Abstract:

This essay uses topographic map evidence to interpret landform origins along the continental divide between Medicine Lodge Creek in Beaverhead County, Montana and the Lemhi River in Lemhi County, Idaho. The continental divide follows the crest of the Beaverhead Mountains. The Lemhi River flows in a north-northwest direction on the west side of the Beaverhead Mountains on the north end of a former south-southeast oriented flood flow channel with the south end of the former flow channel draining to south-southeast oriented Birch Creek, which eventually disappears as a surface stream, but which is headed toward the southwest oriented Snake River in eastern Idaho. Medicine Lodge Creek flows in a north direction on the east side of the Beaverhead Mountains in a former south-southeast oriented flood flow channel with the middle section of the former flood flow channel now being drained by southeast oriented Cabin Creek and north oriented Deadman and Big Sheep Creeks and the south end of the former flood flow channel being drained by south-southeast oriented Medicine Lodge Creek, which eventually disappears as a surface stream, but which also is headed toward the southwest oriented Snake River in eastern Idaho. The floor of Bannock Pass, which is where the eastern former flood flow channel crosses the present day continental divide, is approximately 1000 meters lower than adjacent mountain tops. In addition to the large through valleys on either side of the Beaverhead Mountains and Bannock Pass where the eastern through valley crosses the present day continental divide there are numerous other through valleys or passes crossing the continental divide and other drainage divides in the Beaverhead Mountains. These other through valleys provide evidence of numerous diverging and converging flood flow channels that crossed the region as the Beaverhead Mountains were beginning to emerge. Emergence of the high Beaverhead Mountains eventually caused floodwaters to flow in a south-southeast direction on either side of the emerging mountain mass. In time regional uplift including Beaverhead Mountains uplift created topographic barriers to the south-southeast oriented flood flow while headward erosion of deep valleys north of the south-southeast oriented flood flow channels caused massive flood flow reversals on both sides of the Beaverhead Mountains to create the north oriented drainage systems seen today. Floodwaters are interpreted to have been derived from the western margin of a thick North American ice sheet and were flowing in a south and south-southeast direction from western Canada across western Montana to and across the study region.

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 Medicine Lodge 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 Medicine Lodge 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.

Medicine Lodge Creek-Lemhi River drainage divide area location map

Figure 1: Medicine Lodge 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 Medicine Lodge Creek-Lemhi River drainage divide along the continental divide in Beaverhead County, Montana and Lemhi County, Idaho and illustrates in the north a region in southwest Montana with Idaho to the south and west and the northwest corner of Wyoming appearing along the south half of the east edge of figure 1. Dillon is a Montana community located near the center of figure 1 and the Beaverhead River flows in a north-northeast direction through Dillon and joins the south and northeast oriented Big Hole River near Twin Bridges to form the northeast and east oriented Jefferson River, which north of figure 1 joins the Madison and Gallatin Rivers to form the north oriented Missouri River with water eventually reaching the Gulf of Mexico. The Beaverhead River is formed at an unlabeled reservoir (Clark Canyon Reservoir) at the confluence of the north-northwest oriented Red Rock River and an unlabeled east oriented tributary (Horse Prairie Creek). The unlabeled north oriented Horse Prairie Creek tributary west of the Tendoy Mountains is the Medicine Lodge Creek of primary concern in this essay. West of the north oriented Medicine Lodge Creek valley are the Beaverhead Mountains, the crest of which is the east-west continental divide which serves as the Montana-Idaho state line. West of the Beaverhead Mountains is the northwest oriented Lemhi River, which joins the north oriented Salmon River near the town of Salmon. The Salmon River flows in a north direction from the west edge of figure 1 (near southwest corner) to the towns of Challis, Ellis, Salmon, and North Fork and then makes an abrupt turn to flow in a west direction. West of figure 1 the Salmon River eventually joins the north oriented Snake River with water ultimately reaching the Pacific Ocean. The Snake River originates in Yellowstone National Park east of figure 1 and flows in a south, southwest and west direction south of figure 1 before turning to flow in a north direction west of figure 1. The Medicine Lodge Creek-Lemhi River drainage divide area investigated in this essay is located in the Beaverhead Mountains and is west of north oriented Medicine Lodge Creek headwaters and east of the northwest oriented Lemhi River headwaters.

Before looking at detailed maps of the Medicine Lodge Creek-Lemhi River drainage divide area a brief look at the big picture erosion history will help understand discussions related to the more detailed maps shown below. 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 Beaverhead Mountains and also on both sides of the Tendoy Mountains were initially developed as south and southeast oriented flood flow channels on either side of the rising mountain ranges. Prior to development of deep flood flow channels on both sides of the emerging Beaverhead Mountains floodwaters flowed along and across what is today the Beaverhead Mountains crest ridge, which is today the east-west continental divide, to a southeast oriented flood flow channel on the Lemhi River alignment and then on the alignment of south-southeast oriented Birch Creek (seen near south edge of figure 1, just south of Beaverhead Mountains) to the southwest oriented Snake River valley south of figure 1.

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 with headward erosion of a deep west oriented valley to the present day location of North Fork, Idaho eventually caused a massive flood flow reversal that created the present day north oriented Salmon River drainage system seen in figure 1. Perhaps at about the same time headward erosion of a deep southeast oriented valley along the Pioneer Mountains north flank (now the southeast oriented Big Hole River valley segment) caused a major flood flow reversal west of the Pioneer Mountain that created the present day north oriented Big Hole River drainage system there. A major reversal of flood flow in what is today the Missouri River drainage basin in southwest Montana occurred when a deep northeast oriented valley eroded headward across the south and southeast oriented flood flow routes in Montana from space in the deep “hole” the melting ice sheet was opening up. Headward erosion of this deep northeast oriented valley beheaded south and southeast oriented flood flow channels and floodwaters on north and northwest ends of those beheaded flood flow routes reversed flow direction to create what are today north oriented drainage systems. During these massive flood flow reversals south oriented flood flow along one route would 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 the rising mountain ranges contributed to the emergence of present day mountain ranges.

Detailed location map for Medicine Lodge Creek-Lemhi River drainage divide area

Figure 2: Detailed location map Medicine Lodge 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 Medicine Lodge 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 in a south-southeast direction along the Beaverhead Mountains crest from the north edge of figure 2 (just north of Lemhi Pass near northwest corner) to the south center edge of figure 2 and then in a north, east, and northeast direction to the east edge of figure 2 (just east of Monida Pass). Beaverhead County in Montana is east and north of the continental divide and Lemhi County in Idaho is west of the continental divide and Clark County, Idaho is located south of the continental divide in the east half of figure 2. Green shaded areas are National Forest lands, which generally are located in mountainous regions. Clark Canyon Reservoir can be seen near the north center edge of figure 2. The Red Rock River flows in a west and north-northwest direction from Lima Reservoir (along east edge of figure 2, south half) to Clark Canyon Reservoir and joins northwest, north, and east oriented Horse Prairie Creek (the east oriented segment is mostly north of figure 2) to form the north-northeast oriented Beaverhead River (not seen in figure 2). The Tendoy Mountains are located just west of the north-northwest oriented Red Rock River segment and are south of Clark Canyon Reservoir. Medicine Lodge Creek is the north oriented stream west of the Tendoy Mountains, which flows to a southeast oriented Horse Prairie Creek segment just west of Clark Canyon Reservoir. Note how a southeast oriented Medicine Lodge Creek tributary is aligned with the northwest oriented Horse Prairie Creek headwaters on the northeast side of the continental divide. South and west of the continental divide in the west half of figure 2 the Lemhi River is formed near the town of Leadore at the confluence of northwest and north-northwest oriented Eighteenmile Creek and west oriented Hawley Creek. South of the north-northwest oriented Eighteenmile Creek segment is northeast and northwest oriented Divide Creek and south of Divide Creek is south-southeast oriented Mud Creek, which south of figure 2 flows to south-southeast oriented Birch Creek, which eventually disappears as a surface stream, but which is headed toward the southwest oriented Snake River headwaters located south of figure 2.

Medicine Lodge Creek-Hawley Creek drainage divide area

Figure 3: Medicine Lodge Creek-Hawley 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 Medicine Lodge Creek-Hawley Creek drainage divide area. The east-west continental divide and state line is shown with a dashed line and extends from the north center edge of figure 3 to the south edge of figure 3 (east of center). East of the continental divide Medicine Lodge Creek flows in a north-northwest and north direction from the east edge of figure 3 to the north edge of figure 3 (near northeast corner). Note the southeast oriented barbed tributaries flowing to north oriented Medicine Lodge Creek near the north edge of figure 3. North of figure 3 Medicine Lodge Creek water eventually reaches the Missouri River and then flows to the Gulf of Mexico. West of the continental divide north-northwest oriented Eighteenmile Creek can be seen in the southwest corner of figure 3. Further north is southwest and west-northwest oriented Hawley Creek. Canyon Creek is a south-southwest oriented stream flowing from the north edge of figure 3 (near northwest corner) to the west edge of figure 3. West of figure 3 Eighteenmile Creek, Hawley Creek, and Canyon Creek join to form the north-northwest oriented Lemhi River. Note how Canyon Creek is a barbed Lemhi River tributary. Also note how west oriented Cruikshank Creek joins south oriented Canyon Creek near the northwest corner of figure 3 and how north oriented Wildcat Creek and Frank Hall Creek flow to Cruikshank Creek and are linked by through valleys to south oriented streams flowing to southwest and west-northwest oriented Hawley Creek. The map contour interval for figure 3 is 40 meters and the through valleys are defined by more than 6 contour lines on a side suggesting they are at least 200 meters deep. The through valleys were eroded as south and southeast oriented flood flow channels moving floodwaters adjacent to and parallel to the present day continental divide. At that time the Beaverhead Mountains did not stand high above the surrounding valleys and floodwaters could freely flow across what is today a high mountain range. To see where the flood flow channels were headed look at the drainage divides to the south and southeast. Hawley Creek is formed at the confluence of south-southeast oriented Reservoir Creek and of northwest and west oriented Big Bear Creek. Note how northwest-to-southeast oriented through valleys link the Reservoir Creek valley with the northwest oriented Big Bear Creek valley segment. Headward erosion of the deep southwest oriented Hawley Creek valley captured the south and southeast oriented flood flow and floodwaters on the northwest end of the beheaded flood flow channel reversed flow direction to create the northwest oriented Big Bear Creek drainage system. Next headward erosion of the deep south-southwest oriented Reservoir Creek valley captured the southeast oriented flood flow. Finally (at least in this sequence of captures) headward erosion of the west oriented Cruikshank Creek valley beheaded and reversed the south and southeast oriented flood flow to create the north oriented Wildcat and Frank Hall Creek drainage routes. Many similar captures of flood flow routes are recorded in the figure 3 evidence, including the beheading and reversal of flood flow routes that once moved floodwaters across the present day east-west continental divide.

Detailed map of Reservoir Creek-Big Bear Creek drainage divide area

Figure 4: Detailed map of Reservoir Creek-Big Bear 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 Reservoir Creek-Big Bear Creek drainage divide area seen is less detail in figure 3. The east-west continental divide serves as the state and county line and is located along the crest of the Beaverhead Mountains extending from the north edge of figure 4 (east of center) to near the southeast corner of figure 4. Reservoir Creek flows in a southwest and south-southwest direction from the north edge of figure 4 (west of center) to the west edge of figure 4 (south half) where it is joined by Big Bear Creek to form southwest and west-northwest oriented Hawley Creek (not seen in figure 4). Big Bear Creek flows in a northwest, west-southwest, and northwest direction from the south center edge of figure 4 to join Reservoir Creek at the west edge of figure 4. Poison Creek is a south-southwest oriented stream originating in section 22 and joining Big Bear Creek in section 33. South-southwest oriented Big Bear Creek tributaries further east of Poison Creek are south-southwest oriented Bog Creek and Meadow Creek. Note through valleys crossing the Reservoir Creek-Poison Creek drainage divide. For example near the south edge of section 21 a through valley links a north-northwest oriented Reservoir Creek tributary valley with a south oriented Poison Creek tributary valley. The map contour for figure 4 is 40 feet and the through valley floor elevation at the drainage divide is between 7960 and 8000 feet. The ridge to the southwest rises to 8294 feet and further to the southwest to 8531 feet while to the east elevations rise much higher suggesting the through valley may be more than 500 feet deep. A similar through valley in the southwest corner of section 27 links the Poison Creek valley with the Bog Creek valley and is approximately 200 feet deep. Perhaps just as interesting are through valleys (or high mountain passes) crossing the continental divide and linking the south-southwest oriented Big Bear Creek tributary valleys with northeast oriented Medicine Lodge Creek tributary valleys. For example, note how the south-southwest oriented Meadow Creek valley is linked by a through valley (or mountain pass) with the northeast oriented Morrison Creek valley.  The through valley is defined by at least 7 contour lines on each side suggesting it is at least 240 feet deep. Further to the northwest along the continental divide an even deeper through valley links the south-southwest oriented Bog Creek valley with the valley of the northwest oriented stream flowing to the northeast corner of figure 4. Diverging and converging south oriented flood flow channels eroded these and other through valleys seen in figure 4 at a time when the Beaverhead Mountains did not form the topographic barrier they do today. At the time floodwaters flowed across the region the Beaverhead Mountains were just beginning to emerge and the deep valley s were being eroded into the emerging mountain mass.

Big Bear Creek-Cabin Creek drainage divide area

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

Figure 5 illustrates the Big Bear Creek-Cabin Creek drainage divide area south and east of figure 3 and includes an overlap area with figure 3. The east-west continental divide is shown with a dashed line extending from the north edge of figure 5 (west half) to the south edge of figure 5 (west half). West of the continental divide Big Bear Creek flows in a northwest and west direction to join south-southwest oriented Reservoir Creek near the west edge of figure 5 (west half). Note north oriented Wheetip Creek, which flows to northwest oriented Big Bear Creek, and how a through valley links the Wheetip Creek valley with the southwest oriented Dry Canyon valley (see southwest corner of figure 5). The map contour interval for figure 5 is 40 meters in the west and 50 meters in the east and the through valley is more than 400 meters deep. The through valley was eroded by south oriented flood flow prior to the reversal of flood flow that created the northwest oriented Big Bear Creek drainage route. East of the continental divide Medicine Lodge Creek flows in a north-northwest direction to the north center edge of figure 5. South of the north oriented Medicine Lodge Creek valleys is the southeast oriented Cabin Creek valley, with Cabin Creek flowing to the southeast corner of figure 5. Note how a deep northwest-to-southeast oriented through valley links the north oriented Medicine Lodge Creek valley with the south oriented Cabin Creek valley. The through valley floor elevation at the drainage divide is between 2400 and 2450 meters. The mountain ridge east of the through valley is in the Tendoy Mountains and Sourdough Peak reaches an elevation of 2917 meters and elevations in the Beaverhead Mountains to the west rise even higher suggesting the through valley is at least 450 meters deep. Interestingly south and east of figure 5 Cabin Creek joins north oriented Big Sheep Creek, which flows to the north-northwest oriented Red Rock River. Before leaving figure 5 note Indian Creek in the south center region. Indian Creek flows in a southeast direction from near the continental divide and then makes an interesting jog to the north around Island Butte to join south oriented Cow Creek and to form southeast oriented Cabin Creek. Island Butte appears to be an erosional residual, which was located between diverging and converging south oriented flood flow channels. Headward erosion of a deeper flood flow channel on the Cabin Creek alignment beheaded and reversed flood flow on the western flood flow channel, which resulted in a reversal of flood flow that captured flood flow on the southeast oriented Indian Creek alignment and diverted that captured flood flow in a north direction to join the southeast oriented flood flow east of Island Butte. You may want to ask where did the flood flow on the southeast oriented Indian Creek alignment come from? Note how the southeast-oriented Indian Creek valley on the east side of the continental divide is on the same alignment as the northwest oriented Big Bear Creek valley on the west side of the continental divide. Also note the northwest-to-southeast oriented through valley linking the Big Bear Creek and Indian Creek valleys, which was eroded by the southeast oriented flood flow channel that supplied the flood water.

Detailed map of Big Bear Creek-Indian Creek drainage divide area

Figure 6: Detailed map of Big Bear Creek-Indian 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 Big Bear Creek-Indian Creek drainage divide area seen in less detail in figure 5. The east-west continental divide serves as the county and state line and extends from the north edge of figure 6 (west half) to the south edge of figure 6 (west half). Big Bear Creek flows in a northwest direction to the northwest corner of figure 6. East of the continental divide the east oriented Cabin Creek headwaters can be seen in the northeast quadrant of figure 6. Indian Creek is the east and south-southeast oriented stream near the south edge of figure 6. Note how a through valley links the northwest oriented Big Bear Creek valley (in section 13 west of the continental divide) with a south-southeast oriented Indian Creek tributary valley (in section 15 east of the continental divide). The map contour interval for figure 6 is 40 feet and the through valley floor elevation is between 8880 and 8920 feet (where it crosses the continental divide). North of the through valley the continental divide rises to 9541 feet near the north edge of figure 6. South of the through valley the continental divide rises to more than 9600 feet near the south edge of figure 6. These elevations suggest the through valley is as much as 600 feet deep. The through valley was eroded by southeast oriented flood flow moving across what is today the east-west continental divide. The flood flow was moving in flood flow channels that paralleled and crossed the present day continental divide. At that time there was no continental divide and there were no high Beaverhead Mountains. The Beaverhead Mountains emerged as the floodwaters were flowing across them. At first floodwaters eroded deep valleys or flood flow channels into the emerging mountain mass, but in time the rising mountains caused floodwaters to be diverted in different directions to deeper flood flow channels being eroded on either side of the emerging mountain range. The flood flow diversion process was complex as deep valleys eroded headward from the deeper valleys on either side of the emerging mountain mass into the mountain mass to capture the southeast oriented flood flow and beheaded and reversed the southeast oriented flood flow channels. Reversals of flood flow on north ends of beheaded flood flow routes created north oriented drainage routes seen today. Often these flood flow reversals captured flood flow from nearby yet to be beheaded flood flow channels, which supplied water volumes required to create significant north oriented drainage systems and erode deep north oriented valleys.

Eighteenmile Creek-Meadow Creek drainage divide area

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

Figure 7 illustrates the Eighteenmile Creek-Meadow Creek drainage divide area south of figure 5 and includes an overlap area with figure 5. The continental divide serves as the state and county line and is shown with a dashed line extending from the north edge of figure 7 (west of center) to the south edge of figure 7 (east of center). The southeast oriented stream in the northeast corner of figure 7 is Cabin Creek, which east of figure 7 joins north oriented Big Sheep Creek with water flowing to the north-northwest oriented Red Rock River. Northeast, north-northeast, and north oriented streams east of the continental divide are Cabin Creek or Big Sheep Creek tributaries. Meadow Creek is a northeast oriented Big Sheep Creek tributary originating near the continental divide (south of center of figure 7) and flowing to the east edge of figure 7 (north half). West of the Meadow Creek headwaters, on the continental divide west side, are headwaters of southwest and west oriented Pass Creek, which flows to west-northwest oriented Eighteenmile Creek. West of figure 7 Eighteenmile Creek joins Hawley Creek and Canyon Creek to form the northwest oriented Lemhi River. Note how a well-defined through valley links the west oriented Pass Creek valley with the east oriented Meadow Creek valley. The map contour interval for figure 7 is 50 meters and the through valley (or pass) elevation at the continental divide is between 2900 and 2950 meters. Eighteenmile Peak to the south rises to 3386 meters and the continental divide rises to more than 3250 meters near the north edge of figure 7 suggesting the through valley or pass could be as much as 300 meters deep. North of the Meadow Creek-Pass Creek through valley is a through valley linking the Meadow Creek valley with the southwest oriented Chamberlain Creek valley and north of that is a through valley linking the northeast oriented Coyote Creek valley with the southwest oriented Chamberlain Creek valley. Converging and diverging flood flow channels eroded these multiple through valleys (or passes) across the continental divide at a time when the Beaverhead Mountains were just beginning to emerge. Floodwaters were probably flowing in a southeast direction along the present day northwest oriented Lemhi River alignment and were flowing to the south-southeast oriented flood flow channel that crossed the continental divide again at Bannock Pass seen in figure 10. Later the floodwaters were captured by reversed flood flow on what is today the north oriented Big Sheep Creek alignment (north and east of figure 7) and  flowed to the present day north-northwest oriented Red Rock River alignment, which at that time may still have been a south-southeast oriented flood flow channel, although subsequently the flood flow in that channel was reversed to create the present north-northwest oriented Red Rock River drainage route. West of the continental divide flood flow was also subsequently reversed to create the northwest oriented Lemhi River drainage system. These flood flow reversals occurred as the Beaverhead Mountains were emerging and the headward erosion of deep valleys on both sides of the rising Beaverhead Mountains contributed to the Beaverhead Mountains emergence as a high mountain range.

Detailed map of Pass Creek-Meadow Creek drainage divide area

Figure 8: Pass Creek-Meadow 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 Pass Creek-Meadow Creek drainage divide area seen in less detail in figure 7. The east-west continental divide serves as the state and county line and extends from the north edge of figure 8 (east of center) to the south edge of figure 8 (slightly east of center). The map contour interval for figure 8 is 40 feet except for a region in the northeast quadrant where the contour interval is 20 feet. Eighteenmile Creek flows in a northwest direction across the southwest quadrant of figure 8 and west of figure 8 flows to the northwest oriented Lemhi River. Pass Creek originates just west of the continental divide in section 5 and flows in a southwest and west direction to join Eighteenmile Creek near west edge of figure 8. East of the Pass Creek headwaters, on the continental divide east side, are headwaters of Meadow Creek, which flows in an east, north-northeast, and east direction to the east edge of figure 8 (north half) and east of figure 8 flows to north oriented Big Sheep Creek, which then flows to the north-northwest oriented Red Rock River. Note the well-defined mountain pass or through valley linking the east oriented Meadow Creek valley with the west oriented Pass Creek valley. The elevation at the pass is shown as being 9592 feet. Near the south edge of figure 8 the continental divide rises to 11,024 feet. The high point on the continental divide north of the pass and seen in figure 8 is 9911 feet, although a short distance north of figure 8 the continental divide rises to 10,684 feet. Depending on which elevations are used the pass could be 300 feet deep or could be more than 1000 feet deep. As seen in figure 7 north of the Pass Creek-Meadow Creek pass are two other deep passes, one of which can be seen in figure 8. The Chamberlain Creek-Meadow Creek pass or through valley is located on the continental divide segment between section 29 in Idaho and section 24 in Montana and has an elevation at the drainage divide of between 9120 and 9160 feet, which is considerable lower than the Pass Creek-Meadow Creek pass elevation to the south. This Chamberlain Creek-Meadow Creek Pass could be from 600 to 1500 feet deep. The Chamberlain Creek-Coyote Creek pass is located just north of figure 8 and has an elevation at the drainage divide of 9243 feet. These passes are remnants of water-eroded valleys that once crossed the emerging Beaverhead Mountains and that were eroded by diverging and converging flood flow channels. Floodwaters were probably flowing in southeast direction to the region seen in figure 8 and east of figure 8 were eventually captured by the reversal of flood flow that created the present day north oriented Big Sheep Creek drainage system.

Divide Creek-Mud Creek drainage divide area

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

Figure 9 illustrates the Divide Creek-Mud Creek drainage divide area south and west of figure 7 and includes an overlap area with figure 7. The east-west continental divide is located near the northeast corner of figure 9 and serves as the county and state line. With the exception of the northeast corner region figure 9 illustrates an area west of the continental divide. The Beaverhead Mountains are located along the east margin of figure 9 while the Lemhi Range can be seen along the west margin of figure 9. Northwest oriented streams in the north half of the large valley or basin between the two mountain ranges converge north of figure 9 to form the northwest oriented Lemhi River. Eighteenmile Creek can be seen flowing in a west-northwest direction to the north center edge of figure 9. Divide Creek is the west and northwest oriented stream located south and west of Eighteenmile Creek, which originates on the west flank of the Beaverhead Mountains and which flows to the north edge of figure 9 (west half) and then to join Eighteenmile Creek. South of Divide Creek is south oriented Mud Creek, which flows to the south edge of figure 9 (just east of center) and then to south-southeast oriented Birch Creek, which eventually disappears as a surface stream, but which is headed toward the southwest oriented Snake River valley. [The Snake River begins in northwest Wyoming and eastern Idaho as a south and southwest oriented drainage route, but turns to flow in a west direction across southern Idaho before turning to flow in a north and northwest direction to join the west oriented Columbia River.] South-southwest oriented Mud Creek tributaries can be seen originating along the west side of the Beaverhead Mountains. Today the valley or basin between the Lemhi Range and the Beaverhead Mountains is a major northwest-to-southeast oriented through valley and links a northwest oriented drainage system with a southeast oriented drainage system. The map contour interval for figure 9 is 50 meters and the through valley floor elevation at the drainage divide is between 2150 and 2200 meters. Elevations in the Lemhi Range to the west rise to more than 3300 meters and elevations in the Beaverhead Mountains to the east also rise to more than 3300 meters suggesting the valley or basin floor is as much as 1100 meters lower in elevation than the surrounding mountain peaks. While the through valley or basin is almost certainly a structural feature it is also a former southeast oriented flood flow channel route. Regional uplift of the drainage divide area seen in figure 9 combined with headward erosion of the deep west oriented Salmon River valley segment seen in figure 1 caused a massive flood flow reversal that resulted in the northwest oriented Lemhi River drainage system and the creation the Lemhi River-Birch Creek (or Divide Creek-Mud Creek) drainage divide seen in figure 9.

Cabin Creek-Divide Creek drainage divide area at Bannock Pass

Figure 10: Cabin Creek-Divide Creek drainage divide area at Bannock Pass. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 10 provides a detailed topographic map of the Cabin Creek-Divide Creek drainage divide area at Bannock Pass located east of figure 7 and includes an overlap area with figure 7. The east-west continental divide is still shown with a dashed line and still serves as the state and county line and can be seen in the south half of figure 10, but is oriented in a west-to-east direction in the east half of figure 10 while west of figure 10 the continental divide is oriented in more of a north-to-south direction as seen in figure 7. Areas north of the continental divide including the entire west half of figure 10 drain to the north-northwest oriented Red Rock River with water eventually reaching the Gulf of Mexico. Cabin Creek can be seen flowing in a southeast direction from the northwest corner of figure 10 to join northeast oriented Meadow Creek and north oriented Deadman Creek (among others) and then making an abrupt turn to flow in a north direction across the mountain ridge to the north edge of figure 10 (west of center) as Big Sheep Creek (not labeled in figure 10). North of figure 10 Big Sheep Creek turns to flow in a northeast direction to join the north-northwest oriented Red Rock River. Little Sheep Creek flows in a west-northwest direction along the continental divide in the southeast quadrant of figure 10 and then turns to flow in a north-northeast direction to the north edge of figure 10 (near northeast corner) and then turns to flow in a north direction to join the north-northwest oriented Red Rock River north of figure 10. The Divide Creek [not the same Divide Creek as seen in figure 9] seen near the south center edge of figure 10 is south of the continental divide and drains to south-southeast oriented Medicine Lodge Creek [not the same Medicine Lodge Creek as seen north of the continental divide], which eventually disappears as a surface stream, but which is headed toward the southwest oriented Snake River, which makes a U-turn in Idaho to eventually flow in a north and northwest direction to join the west oriented Columbia River. Note Bannock Pass located on the drainage divide between north-oriented Deadman Creek and east and southeast oriented Divide Creek near the south center edge of figure 10. The map contour interval for figure 10 is 50 meters and the Bannock Pass elevation at the drainage divide is between 2300 and 2350 meters. Elevations on the continental divide south and west of figure 10 rise to more than 3300 meters while Garfield Mountain near the east edge of figure 10 rises to 3341 meters. These elevations suggest Bannock Pass could be as much as 1000 meters deep. Bannock Pass was eroded by south-southeast oriented flood flow moving from the present day north oriented Medicine Lodge Creek valley seen in earlier figures in this essay to the south oriented Medicine Lodge Creek valley south of figure 10. The south-southeast oriented flood flow route was dismembered when headward erosion of the deep Red Rock River valley to the north of figure 10 beheaded and reversed a south oriented flood flow channel on the present day north oriented Big Sheep Creek alignment. The flood flow reversal and capture was probably greatly aided by Beaverhead Mountain uplift, which was occurring as floodwaters flowed across the region. Later the south-southeast oriented flood flow channel was beheaded and reversed to create the north oriented Medicine Lodge Creek drainage system at the north end of the dismembered flood flow channel.

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.