


Abstract:

This essay uses topographic map evidence to interpret landform origins along the continental divide between the Big Hole River in Beaverhead County, Montana and the Salmon River in Lemhi County, Idaho. The Big Hole River flows in a north direction in Montana on the east side of the Beaverhead Mountains while on the west side of the Beaverhead Mountains the Salmon River flows in a north direction in Idaho to North Fork, Idaho where it joins the south oriented North Fork Salmon River and then flows in a west direction. The broad Big Hole River valley is approximately 800 meters higher in elevation than the floor of the narrow Salmon River valley. Tributaries to the north oriented Big Hole River are oriented in northeast directions, although north of the Beaverhead Mountains the Big Hole River tributaries tend to be oriented in south directions. Tributaries to the north oriented Salmon River flow in southwest and south-southwest directions and join the Salmon River as barbed tributaries. Notches, mountain passes, or through valleys cross the continental divide and link Big Hole River tributary valleys with Salmon River tributary valleys. While probably altered by glacial erosion these notches, mountain passes, and through valleys were eroded by south oriented flood flow channels moving floodwaters in large volumes from the present day north oriented Big Hole River valley to a south oriented flood flow channel on the present day north oriented Salmon River alignment. At that time the Beaverhead Mountains did not exist and the Beaverhead Mountains were uplifted as the south oriented floodwaters flowed across them. Beaverhead Mountain uplift and regional uplift south of the study region caused massive flood flow reversals in the Big Hole River drainage basin on the east side of the rising Beaverhead Mountain and also in the Salmon River drainage basin on the west side of the rising Beaverhead Mountains to create the present day north oriented drainage systems. These flood flow reversals combined with the Beaverhead Mountain uplift caused reversals of flood flow on northeast ends of what had been southwest oriented flood flow channels and created the present day northeast oriented Big Hole River tributaries and the Big Hole River-Salmon River drainage divide. The flood flow reversal west of the Beaverhead Mountains probably occurred in a flood flow channel at a much higher elevation than the present day deep Salmon River valley with flood flow first moving north to the Bitterroot River valley. Headward erosion of the deep west oriented Salmon River valley then captured the north oriented flood flow and the deep Salmon River then eroded headward into the region west of the Beaverhead Mountains.

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 are listed on the sidebar category list under their appropriate Missouri River tributary drainage basin, Missouri River segment drainage basin (by state), and/or state in which the Missouri River drainage basin is located.

Introduction

The purpose of this essay is to use topographic map interpretation methods to explore the Big Hole River-Salmon River drainage divide area landform origins along the continental divide in Beaverhead County, Montana and Lemhi County, Idaho and 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 leaving 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 Missouri River drainage basin landform origins research project essays 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 Big Hole River-Salmon River drainage divide area landform evidence along the continental divide in Beaverhead County, Montana and Lemhi County, Idaho will be regarded as evidence supporting the “thick ice sheet that melted fast” paradigm.

Big Hole River-Salmon River drainage divide area location map

Figure 1: Big Hole River-Salmon River 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 Big Hole River-Salmon River drainage divide area along the continental divide in Beaverhead County, Montana and Lemhi County, Idaho and in the east half  illustrates a region in southwestern Montana with Idaho in the west half and also appearing along the south edge of figure 1. The Missouri River is formed near the town of Three Forks (located near east center edge of figure 1) at the confluence of the Gallatin, Madison, and Jefferson Rives and then flows in a north and north-northwest direction to the north edge of figure 1. North of figure 1 the Missouri River turns to flow in a northeast and then east direction to North Dakota where it turns to flow in a southeast and south direction with water eventually reaching the Gulf of Mexico. The Jefferson River is formed in the southeast quadrant of figure 1 at the confluence of the Big Hole and Beaverhead Rivers near Twin Bridges, Montana. The Big Hole River flows in a north and northeast direction west of the Pioneer Mountains and then turns to flow in an east-southeast direction to the town of Divide. From Divide the Big Hole River flows in a south and northeast direction to join the north-northeast oriented Beaverhead River. The Salmon River originates in Idaho south of figure 1 and flows in a north-northeast direction from the south edge of figure 1 (just west of center) to the town of Salmon and then to the town of North Fork where it turns to flow in a west direction to the west edge of figure 1 (south half). The east-west continental divide is located along the Idaho-Montana border in the Beaverhead Mountains south of Lost Trail Pass (on Idaho-Montana border just north of North Fork where north oriented Salmon River turns to flow in a west direction), although Lost Trail Pass is not on the continental divide, but is instead on the drainage divide between the north oriented Bitterroot River and a south oriented Salmon River tributary with the Bitterroot River flowing to northwest oriented Clark Fork.  The Big Hole-Salmon River drainage divide area investigated in this essay is located along the north end of the Beaverhead Mountains and is south of Lost Trail Pass and north of the town of Salmon, Idaho.

Before looking at detailed maps of the Big Hole River-Salmon River drainage divide area a brief look at the big picture erosion history is appropriate. Large volumes of south and southeast oriented floodwaters once flowed across the region shown by figure 1. Floodwaters were derived from the western margin of a rapidly melting thick North American ice sheet and were flowing in a south and southeast direction from southwest Alberta and southeast British Columbia to and across the figure 1 region. North oriented rivers in figure 1 are generally flowing in valleys that originated as south oriented flood flow channels. The north oriented drainage system seen on both sides of the east-west continental divide developed sequentially during massive flood flow reversals that occurred as mountain ranges and high plateaus were uplifted by ice sheet related crustal warping and occurred as floodwaters flowed across the region. During these flood flow reversals flood flow moving in one direction along one route would be captured so as to flow in an opposite direction along an adjacent route. In addition, deep flood water erosion of valleys and basins surrounding the rising mountain ranges contributed to the emergence of present day mountain ranges. At the same time ice sheet related crustal warping combined with deep glacial erosion under the ice sheet was creating a deep “hole” in which the ice sheet was located. Eventually as the ice sheet melted there came a time when elevations on the ice sheet surface (at least in the south) were lower than elevations along the deep “hole” southwest rim in Montana where the immense south and southeast oriented ice marginal melt water floods were flowing. Deep northeast oriented valleys then eroded headward from space in the deep “hole” being opened up by the ice sheet melting to capture the south and southeast oriented melt water floods in present day eastern and central Montana. At the same time headward erosion of the Columbia River valley and tributary valleys from the Pacific Ocean beheaded and reversed south and southeast oriented flood flow channels moving floodwaters to western Montana.

The northeast oriented Missouri River valley (north and east of figure 1) and its east and northeast oriented tributary valleys eroded headward from the deep “hole” across the south and southeast oriented flood flow. Northwest oriented Missouri River tributary valleys and the north-northwest oriented Missouri River valley segment were eroded by reversals of flood flow on north and northwest ends of beheaded flood flow channels. North oriented Jefferson River tributaries alignments were established initially as south oriented flood flow channels and were reversed and deepened during the massive upper Missouri River drainage basin flood flow reversal. Reversal of flood flow in the Jefferson River valley captured south oriented flood flow on the present day north oriented Clark Fork-Big Hole River alignment, which created the present day Big Hole River U-turn east of the Pioneer Mountains. The north oriented Big Hole River segment west of the Pioneer Mountains flows on the alignments of what began as south oriented flood flow channels, but which were reversed to create the north oriented Big Hole River valley west of the Pioneer Mountains.

The north oriented Bitterroot River alignment in western Montana and the north oriented Salmon River alignment in Idaho (south of Lost Trail Pass seen in figure 1) were established by one or more south oriented flood flow channels. That south oriented flood flow channel may have been joined by an east oriented flood flow channel on the present day west oriented Salmon River valley segment (west of Salmon, Idaho). At that time none of the deep valleys seen today existed and floodwaters were flowing on surfaces equivalent in elevation to the crests of the present day mountain range crests. The south oriented flood flow channel was dismembered first by crustal warping that uplifted regions in the south of figure 1 and that probably caused a major flood flow reversal on the previous south oriented flood flow channel with floodwaters flowing to the north oriented Bitterroot River valley. This newly formed north oriented flood flow channel probably was dismembered when crustal warping caused floodwaters in what may have been an east oriented flood flow channel on the present day west oriented Salmon River alignment to reverse flow direction and to erode the west oriented Salmon River valley segment. Floodwaters eroding the present day deep north and west oriented Salmon River valley probably were captured from floodwaters that continued to flow in south directions on the present day Missouri River alignment to the Snake River headwaters area (in Yellowstone National Park region), which then flowed in northwest and north directions to the Salmon River alignment, although at the same time floodwaters were probably flowing along on the present day Snake River alignment (south and west of figure 1) and eventually captured all of the flood flow.

Detailed location map for Big Hole River-Salmon River drainage divide area

Figure 2: Detailed location map Big Hole Rive-Salmon River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 2 provides a more detailed location map for the Big Hole River-Salmon River drainage divide area along the continental divide in Beaverhead County, Montana and Lemhi County, Idaho and shows drainage routes not seen in figure 1. The Montana-Idaho state line is shown with a dashed line and extends in a southeast and northeast direction from the west edge of figure 2 to near Lost Trail Pass and then in a south-southeast direction along the Beaverhead Mountains to the south edge of figure 2 (east of center). South of the Lost Trail Pass region the east-west continental divide serves as the state line, north of the Lost Trail Pass area the continental divide extends in a northeast direction to the north center edge of figure 2 and serves as the boundary between two Montana counties with Beaverhead County occupying much of the east half of figure 2 and Ravalli County being located in the northwest corner region of figure 2. All Idaho areas seen in figure 2 are located in Lemhi County. Green shaded areas are National Forest lands, which generally are located in mountainous regions.

The Big Hole River flows in a north direction from the southeast quadrant figure 2 to the town of Wisdom and then to the north edge of figure 2 (east half). East of the north oriented Big Hole River along the east edge of figure 2 are the Pioneer Mountains. The North Fork Big Hole River flows in a northeast direction from the Big Hole National Battlefield (west of Wisdom) to join the Big Hole River near the north edge of figure 2. Note how near the north edge of figure 2 there are multiple southeast oriented streams flowing from the continental divide to join the northeast oriented North Fork Big Hole River as barbed tributaries. Joseph Creek is an east-southeast oriented North Fork Big Hole River tributary originating near Lost Trail Pass. The north oriented stream originating near Lost Trail Pass is a tributary to the north oriented Bitterroot River. South of Joseph Creek Big Hole River tributaries from the west are oriented in northeast directions and originate in the Beaverhead Mountains. The town of Salmon is located in Idaho near the south center edge of figure 2. The Salmon River flows in a north direction from the south edge of figure 2 to the town of Salmon and then to the town of North Fork before turning to flow in a west and west-southwest direction to the west edge of figure 2. The North Fork Salmon River flows in a south direction from near Lost Trail Pass to join the Salmon River as a barbed tributary near the elbow of capture where the north oriented Salmon River turns to flow in a west direction. Note southwest oriented streams originating in the Beaverhead Mountains flowing to the north oriented Salmon River as barbed tributaries south of the town of North Fork. These south oriented barbed tributaries suggest the present day north oriented Salmon River alignment was initially established as a south oriented flood flow channel. The northeast oriented Big Hole River tributaries were probably formed by reversals of flood flow on northeast ends of beheaded southwest oriented flood flow channels.

East Fork Camp Creek-North Fork Salmon River drainage divide area

Figure 3: East Fork Camp Creek-North Fork Salmon River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 3 provides a topographic map of the East Fork Camp Creek-North Fork Salmon River drainage divide area. Lost Trail Pass is located near the center of figure 3 at the highway junction and marks the drainage divide between the north-northeast and north-northwest oriented East Fork Camp Creek, which flows to the north oriented Bitterroot River (not seen in figure 3), and south-southwest oriented Moose Creek, which flows to the south oriented North Fork Salmon River, which flows to the south edge of figure 3 (slightly west of center). The Montana-Idaho state line extends in a northeast and east direction along the Ravalli-Lemhi County border from near the southwest corner of figure 3 to Lost Trail Pass and then extends in a south-southeast and southeast direction along the Beaverhead Mountain crest, which is the continental divide. Chief Joseph Pass is on the continental divide near Lost Trail Pass and the east-west continental divide extends in a north direction from Chief Joseph Pass to the north center edge of figure 3. Joseph Creek is the east-southeast oriented stream flowing from near Chief Joseph Pass to the east edge of figure 3 (the highway follows it) and east of figure 3 joins the east-northeast North Fork Big Hole River, which flows to the north oriented Big Hole River. Trail Creek is a south and southeast oriented Joseph Creek tributary flowing adjacent to the continental divide from near the north center edge of figure 3. Note how near the north edge of figure 3 south oriented Trail Creek and the continental divide are both located on the edge of the deep north oriented East Fork Camp Creek valley. The map contour interval for figure 3 is 50 meters and the East Fork Camp Creek valley is shown with at least five contour lines in that region. This remarkable situation probably occurred because the deep north oriented East Fork Camp Creek valley was eroded by a massive reversal of south oriented flood flow that did not affect the region immediately to the east. In other words, water continued to flow in a south direction in the Trail Creek valley while much larger volumes of flood water were flowing in a north direction in the East Fork Camp Creek valley. If correctly interpreted the south oriented Moose Creek and North Fork Salmon River valleys are on the alignments of flood flow channels that originally flowed in south directions, then reversed flow direction to flow in north directions, and finally reversed flow to flow in south directions. Study of the figure 3 map area reveals numerous through valleys or mountain passes crossing drainage divides. These through valleys or mountain passes were eroded by flood flow channels that initially were oriented in south directions, but that as crustal warping raised the regional mountains and as deeper valleys eroded headward into the region the flood flow movements became complex with multiple flood flow reversals such as occurred on the East Fork Camp Creek-North Fork Salmon River alignment.

Detailed map of East Fork Camp Creek-North Fork Salmon River drainage divide area

Figure 4: Detailed map of East Fork Camp Creek-North Fork Salmon River 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 East Fork Camp Creek-North Fork Salmon River drainage divide area seen is less detail in figure 3. Chief Joseph Pass is located near the south center edge of figure 4 and the east-west continental divide extends in a north, northeast, and north direction from Chief Joseph Pass to the north edge of figure 4 (east of center). Joseph Creek originates north of Chief Joseph Pass and flows in a southeast direction to the south edge of figure 4 (east half) and joins the east-northeast oriented North Fork Big Hole River south and east of figure 4. Trail Creek flows in a southeast direction from the north edge of figure 4 (just east of the continental divide) to the east edge of figure 4 (just south of center) and joins Joseph Creek south and east of figure 4. Lost Trail Pass is located at the highway intersection (northwest of Chief Joseph Pass). The East Fork Camp Creek flows in a deep north-northeast oriented valley from near Lost Trail Pass almost to the north edge of figure 4 and then turns to flow in a north-northwest direction to the north center edge of figure 4. North of figure 4 the East Fork Camp Creek flows to the north oriented Bitterroot River. South of Lost Trail Pass are south oriented headwaters of Moose Creek, which south of figure 4 flows to the south oriented North Fork Salmon River. The south oriented stream in the southwest corner of figure 4 is the North Fork Salmon River. The north-northeast oriented stream originating just north of the North Fork Salmon River headwaters is the West Fork Camp Creek, which joins the East Fork Camp Creek north of figure 4. The contour interval for figure 4 is 40 feet and the Lost Trail Pass elevation at the drainage divide is shown as 7014 feet. Follow the Ravalli County-Lemhi County line west and note additional higher level passes linking the north-oriented Camp Creek headwaters valleys with the south oriented North Fork Salmon River headwaters valleys. Also follow the continental northward to near the north edge of figure 4 and the elevation of the East Fork Camp Creek-Trail Creek drainage divide elevation drops to about 6800 feet. These passes are all water eroded features and record a complex history of flood flow movements. As described in the figure 3 discussion to East Fork Camp Creek-Trail Creek drainage divide near the north edge of figure 4 suggests flood flow was moving in opposite directions, with south oriented flood flow east of the present day continental divide eroding the south oriented Trail Creek and Joseph Creek valleys while west of the present day continental divide north oriented flood flow eroded the north oriented Camp Creek headwaters valleys and then in the southwest quadrant of figure 4 was reversed to erode the south oriented North Fork Salmon River headwaters valleys.

Pioneer Creek-Sheep Creek drainage divide area

Figure 5: Pioneer Creek-Sheep Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 5 illustrates the Pioneer Creek-Sheep Creek drainage divide area south and east of figure 3 and does not include an overlap area with figure 3.  The continental divide is also the Lemhi County (Idaho)-Beaverhead County (Montana) border line and is shown with a dashed line extending from near the north center edge of figure 5 to the south edge of figure 5 (just east of center). The highway located near the west edge of figure 5 follows the valley of the south oriented North Fork Salmon River, which south and west of figure 5 joins the north and west oriented Salmon River as a barbed tributary at the Salmon River elbow of capture (see figure 2). Sheep Creek is the northwest and west oriented tributary joining the North Fork Salmon near where the North Fork Salmon River flows across the west edge of figure 5 (south half). Sheep Creek is formed south of Eagle Mountain at the confluence of its northwest oriented South Fork and its north and southwest oriented North Fork. Bradley Gulch is a southwest and south oriented tributary to the North Fork Sheep Creek located west of Morgan Mountain and originates directly south of the continental divide. Directly north of Bradley Gulch and the continental divide are headwaters of north, northeast, and east oriented Pioneer Creek, which flows to join south and northeast oriented Ruby Creek, which flows to near the northeast corner of figure 5. North and east of figure 5 Ruby Creek flows to the east-northeast oriented North Fork Big Hole River, which then flows to the north oriented Big Hole River. West of Bradley Gulch the North Fork Sheep Creek flows in a north direction along the west side of the continental divide before turning to flow in a west direction just south of the continental divide. Directly east of the North Fork Sheep Creek elbow of capture (where it turns from flowing in a north direction to flowing in a west direction) and of the continental divide are headwaters of east and north oriented Big Moosehorn Creek, which flows to northeast oriented Ruby Creek. Note how the North Fork Sheep Creek and Bradley Gulch valleys are much deeper than the adjacent headwaters valleys of Pioneer Creek and Big Moosehorn Creek. West of the Pioneer Creek elbow of capture (where it turns from flowing in a north direction to flowing in a northeast and east direction) is Big Hole Pass, which links the Pioneer Creek valley with the west oriented Dahlonega Creek valley (with Dahlonega Creek flowing to the North Fork Salmon River near Gibbonsville). The map contour interval for figure 5 is 50 meters and the Big Moosehorn Creek valley appears to be a hanging valley almost 250 meters higher than the deep North Fork Sheep Creek valley just west of the continental divide. Likewise the Pioneer Creek valley appears to be hanging valley almost 250 meters higher than the deep Bradley Gulch valley directly across the continental to the south and the Pioneer Creek valley near Big Hole Pass appears to be in a hanging valley almost 500 meters higher than the Dahlonega Creek directly to the west of the continental divide. These apparent hanging valleys were probably formed as south oriented flood flow moving from the Big Hole River valley moved to actively eroding deep valleys, which were eroding headward from the deeper North Fork Salmon River valley. While the maze of valleys and through valleys (or mountains passes) seen in figure 5 suggests a complex history of flood flow movements probably preceded what was probably the final flood flow movement from the Big Hole River valley to the North Fork Salmon River valley, the final movement did erode the deep North Fork Sheep Creek, Bradley Gulch, and Dahlonega Creek valleys before a massive flood flow reversal created the north oriented Big Hole River drainage system and the Big Hole River-Salmon River drainage divide, which is today the east-west continental divide.

Detailed map of Pioneer Creek-Sheep Creek drainage divide area

Figure 6: Detailed map of Pioneer Creek-Sheep Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 6 provides a detailed topographic map of the Pioneer Creek-Sheep Creek drainage divide area seen in less detail in figure 5. The continental divide is a well-marked state and county line and extends from the north edge of figure 6 near Big Hole Pass in a south direction to Morgan Mountain and then in an east direction before turning in a south direction to the south edge of figure 6 (east half). Pioneer Creek originates east of Morgan Mountain on the north side of the continental divide and then flows in a north, northeast, and east-northeast direction to near the north edge of figure 6 where it joins southeast and northeast oriented Ruby Creek, which flows to the northeast corner of figure 6. South of the north oriented Pioneer Creek headwaters and of the continental divide is south oriented Bradley Gulch, which flows to southwest oriented North Fork Sheep Creek. Note how the north oriented Pioneer Creek valley is linked by a through valley or mountain pass with the south oriented Bradley Gulch valley. The map contour interval for figure 6 is 40 feet and the through valley floor elevation at the drainage divide is 7140 feet. Morgan Mountain to the west rises to 8308 feet while the continental divide to the east rises to 7911 feet, suggesting the through valley is approximately 800 feet deep. The through valley is a water-eroded feature and the much steeper gradient on the south side suggests the final direction of flow was from the Big Hole River valley to the Sheep Creek valley. In the southeast quadrant of figure 6 is a through valley linking the west and southwest oriented North Fork Sheep Creek valley and the east and north oriented Big Moosehorn Creek valley. The through valley elevation at the continental divide is between 7400 and 7440 feet. Just north of Big Moosehorn Creek the ridge rises to 8105 feet and to the south the continental divide rises to more than 8600 feet, suggesting the though valley is approximately 600 feet deep. Again the steeper gradient on the west side of the continental divide suggests the final water movement was from the Big Hole River valley to east and north to the Sheep Creek valley to the west and south. Big Hole Pass near the north edge of figure 6 is an even lower through valley with an elevation of between 7040 and 7080 feet. While not seen in figure 6 the continental divide north and west of figure 6 the continental divide rises to 8286 feet suggesting Big Hole Pass is approximately 1200 feet deep. In fact elevations along much of the continental divide in figure 6 are lower than high points both to the north and to the south suggesting a much broader and deeper through valley once carried large volumes of water across what is now the east-west continental divide.

Sheep Creek-Rock Creek drainage divide area

Figure 7: Sheep Creek-Rock Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 7 illustrates the Sheep Creek-Rock Creek drainage divide area south of figure 5 and includes a significant overlap area with figure 5. The continental divide is also the state and county line and extends from just west of the north center edge of figure 7 to east of the south center edge of figure 7. The highway in the northwest corner region of figure 7 follows the south oriented North Fork Salmon River valley. Northwest and west oriented Sheep Creek joins the North Fork Salmon River near the point where the North Fork and highway cross the west edge of figure 7 (north half). Note north and northwest oriented Sheep Creek headwaters near the continental divide in the center region of figure 7. These north and northwest oriented Sheep Creek headwaters valleys were probably initiated as south and southeast oriented flood flow channels and reversed flow direction as crustal warping raised the Beaverhead Mountain range and as headward erosion of the much deep west oriented Sheep Creek and southwest oriented North Fork Sheep Creek valleys beheaded the south oriented flood flow channels. South oriented streams in the southwest quadrant of figure 7 flowing to the south edge of figure 7 (west of continental divide) flow to the north oriented Salmon River as barbed tributaries. N Mountain is a labeled high point in the southwest quadrant of figure 7. East of N Mountain is south oriented Blacktail Creek, which flows to south-southwest oriented Fourth of July Creek. Note how the Blacktail Creek valley is linked by a through valley with the northwest oriented South Fork Sheep Creek valley. The map contour interval for figure 7 is 50 meters and the through valley floor elevation at the drainage divide is 2292 meters. N Mountain rises to 2704 meters and the ridge west of the through valley rises to 2716 meters suggesting the through valley is at least 400 meters deep. East of the Fourth of July Creek and South Fork Sheep Creek headwaters and the continental divide are headwaters of east-northeast oriented Rock Creek, which east of figure 7 turns to flow in a northeast direction to the north oriented Big Hole River. South of Rock Creek is east oriented Big Lake Creek, which east of figure 7 also turns to flow in a northeast direction to the north oriented Big Hole River. While today a well-defined mountain ridge separates the Rock Creek headwaters valley from the northwest oriented South Fork Sheep Creek valley and the west and south-southwest oriented Fourth of July Creek valley a close look reveals significant notches in that ridge. For example the through valley (or notch) linking the South Fork Sheep Creek and Rock Creek valleys has an elevation of between 2700 and 2750 meters. Pyramid Peak to the north rises to 2931 meters while an unnamed mountain to south rises to 2899 meters suggesting the notch or through valley is approximately 150 meters deep. The through valley or notch linking the Rock Creek valley with the Fourth of July Creek valley is slightly higher in elevation, although it has a similar depth. These through valleys or notches are evidence of earlier flood flow channels, which crossed what is today the Beaverhead Mountains. At that time the Beaverhead Mountains were being uplifted and deep valleys were eroding headward into the region from both the east and the west, which resulted in complex flood flow movements that eroded the present day deep valleys on both sides of the continental divide.

Detailed map of Sheep Creek-Rock Creek drainage divide area

Figure 8: Sheep Creek-Rock Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 8 provides a topographic map of the Sheep Creek-Rock Creek drainage divide area seen in less detail in figure 7. The continental divide is labeled and well-marked and extends from the north edge to the south edge of figure 8. Pyramid Peak is located on the continental divide a short distance south of the center of figure 8. Moose Creek originates east of Pyramid Peak and flows in an east-northeast direction to the east edge of figure 8 (north of center). South of Moose Creek is Rock Creek, which also flows to the east edge of figure 8. East of figure 8 Moose Creek and Rock Creek flow to the north oriented Big Hole River. West of Pyramid Peak are headwaters of north-northwest oriented South Fork Sheep Creek, which joins south-southwest oriented North Fork Sheep Creek near the northwest corner of figure 8 to form west oriented Sheep Creek. The north-northwest oriented stream north and east of the South Fork Sheep Creek is Pruvan Creek, which is a North Fork Sheep Creek tributary. Sheep Mountain is a labeled mountain peak on the continental divide near the north edge of figure 8. The north oriented stream originating west of Sheep Mountain is the North Fork Sheep Creek, which north of figure 8 turns to flow in a west and southwest direction and to join the South Fork Sheep Creek near the northwest corner of figure 8. Today the continental divide and other figure 8 drainage divides are high narrow mountain ridges, which have probably been carved by valley glaciers. However, the valleys existed before the valley glaciers and notches linking valleys on opposite sides of the present day drainage divides probably are remnants of water-eroded through valleys that were eroded by flood flow channels, which once crossed the region. For example, south of Pyramid Peak is a notch linking the east-northeast oriented Rock Creek valley with the north-northwest oriented South Fork Sheep Creek valley. The map contour interval is 40 feet and the notch floor elevation at the drainage divide is between 9000 and 9040 feet. Pyramid Peak rises to 9676 feet while the peak south of the notch rises to 9680 feet suggesting the notch is more than 600 feet deep. Just as interesting is the through valley or notch east of Pyramid Peak linking the east-northeast oriented Moose Creek valley with the east-northeast oriented Rock Creek valley. This through valley or notch in the southwest corner of section 30 has a floor elevation of between 8640 and 8680 feet. Jumbo Mountain on the ridge to the east rises to 9725 feet suggesting the through valley or notch is more than 1000 feet deep. The through valley or notch on the continental divide was probably eroded by flood flow moving from the Big Hole River valley to the Sheep Creek valley, although alternate interpretations are possible. The through valley in section 30 may have been eroded by flood flow moving from the Rock Creek valley to the Moose Creek valley after southwest oriented flood flow in the Moose Creek valley had been beheaded and reversed, but before southwest oriented flood flow in the Rock Creek had been beheaded and reversed. Again alternate interpretations are possible.

Big Lake Creek-Carmen Creek drainage divide area

Figure 9: Big Lake Creek-Carmen Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 9 illustrates the Big Lake Creek-Carmen Creek drainage divide area south and east of figure 7 and includes an overlap area with figure 7. The continental divide serves as the state and county line and extends from near the north center edge to the near the south center edge of figure 9. The north oriented Salmon River can be seen in the southwest corner of figure 9. The map contour interval for figure 9 is 50 meters and elevations in the Salmon River valley near the west edge of figure 9 are less than 1150 meters. Tributaries from the east are oriented in south directions and join the north oriented Salmon River as barbed tributaries. Fourth of July Creek is the south-southwest and southwest oriented tributary joining the Salmon River just west of the west center edge of figure 9. Carmen Creek originates on the southwest side of the continental divide just north of the center of figure 9 and flows in a south-southwest direction to the south edge of figure 9 and joins the Salmon River south of figure 9. Streams east of the continental divide flow in northeast directions to join the north oriented Big Hole River, which is located east of figure 9. Big Lake Creek is the Big Hole River tributary originating north and east of the Carmen Creek headwaters and flows in a northeast direction to the north edge of figure 9 (near northeast corner). Big Lake Creek crosses the 2000-meter contour line just before reaching the north edge of figure 9, which means the broad north oriented Big Hole River valley on the east side of continental divide is approximately 800 meters higher than the narrow north oriented Salmon River valley on the west side of the continental divide. The northeast oriented Big Hole River tributaries and the southwest and south-southwest oriented Salmon River tributaries probably are flowing on alignments initially established as diverging and converging south oriented flood flow channels moving floodwaters to a south oriented flood flow channel on the present day north oriented Salmon River alignment. At that time the Beaverhead Mountains did not present an obstacle to the south-southwest oriented flood flow, although as floodwaters flowed across the region the Beaverhead Mountains were uplifted forcing flood flow reversals on the east side of the drainage divide. There may also have been flood flow reversals on the west side of the drainage divide on the Salmon River alignment before the deep Salmon River valley was eroded, which initially caused water to flow in a north direction to the north oriented Bitterroot River valley as seen in figures 3 and 4. However, headward erosion of the deep west oriented Salmon River valley segment captured the north oriented flood flow and the deep valley then eroded headward in a south direction along the north oriented flood flow, which was probably being captured from south oriented flood flow channels east of figure 9.

Detailed map of Big Lake Creek-Carmen Creek drainage divide area

Figure 10: Detailed map of Big Lake Creek-Carmen Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 10 provides a detailed topographic map of the Big Lake Creek-Carmen Creek drainage divide area seen in less detail in figure 9. The continental divide serves as the state and county line and extends from the north edge of figure 10 (west half) to the south center edge of figure 10. Carmen Creek is the west and south-southwest oriented Salmon River tributary seen west of the continental divide. Big Lake Creek is the east oriented Big Hole River tributary flowing near the north edge of figure 10 on the east side of the continental divide and has east-northeast and northeast oriented tributaries. Stag-a-melt Creek is the east-northeast oriented stream flowing near the south edge of the southeast quadrant of figure 10. East of figure 10 Big Lake Creek turns to flow in a northeast direction and Stag-a-melt Creek joins a northeast oriented stream with water eventually reaching the north oriented Big Hole River. Note the deep notch in the continental divide just east of the Carmen Creek west oriented headwaters, which links the Carmen Creek valley with an east-northeast oriented Big Lake Creek tributary valley. While valleys in this region have probably been altered by valley glaciers the notch is probably a remnant of what was once a water-eroded through valley. The map contour interval for figure 10 is 40 feet and the notch or through valley floor elevation at the drainage divide is between 8320 and 8360 feet. The unnamed peak in section 33 or 29 to the south rises to more than 9700 feet while the continental divide near the north edge of figure 10 rises to 9450 feet suggesting the notch or through valley may be as much as 1100 feet deep. The notch or through valley was probably eroded by southwest oriented flood flow moving from what is today the north oriented Big Hole River valley to what is today the north oriented Salmon River valley. At that time flood flow on the Salmon River alignment was flowing in a south direction. Uplift of the Beaverhead Mountains (seen in figure 10) caused flood flow reversals that created the Big Hole River-Salmon River drainage divide. Regional uplift also caused flood flow reversals on both sides of that drainage divide that reversed flood flow in the Big Hole valley to create the north oriented drainage system and that reversed flood flow in the Salmon River valley that created the north oriented Salmon River drainage system. The Big Hole River and the Salmon River flood flow reversals may have been taking place as the drainage divide between the Big Hole River and the Salmon River was being created.

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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