Horse Prairie Creek-Lemhi 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 Horse Prairie Creek drainage basin in Beaverhead County, Montana and the Lemhi River drainage basin in Lemhi County, Idaho. The east-west continental divide in the region is located along the crest of the Beaverhead Mountains and serves as the state boundary. The Lemhi River flows in a northwest and north-northwest direction on the west side of the Beaverhead Mountains. Lemhi River tributaries from the Beaverhead Mountains flow in southwest directions and join the Lemhi River as barbed tributaries. Horse Prairie Creek is a northwest, north, and east oriented stream on the northeast side of the continental divide, which joins the north-northwest oriented Red Rock River to form the north-northeast oriented Beaverhead River and which has southeast, northeast, and north oriented tributaries flowing along or originating along the continental divide. Major passes crossing the continental divide in the Horse Prairie Creek-Lemhi River drainage divide area include Lemhi Pass, Bannock Pass, and Deadman Pass. These passes and other shallower passes across the continental divide and other drainage divides provide evidence of former south and southeast oriented flood flow channels that initially were eroded into a surface equivalent in elevation to elevations along the continental divide today. Floodwaters were derived from the western margin of a melting thick North American ice sheet and were flowing from southeast British Columbia across western Montana and then further south. At that time the Beaverhead Mountains did not stand high above surrounding regions as they do today. The Beaverhead Mountains emerged as ice sheet related crustal warping raised the region and as melt water flood erosion carved deep valleys around the rising mountain range. Floodwaters initially flowed in a south-southeast direction along what is today the continental divide alignment, however deep flood flow channels were eroded into the emerging Beaverhead Mountains until the mountain ridge became an insurmountable barrier and floodwaters were forced to erode deep southeast oriented valleys on either side of the emerging mountain range. Continued regional uplift combined with headward erosion of deep valleys from both the east and the west subsequently caused massive flood flow reversals on both sides of the emerging Beaverhead Mountains and that eventually created the east-west continental divide. The flood flow reversal north and east of the Beaverhead Mountains resulted in the diversion of southeast and south oriented flood flow moving into the Horse Prairie Creek drainage basin in a north-northeast direction toward space in the deep “hole” the melting ice sheet had occupied (forming what is today the upper Missouri River drainage basin in Montana). The flood flow reversal south and west of the Beaverhead Mountains occurred when headward erosion of the deep west oriented Salmon River valley beheaded and reversed the south and southeast oriented flood flow channel on the present day northwest and north oriented Lemhi River and Salmon River alignment.

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 Horse Prairie Creek-Lemhi 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 Horse Prairie Creek-Lemhi 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.

Horse Prairie Creek-Lemhi River drainage divide area location map

Figure 1: Horse Prairie Creek-Lemhi 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 Horse Prairie Creek-Lemhi River drainage divide along the continental divide in Beaverhead County, Montana and Lemhi County, Idaho and illustrates in the east half a region in southwest Montana with Idaho in the west half and appearing along the south edge of figure 1. The well-marked Montana-Idaho state line follows the east-west continental divide along the crest of the Beaverhead Mountains and the Centennial Mountains with Montana and the east oriented Missouri River drainage basin located north and east of the continental divide and the west oriented Snake River drainage basin located south and west of the continental divide. The Salmon River originates west of figure 1 in central Idaho and flows in an east direction to the southwest corner of figure 1 where it turns to flow in a north-northeast direction to the towns of Challis, Ellis, Salmon, and North Fork, where it turns to flow in a west-southwest direction to the west edge of figure 1 and then eventually to join the Snake River. The Lemhi River is a northwest oriented tributary flowing along the Beaverhead Mountains southwest flank to join the north oriented Salmon River near the town of Salmon, Idaho. Horse Prairie Creek is shown, but not labeled in figure 1 and is a northwest, north, northeast, and east oriented stream originating almost directly north Leadore, Idaho (but on the north side of continental divide) and joining the west and northwest oriented Red Rock River at an unlabeled reservoir near the Tendoy Mountains north end. The north-northeast oriented river flowing from that unlabeled reservoir is the Beaverhead River, which joins the north, northeast, southeast, south, and northeast oriented Big Hole River (which flows around the Pioneer Mountains north of figure 1) to form the northeast and east oriented Jefferson River, which is a Missouri River tributary. The Horse Prairie Creek-Lemhi River drainage divide area investigated in this essay is located west of the Tendoy Mountains, south of the east-oriented Horse Prairie Creek alignment and east and north of the Lemhi River segment upstream from the town of Tendoy, Idaho.

Before looking at detailed maps of the Horse Prairie Creek-Lemhi River drainage divide area a brief look at the big picture erosion history is needed. 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 systems seen on both sides of Beaverhead Mountains were initially developed as southeast oriented flood flow channels on either side of the emerging mountain range. Prior to development of deep southeast oriented flood flow channels on the Beaverhead Mountains northeast side floodwaters flowed along and across the emerging Beaverhead Mountains crest to a southeast oriented flood flow channel on the Lemhi River alignment. The southeast oriented Lemhi River flood flow channel diverged from a south oriented flood flow channel on the present day north oriented Salmon River alignment. Uplift of high mountain ranges and high plateaus by ice sheet related crustal warping combined headward erosion of a deep west oriented valley to the present day of North Fork, Idaho eventually caused a massive flood flow reversal that created the present day north oriented Salmon River drainage system and northwest oriented Lemhi River drainage route seen in figure 1. Perhaps at about the same time headward erosion of the deep southeast oriented Big Hole River valley along the Pioneer Mountains north flank caused a major flood flow reversal west of the Pioneer Mountains that created the present day north oriented Big Hole River drainage system there.

Prior to that flood flow reversal west of the Pioneer Mountains south and southeast oriented floodwaters moved in southeast directions to the present day Horse Prairie Creek drainage basin and it is possible southeast oriented floodwaters flowing on the present day northwest oriented Lemhi River alignment crossed the present day continental divide to enter the Horse Prairie Creek headwaters region east of Tendoy, Idaho. Further south and east through valleys or mountain passes across the continental divide suggest headward erosion of the deep east oriented Horse Prairie Creek valley beheaded and reversed south oriented flood flow channels to the Lemhi River headwaters area. During these flood flow reversals south oriented flood flow along one route would sometimes be captured so as to flow in a north direction along an adjacent route. In addition, deep flood water erosion of valleys and basins surrounding present day mountain ranges contributed to the emergence of the mountain ranges, which was occurring as floodwaters flowed across and later around them. Subsequently a reversal of flood flow in the Missouri River drainage basin (north and east of figure 1) resulted in headward erosion of the deep northeast and east oriented Jefferson River valley (north and west of the Tobacco Root Mountains in northeast quadrant of figure 1), which beheaded and reversed south oriented flood channels east of the Pioneer Mountains, which beheaded and reversed south oriented flood flow routes in that region so as to create the north-northeast oriented Beaverhead River and the Big Hole River U-turn east of the Pioneer Mountains.

Detailed location map for Horse Prairie Creek-Lemhi River drainage divide area

Figure 2: Detailed location map for Horse Prairie Creek-Lemhi 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 Horse Prairie Creek-Lemhi 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 follows the east-west continental divide and is shown with a dashed line and extends from the north edge of figure 2 (west half) to the south edge of figure 2 (east of center) with Lemhi Pass and Bannock Pass being major passes shown. Beaverhead County in Montana is east of the continental divide and Lemhi County in Idaho is west of the continental divide. Green shaded areas are National Forest lands, which generally are located in mountainous regions. The Lemhi River flows in a northwest and north-northwest direction from the south edge of figure 2 (west of continental divide) to the towns of Leadore, Lemhi, and Tendoy and then to the northwest corner of figure 2. Note southwest oriented barbed tributaries flowing to the north oriented Lemhi River from the Beaverhead Mountains, the crest of which is where the continental divide is located. Agency Creek flows in a southwest and west direction from Lemhi Pass to join the Lemhi River near Tendoy. Canyon Creek is a south-southwest and southwest oriented stream flowing from near Bannock Pass to join the northwest oriented Lemhi River near Leadore. Hawley Creek is a southwest and west oriented Lemhi River tributary located east of Leadore. Horse Prairie Creek is a northwest, north, and east oriented stream flowing from southeast of Bannock Pass (north of continental divide) to Clark Canyon Reservoir where it joins the north-northwest oriented Red Rock River to form the north-northeast oriented Beaverhead River. Frying Pan Creek is an east-northeast oriented Horse Prairie Creek tributary originating near Lemhi Pass. Medicine Lodge Creek is a north oriented Horse Prairie Creek tributary originating in the southeast quadrant of figure 2 and joining Horse Prairie Creek just west of Clark Canyon Reservoir. Note how the north oriented Medicine Lodge Creek headwaters are located east of the continental divide and the southwest oriented Hawley Creek headwaters and south oriented Canyon Creek headwaters are located directly to the west on the west side of the continental divide. The south oriented Lemhi River tributaries or tributary valley segments are relics of south oriented flood flow channels that existed prior to massive flood flow reversals on both sides of the continental divide. Most north oriented valleys are located on alignments of what were initially south oriented flood flow channels, which were beheaded and reversed so as to become north oriented drainage routes. Crustal warping that raised mountain ranges and otherwise uplifted  areas in the south played a major role in the massive flood flow reversals.

Agency Creek-Trail Creek drainage divide area

Figure 3: Agency Creek-Trail Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 3 provides a topographic map of the Agency Creek-Trail Creek drainage divide area. The east-west continental is the marked county line extending from the north edge of figure 3 (east half) to the south edge of figure 3 (east half). The Lemhi River flows in a north direction near the west edge of figure 3 and is on the west side of continental divide with water eventually reaching the Pacific Ocean. Lemhi Pass is located on the continental divide near the north edge of figure 3. The map contour interval for figure 3 is 40 meters and the Lemhi Pass elevation is given as 2247 meters, which is more than 200 meters lower than adjacent continental divide elevations both to the north and the south (and much lower than elevations further to the north and to the south). Agency Creek originates near Lemhi Pass and flows in a west, southwest, and west direction to join the north oriented Lemhi River near the town of Tendoy. Cow Creek is a northwest oriented Agency Creek tributary. South of Cow Creek is southwest and south oriented Yearian Creek (not labeled in figure 3), which flows to the south edge of figure 3 and which joins the north oriented Lemhi River south of figure 3 as a barbed tributary. East of Lemhi Pass Trail Creek flows in an east-southeast direction, with a northward jog to the east edge of figure 3 and joins Horse Prairie Creek east of figure 3. Frying Pan Creek, which has a North and South Fork, is located south of Trail Creek and flows in an east-northeast direction to join Trail Creek. Bear Creek is the north, east, and north oriented stream joining Trail Creek at the point where Trail Creek makes its jog to the north. Lemhi Pass is a major through valley eroded across the continental divide and was probably eroded by an east-southeast oriented flood flow diverging from a south oriented flood flow channel on the present day north oriented Lemhi River alignment, which was flowing to an east and then south oriented flood flow channel on the east oriented Horse Prairie Creek and present day north-northwest oriented Red Rock River alignments. The power line west of Lemhi Pass probably follows the route of the former flood flow channel, which has since been dismembered by headward erosion of south oriented Agency Creek tributary valleys. The south oriented Agency Creek tributary valleys suggest the presence of other diverging and converging south oriented flood flow channels on the west side of the continental divide. Perhaps the most obvious of these south oriented flood flow channels was on the alignment of the north oriented Cow Creek valley and the south oriented Yearian Creek valley. That flood channel was dismembered when the south oriented flood flow channel on the Lemhi River alignment eroded a much deeper valley and the deep west oriented Agency Creek valley then eroded headward from that deeper valley. The north-northwest oriented Cow Creek valley segment was probably eroded by a reversal of flood flow on the north and northwest end of the beheaded flood flow route to what had been the actively eroding Yearian Creek valley.

Detailed map of Cow Creek-Yearian Creek drainage divide area

Figure 4: Detailed map of Cow Creek-Yearian Creek 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 Cow Creek-Yearian Creek drainage divide area seen is less detail in figure 3. The north oriented Lemhi River can be seen along the west center edge of figure 4. The east-west continental divide is located east of figure 4. Cow Creek flows in a northwest direction in the northwest quadrant of figure 4 and north of figure 4 flows to west oriented Agency Creek. Yearian Creek flows in a southwest direction from the east edge of figure 4 (north half) to section 24 and then flows in a south direction to the south edge of figure 4 (east of center). South of figure 4 south oriented Yearian Creek joins the north oriented Lemhi River as a barbed tributary. Note how a north-to-south oriented through valley links the south oriented Yearian Creek valley with a north oriented Cow Creek tributary valley. The map contour interval for figure 4 is 40 feet and the through valley floor elevation at the drainage divide in the southeast quadrant of section 12 is 6754 feet. The hill in section 11 to the west rises to 7842 feet while a hill in section 8 to the east rises to 8150 feet suggesting the through valley is at least 1100 feet deep. The through valley is a water-eroded feature and was eroded by south oriented flood flow at a time when the deep Lemhi River valley to the west did not exist. At that time there was probably a south oriented flood flow channel on the present day north oriented Lemhi River valley alignment and the south oriented flood flow channel on the present day Cow Creek-Yearian Creek alignment diverged from and converged with that south oriented flood flow channel on the Lemhi River alignment. These diverging and converging flood flow channels were just a small part of a much larger complex of diverging and converging flood flow channels crossing the entire region. Another diverging flood flow channel carried floodwaters across the present day continental divide in the Lemhi Pass valley to the present day Trail Creek-Horse Prairie Creek alignment and then probably in a south-southeast direction on the present day Red Rock River alignment with the water again crossing the present day continental divide at Monida Pass south and east of Lima, Montana (see figure 1 for Interstate highway route).

Nip and Tuck Creek-Little Eightmile Creek drainage divide area

Figure 5: Nip and Tuck Creek-Little Eightmile Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 5 illustrates the Nip and Tuck Creek-Little Eightmile Creek drainage divide area south and east of figure 3 and includes an overlap area with figure 3. The east-west continental divide serves as the state line and extends from the north edge of figure 5 (west half) in a southeast direction to Grizzly Hill in the southeast quadrant of figure 5 and then in a northeast direction to the east edge of figure 5 (just south of center). The Lemhi River flows in a northwest direction across the southwest corner of figure 5. Yearian Creek flows in a southwest and south direction across the northwest corner of figure 5 and joins the northwest and north oriented Lemhi River as a barbed tributary a short distance west of the west center edge of figure 5. Little Eightmile Creek is a south-southwest oriented stream located west of Grizzly Hill in the south center area of figure 5 and flowing to the south edge of figure 5 (west of center) to join the northwest oriented Lemhi River as a barbed tributary. The South Fork Divide Creek originates on the northeast side of Grizzly Hill in the southeast quadrant of figure 5 and flows in a northeast direction to join the North Fork Divide Creek and to become Divide Creek and to join north-northwest and north oriented Horse Prairie Creek (not labeled in figure 5), which flows across the northeast corner of figure 5. Nip and Tuck Creek is a northeast oriented Horse Prairie Creek tributary north of Divide Creek. North of Nip and Tuck Creek is northeast and north oriented Black Canyon Creek, which joins Horse Prairie Creek north of figure 5. Note how the east oriented North Fork Divide Creek and Nip and Tuck Creek are linked by through valleys with the Little Eighmile Creek headwaters. The map contour interval for figure 5 is 40 meters and the Nip and Tuck Creek-Little Eightmile Creek through valley floor elevation at the continental divide is between 2520 and 2560 meters. Grizzly Hill to the south rises to more than 2800 meters while even higher elevations can be found on the continental to the northwest suggesting the through valley is at least 240 meters deep. A much deeper through valley can be seen at Bannock Pass, which crosses the continental divide near the east edge of figure 5 and which is better seen and discussed in figures 7 and 8 below. The through valleys crossing the continental divide in the east half of figure 5 were probably eroded by diverging and converging flood flow channels moving floodwaters to a southeast oriented flood flow channel on the present day northwest oriented Lemhi River valley alignment. At that time the Beaverhead Mountains were just beginning to emerge and floodwaters eroded deep valleys into the rising mountain ridge. In time the mountain uplift succeeded in blocking the south oriented flood flow and forced a reversal of flood flow that created the north oriented Horse Prairie Creek drainage system seen north of the continental divide.

Detailed map of Nip and Tuck Creek-Little Eightmile Creek drainage divide area

Figure 6: Detailed map of Nip and Tuck Creek-Little Eightmile 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 Nip and Tuck Creek-Little Eightmile Creek drainage divide area seen in less detail in figure 5. The continental divide serves as the county line and extends from the west edge of figure 6 (near northwest corner) to the south edge of figure 6 (slightly west of center). Little Eighmile Creek flows in a southwest direction from section 9 to near the southwest corner of figure 6. Nip and Tuck Creek flows in an east and east-northeast direction from section 36 to near the northeast corner of figure 6. A through valley near the north edge of section 9 (south of the continental divide-the continental divide serves as the boundary between regions surveyed at different times) and the northwest corner of section 6 (north of the continental divide) links a northeast oriented Nip and Tuck Creek tributary valley with the southwest oriented Little Eightmile Creek valley. The map contour interval for figure 6 is 40 feet and the through valley floor elevation at the drainage divide is between 8360 and 8400 feet. Following the continental divide to the northwest leads to elevations exceeding 9160 feet. Grizzly Hill to the south rises to more 9280 feet suggesting the through valley is at least 760 feet deep. A slightly shallower through valley near the southeast of section 9 (west of continental divide) links the east-northeast oriented North Fork Divide Creek valley with the southwest oriented Little Eightmile Creek valley. The floor elevation of this second through valley at the continental divide is between 8440 and 8480 feet. These through valleys were eroded by diverging and converging flood flow channels probably moving floodwaters in a southwest direction to a southeast oriented flood flow channel on the present day northwest oriented Lemhi River alignment. At that time the deep Lemhi River valley to the southwest did not exist and the southeast oriented flood flow channel on that alignment was at a much higher elevation relative the through valley floor elevations than it is today. Likewise the deep Horse Prairie Creek valley and tributary valleys north and east of the continental divide did not exist at that time and floodwaters were flowing on surfaces equivalent in elevation or higher than the present day through valley floor elevations. Headward erosion of deep valleys on both side of the present day continental divide combined with crustal warping that raised the Beaverhead Mountains and perhaps the entire region caused massive flood flow reversals that created the north oriented drainage systems seen today and the present day continental divide.

Horse Prairie Creek-Canyon Creek drainage divide area

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

Figure 7 illustrates the Horse Prairie Creek-Canyon Creek drainage divide area east of figure 5 and includes an overlap area with figure 5. The continental divide serves as the Montana-Idaho state line and extends from the west edge of figure 7 (north half) to near the southeast corner of figure 7. Bannock Pass is a deep pass eroded across the continental and is located near the center of figure 7. The northeast and south oriented stream located south of Bannock Pass is Canyon Creek, which flows to the south center edge of figure 7 and which south of figure 7 joins the northwest oriented Lemhi River as a barbed tributary. Deadman Pass crosses the continental divide in the southeast quadrant of figure 7 and is located east of Bannock Pass. Whiskey Spring Creek flows in a northwest direction along the continental divide on the south side of Deadman Pass and then turns to flow in a southwest direction to join south oriented Canyon Creek. The northwest oriented stream just north of Deadman Pass is Horse Prairie Creek, which originates near the east edge of figure 7 (just north of continental divide) and which flows to the north center edge of figure 7. North of figure 7 Horse Prairie Creek turns to flow in a north, northeast, and east direction (see figures 1 and 2). The northeast oriented stream flowing parallel to the continental divide just north of Bannock Pass is Divide Creek, which joins north-northwest oriented Horse Prairie Creek in the north center area of figure 7. The map contour interval for figure 7 is 40 meters and the Bannock Pass elevation at the continental divide is between 2320 and 2360 meters. Grizzly Hill to the west rises to more than 2800 meters and elevations greater than 2800 meters can be seen near the southeast corner of figure 7 suggesting Bannock Pass is at least 440 meters deep. The continental divide elevation is significantly lower for a significant distance between Grizzly Hill in the southwest quadrant of figure 7 and the higher mountains in the southeast corner figure 7 with Bannock Pass and Deadman Pass representing two of the lowest points. This lower region along the continental divide was probably eroded by massive south oriented flood flow, with the Bannock Pass and Deadman Pass valleys being eroded as deeper channels in the floor of what was once a major south oriented flood eroded valley. At that time the deep east oriented Horse Prairie Creek valley to the north did not exist and elevations north of the present day continental divide were at least as high in a relative sense as the present day continental divide elevation. Floodwaters were flowing to a south oriented flood flow channel on the present day northwest oriented Lemhi River alignment. Headward erosion of the deep east oriented Horse Prairie Creek valley north of figure 7 beheaded and reversed the flood flow so as to create the northwest and north oriented Horse Prairie Creek drainage system and the drainage divide that has since become the east-west continental divide.

Detailed map of Divide Creek-Canyon Creek drainage divide area

Figure 8: Detailed map of Divide Creek-Canyon Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 8 provides a detailed topographic map of the Divide Creek-Canyon Creek drainage divide area seen in less detail in figure 7 and also includes overlap areas with figure 6. The continental divide serves as the state and county line and extends in a south-southeast direction from the west edge of figure 8 (north half) to Grizzly Hill near the southwest corner of figure 8. From Grizzly Hill the continental divide extends in a northeast direction almost to the east edge of figure 7 before turning in a southeast direction. Nip and Tuck Creek flows in an east-northeast direction near the north edge of the northwest quadrant of figure 8. The South Fork Divide Creek originates east of Grizzly Hill in the southwest quadrant of figure 8 and flows in a northeast direction to join east and east-northeast oriented North Fork Divide Creek and to form northeast oriented Divide Creek, which flows to the north edge of figure 8 (east half) and joins Horse Prairie Creek north of figure 8. Bannock Pass is located in the south half of section 2 (on Montana side of continental divide). The map contour interval for figure 8 is 40 feet and the Bannock Pass elevation is shown as 7681 feet. The top of Grizzly Hill has an elevation greater than 9280 feet and further to the northwest continental divide elevations rise even higher. East of figure 8 the continental divide elevations rise to more than 10,000 feet suggesting the Bannock Pass depth is at least 1400 feet deep. The entire distance between Bannock Pass and Deadman Pass, which is east of figure 8, is lower than 8000 feet suggesting there was once a broad south oriented flood flow channel eroded across the region. At that time elevations north of the present day continental divide were at least as high in a relative sense as the present day continental divide elevation. Crustal warping that raised the Beaverhead Mountains and probably the entire region combined with headward erosion of deep valleys on both sides of the present continental divide caused massive flood flow reversals north and south of the present day continental divide that created the north oriented Horse Prairie Creek and Beaverhead River drainage system north of the continental divide and the northwest and north oriented Lemhi River drainage system south and west of the present day continental divide.

Canyon Creek-Medicine Lodge Creek drainage divide area

Figure 9: Canyon Creek-Medicine Lodge Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 9 illustrates the Canyon Creek-Medicine Lodge Creek drainage divide area south and east of figure 7 and includes an overlap area with figure 7. The east-west continental divide extends in a south-southeast direction from near the north center edge of figure 9 to the south edge of figure 9 (east half). East of the continental divide the north oriented stream flowing from the east edge of figure 9 (south of center) to near the northeast corner of figure 9 is Medicine Lodge Creek, which north of figure 9 flows to Horse Prairie Creek and then to the north-northeast oriented Beaverhead River. Note how Medicine Lodge Creek has several southeast oriented tributaries in the northeast quadrant of figure 9. These barbed tributaries provide evidence the present day north oriented Medicine Lodge Creek valley was probably initiated as a south oriented flood flow channel. Note also how north-northeast oriented Morrison Creek is linked by a shallow through valley or mountain pass across the continental divide with south-southwest oriented Meadow Creek, which flows to northwest oriented Big Bear Creek. That shallow through valley is evidence of what was once a diverging south oriented flood flow channel that continued in a south direction south of the present day Big Bear Creek valley to the south edge of figure 9 and then to what was then a southeast oriented flood flow channel on the present day northwest oriented Lemhi River valley alignment. The edge of the northwest oriented Lemhi River valley is located in the southwest corner of figure 9. Canyon Creek is the south oriented stream flowing from the north edge of figure 9 (west half) to Leadore and then turning to flow in a west direction to the west edge of figure 9 and to join the northwest oriented Lemhi River. Cruikshank Creek is a west oriented tributary to south oriented Canyon Creek and Wildcat Creek and Frank Hall Creek are north and northwest oriented Cruikshank Creek tributaries. Note how the north and northwest oriented Wildcat and Frank Hall Creek valleys are linked by well-defined through valleys with south oriented tributary valleys to southwest and west oriented Hawley Creek. The map contour interval for figure 9 is 40 meters and the Frank Hall Creek-Quaking Asp Creek through valley floor elevation at the drainage divide is between 2560 and 2600 meters. Elevations on the hill to the southwest rise to more than 2880 meters while Elk Mountain to the northeast rises to more than 3000 meters. The Wildcat Creek-Rocky Canyon through valley floor elevation at the drainage divide is between 2600 and 2640 meters and the hill to the southwest rises to 2917 meters. These elevations suggest the through valleys are approximately 300 meters deep. Diverging and converging flood flow channels eroded these through valleys. At that time south and southeast oriented flood flow channels were being eroded along and across the present day continental divide, which was subsequently formed as crustal warping caused floodwaters to converge in deeper flood flow channels on either side of the rising mountain ridge and present day drainage divide.

Detailed map of Frank Hall Creek-Quaking Asp Creek drainage divide area

Figure 10: Detailed map of Frank Hall Creek-Quaking Asp 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 Frank Hall Creek-Quaking Asp Creek drainage divide area seen in less detail in figure 9. The east-west continental divide is the labeled county line and extends in a south-southeast direction from the north edge to the south edge of figure 10 (east half). Frank Hall Creek originates in section 31 and flows in a west and south direction into section 6 where it turns to flow in a northwest direction to section 36 where it enters west oriented Cruikshank Creek and west of figure 10 the water reaches south oriented Canyon Creek before flowing to the northwest oriented Lemhi River. Quaking Asp Creek originates in section 5 near the continental divide and the Frank Hall Creek headwaters and flows in a south direction to the south center edge area of figure 10. South of figure 10 the water joins other streams and turns to flow in a southwest and west direction to join the northwest oriented Lemhi River. Note in the southeast corner of section 6 a deep through valley linking the northwest oriented Frank Hall Creek valley with the south oriented Quaking Asp Creek valley. The map contour interval for figure 10 is 40 feet and the through valley floor elevation at the drainage divide is between 8480 and 8520 feet. The hill in the southeast corner of section 12 to the southwest of the through valley rises to 9501 feet and a hill west of figure 10 on the same ridge rises to 9567 feet while elevations greater than 10,000 feet can be seen on the continental divide ridge to the east. These elevations suggest the through valley is at least 1000 feet deep. The through valley is a water-eroded valley and was eroded by south oriented flood flow moving to the actively eroding Quaking Asp Creek valley. At that time there was no high continental divide ridge to the north and floodwaters could freely flow across the region. Headward erosion of the deeper Canyon Creek valley and its west oriented Cruikshank Creek valley captured the southeast and south oriented flood flow. Floodwaters on the northwest end of the beheaded flood flow route reversed flow direction to erode the northwest oriented Frank Hall Creek valley. Apparently south oriented flood flow was still moving along a surface equivalent in elevation to the present day continental divide elevation east of the reversed flood flow route and that south oriented flood flow eroded the west and south oriented Frank Hall Creek headwaters valley before being captured by headward erosion of the west-southwest oriented Rough Canyon 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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