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 Lemhi River in Lemhi County, Idaho. The east-west continental divide in the study region is located along the crest of the Beaverhead Mountains and serves as the state boundary. The Lemhi River flows in a north-northwest direction on the west side of the Beaverhead Mountains and joins the north oriented Salmon River at the study region north end. Lemhi River tributaries from the Beaverhead Mountains flow in southwest directions and join the Lemhi River as barbed tributaries. A major through valley on the east side of the Beaverhead Mountains links the north oriented Big Hole River valley with the south-southeast oriented Bloody Dick Creek valley. Big Hole River tributaries from the Beaverhead Mountains generally flow in northeast directions, as do some Bloody Dick Creek tributaries, which join south oriented Bloody Dick Creek as barbed tributaries. The southwest oriented Lemhi River tributary valleys are linked by mountain passes (notches or cols) crossing the Beaverhead Mountains crest ridge with northeast oriented Big Hole River and Bloody Dick Creek tributary valleys. Also notches (or cols) cross drainage divides between northeast oriented valleys just east of the present day continental divide. The notches (or cols) just east of the continental divide are interpreted as evidence of a former south-southeast oriented flood flow channel, which flowed along the alignment of the present day continental divide. At that time the Beaverhead Mountains did not stand high above surrounding regions as they do today. The notches (mountain passes or cols) linking southwest oriented valleys with  northeast oriented valleys are interpreted as having been eroded by southwest oriented flood flow channels that eroded headward from a flood flow channel on the present day north oriented Lemhi River alignment and which captured the south-southeast oriented flood flow moving along the present day continental divide alignment and also south oriented flood flow from the present day north oriented Big Hole River drainage basin west of the Pioneer Mountains. Headward erosion of a deep south and southeast oriented flood flow channel on the present day south oriented Bloody Dick Creek and north oriented Big Hole River alignment adjacent to what were then the emerging Beaverhead Mountains next captured the southwest oriented flood flow and diverted the floodwaters to a south oriented flood flow channel on the present day north oriented Red Rock River-Beaverhead River alignment. Continued uplift of the Beaverhead Mountains and other regional mountain ranges next caused major flood flow reversals to form the north oriented Big Hole River drainage system west of the Pioneer Mountains and the north oriented Red Rock and Beaverhead River drainage system east of the Pioneer Mountains.Floodwaters are interpreted to have been derived from southeast British Columbia and southwest Alberta, which at that time were located along the western margin of melting thick North American ice sheet.

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-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 Big Hole River-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.

Big Hole River-Lemhi River drainage divide area location map

Figure 1: Big Hole River-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 Big Hole River-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 southwestern Montana with Idaho in the west half and appearing along the south edge of figure 1. The Jefferson River is formed in the southeast quadrant of figure 1 at the confluence of Big Hole and Beaverhead Rivers near Twin Bridges. The Big Hole River flows in a north and northeast direction west of the Pioneer Mountains and then turns to flow in a southeast direction to the town of Divide. From Divide the Big Hole River flows in a south direction before making a second U-turn to flow in a 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 towns of Challis, Ellis and 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 Lemhi River is the northwest oriented tributary flowing near the Idaho-Montana border and joining the Salmon River near the town of Salmon. The east-west continental divide is located along the Idaho-Montana border in the Beaverhead Mountains. The Big Hole River-Lemhi River drainage divide area investigated in this essay is located along the Beaverhead Mountains crest and is located east of the Lemhi River and west and south of the north oriented Big Hole River headwaters.

Before looking at detailed maps of the Big Hole River-Lemhi 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 Beaverhead Mountains were initially developed as south and southeast oriented flood flow channels on either side of the rising mountain range. Prior to development of deep southeast oriented flood flow channels on the Beaverhead Mountains northeast side floodwaters flowed across the rising Beaverhead Mountains to a south-southeast oriented flood flow channel on the Lemhi River alignment. The south-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 town of North Fork, Idaho eventually caused a massive flood flow reversal that created the present day north oriented Salmon River and Lemhi River drainage system 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 Mountain that created the present day north oriented Big Hole River drainage system there. During these flood flow reversals south oriented flood flow along one route would often 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 the present day mountain ranges. 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 (in northeast corner of figure 1), which beheaded and reversed south oriented flood channels east of the Pioneer Mountains, and which created the Big Hole River U-turn in that region.

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

Figure 2: Detailed location map Big Hole River-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 Big Hole River-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 from the north edge of figure 2 (west half) to the south edge of figure 2 (west of center). 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 Big Hole Guide Meridian is a labeled north-to-south oriented line near the center of figure 2. The Big Hole River originates near the continental divide and flows in a north direction just west of the Big Hole Meridian. Big Hole River tributaries originating in the Beaverhead Mountains are generally oriented in northeast directions and include Lake Creek, Miner Creek, and Pioneer Creek. South of the Big Hole River headwaters and west of Selway Mountain are east and northeast headwaters of southeast and east oriented Bloody Dick Creek, which flows to east oriented Horse Prairie Creek, which then joins the Red Rock River at Clark Canyon Reservoir to form the north-northeast oriented Beaverhead River. Remember the Big Hole River north of figure 2 turns to flow in a northeast, southeast and south direction around the Pioneer Mountains before turning to flow in a northeast direction to join the north-northeast oriented Beaverhead River and to form the northeast oriented Jefferson River (see figure 1). The Salmon River flows in a north direction from the south edge of figure 2 (near southwest corner) to the town of Salmon and then to the town of North Fork (south of northwest corner of figure 2) before turning to flow in a west direction to the west edge of figure 2. The Lemhi River flows in a north-northwest and northwest direction from the south edge of figure 2 (west half) to join the Salmon River near the town of Salmon. Tributaries from the Beaverhead Mountains flow in southwest and south-southwest directions and join the Lemhi River as barbed tributaries. The south oriented barbed tributaries provide evidence the present day north oriented Lemhi River alignment was initially formed by a south oriented flood flow channel. Also the southwest oriented Lemhi River tributaries and northeast oriented Big Hole River tributaries probably flow on alignments of what were at one time southwest oriented flood flow channels to a deeper southeast and south-southeast oriented flood flow channel on the Lemhi River alignment. At that time the Beaverhead Mountains were just beginning to emerge and floodwaters could flow across what is today a major mountain range and topographic barrier.

Miner Creek-Kirtley Creek drainage divide area

Figure 3: Miner Creek-Kirtley 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 Miner Creek-Kirtley Creek drainage divide area. The town of Salmon, Idaho is located near the southwest corner of figure 3 and the Salmon River flows in a north direction along the west edge of figure 3. The northwest oriented stream just barely seen in the southwest corner of figure 3 joining the Salmon River at the town of Salmon is the Lemhi River. The east-west continental divide and Montana-Idaho state line is marked with a dashed line and follows the Beaverhead Mountains crest from the north edge of figure 3 (east of center) to the east edge of figure 3 (near southeast corner). East and north of the continental divide are headwaters of northeast oriented Big Hole River tributaries, with Miner Creek being the stream flowing to the northeast corner of figure 3. Hamby Creek is the northeast oriented stream south of Miner Creek and originates near Hamby Lake. On the west side of the continental divide are headwaters of southwest oriented Salmon River and Lemhi River tributaries. Note how those southwest oriented tributaries flow to the north oriented Salmon River and Lemhi River as barbed tributaries. Carmen Creek is the southwest oriented stream flowing to the north oriented Salmon River near the town of Carmen. Kirtley Creek is the southwest oriented stream originating as a North Fork and an East Fork on the west side of the continental from the Upper Miner Lakes where northeast oriented Miner Creek originates. Kirtley Creek flows to the south edge of figure 3 (east of the town of Salmon) and joins the northwest oriented Lemhi River south of figure 3. Geertson Creek is the south-southwest oriented stream located east of Kirtley Creek and flows to the south edge of figure 3 (east of center) and also joins the Lemhi River as a barbed tributary south of figure 3. While today the Beaverhead Mountains form a giant barrier there is evidence of southwest oriented flood flow channels that once linked the present day northeast oriented Miner Creek valley with the southwest oriented Kirtley Creek valley and that also linked the northeast oriented Hamby Creek valley with the south-southwest oriented Geertson Creek valley. The North Fork Kirtley Creek originates between Freeman Peak and Monument Peak while the East Fork Kirtley Creek originates south of Monument Peak. Note how there are notches (cols or mountain passes) cut in the high mountain ridge between Freeman Peak and Monument Peak and just south of Monument Peak, which link the northeast oriented Miner Creek valley with the southwest oriented North Fork and East Fork Kirtley Creek valleys. A similar notch links the Hamby Creek valley and the Geertson Creek valley. Those notches are probably evidence of former valleys that once crossed the Beaverhead Mountains at a time when the mountains did not stand high like they do today. The North and East Fork Kirtley Creek valleys provide evidence of what were once diverging and converging southwest oriented flood flow channels with floodwaters flowing to a deeper south oriented flood flow channel on the present day north oriented Lemhi River alignment. Uplift of the Beaverhead Mountains occurred as floodwaters were flowing across the region and eventually caused floodwaters on northeast ends of the southwest oriented flood flow channels to reverse flow direction and to flow in northeast directions.

Detailed map of Hamby Creek-Geertson Creek drainage divide area

Figure 4: Detailed map of Hamby Creek-Geertson 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 Hamby Creek-Geertson drainage divide area seen in less detail in figure 3. The east-west continental divide serves as the state and county line and follows the high Beaverhead Mountains crest ridge from the north edge of figure 4 (west half) to the south edge of figure 4 (east half). Geertson Creek originates on the west side of the continental divide in section 18 and flows in a west and southwest direction to the southwest corner of figure 4. As seen in figure 3, south and west of figure 4 Geertson Creek flows to the north oriented Lemhi River as a barbed tributary. East of the continental divide and in the northeast quadrant of figure 4 Hamby Creek originates at Hamby Lake and flows to the northeast corner of figure 4. North and east of figure 4 Hamby Creek flows in a northeast and north-northeast direction to join the north oriented Big Hole River. The Beaverhead Mountains appear to have been glaciated with both Geertson Creek and Hamby valley heads being located in cirques, although glacial activity on the northeast side of the continental divide appears to have been much more intense than on the on the southwest side. Typical alpine glacial features are present in figure 4 with the narrow ridge between the Geertson Creek headwaters and the Hamby Creek headwaters being an arête and the notches in that ridge being cols. In this essay our concern is with the valley formation, which occurred before the glaciation. The deepest notch in the ridge separating the Geertson Creek and Hamby Creek headwaters appears to have an elevation of between 9280 and 9320 feet (the map contour interval for figure 4 is 40 feet). Center Mountain to the southeast rises to 10,364 feet while Sacajawea Peak to the northwest rises to 10,390 feet suggesting the notch is more than 1000 feet deep. It is possible the notch was eroded by glacial activity, although it is more probable the notch is a relic of a through valley that once linked the southwest oriented Geertson Creek valley with the northeast oriented Hamby Creek valley. If the latter hypothesis is correct the through valley was eroded by southwest oriented flood flow moving from the present day north oriented Big Hole River basin to what was then a south oriented flood flow channel on the present north oriented Lemhi River alignment. At that time the Beaverhead Mountains were probably just beginning to emerge and floodwaters eroded a deep valley in the rising mountain mass until Beaverhead Mountain uplift created a barrier to the southwest oriented flood flow, which forced a reversal of flood flow that created the north oriented Big Hole River drainage system west of the Pioneer Mountains (see figure 1).

Pioneer Creek-Lemhi River drainage divide area

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

Figure 5 illustrates the Pioneer Creek-Lemhi River drainage divide area south and east of figure 3 and includes an overlap area with figure 3. The continental divide serves as the state line and is located along the Beaverhead Mountains crest ridge, which extends in a south-southeast and east direction from the north edge of figure 5 (east of center) to the east edge of figure 5 (north of southeast corner). The Lemhi River flows in a northwest direction across the southwest corner of figure 5. Labeled southwest and south-southwest oriented Lemhi River tributaries include Kirtley Creek, Geertson Creek, Bohannon Creek, Wimpey Creek, and Pratt Creek, all of which flow to the northwest oriented Lemhi River as barbed tributaries. East of the continental divide are headwaters of east and northeast oriented streams including Pioneer Creek, Jahnke Creek, Darkhorse Creek, and Park Creek. Darkhorse Creek could be considered the head of the north oriented Big Hole River, which originates at Skinner Meadows (located along east edge of figure 5, north of center). The unnamed northeast oriented stream between Darkhouse Creek and Park Creek is the northeast oriented headwaters of south-southeast oriented Bloody Dick Creek, which south and east of figure 5 turns to flow in an east direction to join east oriented Horse Prairie Creek and to enter the north-northeast oriented Beaverhead River (see figure 2, also see figure 7 and 8 to see the Big Hole River-Bloody Dick Creek drainage divide). The Beaverhead Mountains crest ridge appears to have been glaciated with the most intense glaciation having occurred on the northeast side. Study of the continental divide reveals notches (cols or through valleys) linking the east and northeast oriented stream valleys on the east side of the continental divide with the southwest oriented Lemhi River tributary valleys. The map contour interval for figure 5 is 50 meters and several of these notches (cols or through valleys) are defined by two or more contour lines on a side. Also note notches (cols or through valleys) crossing the drainage divides between headwaters of the east and northeast oriented streams on the east side of the continental divide, which suggest those valleys captured a south-southeast oriented flood flow channel, which was located adjacent to the present day continental divide. If correctly interpreted that high level south-southeast oriented flood flow channel was eroded into a surface now represented by the highest elevations along the continental divide and was captured in sequence by headward erosion of the Park Creek valley, northeast oriented Bloody Dick Creek headwaters valley, Darkhorse Creek valley, Jahnke Creek valley, and Pioneer Creek valley, with headward erosion of each valley beheading flood flow to the newly eroded valley to the southeast. At the time those captures occurred those capturing valleys were probably oriented in southwest directions and were eroding headward from a south oriented flood flow channel on the present day north oriented Lemhi River alignment.

Detailed map of Pioneer Creek-West Wimpey Creek drainage divide area

Figure 6: Detailed map of Pioneer Creek-West Wimpey 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-West Wimpey Creek drainage divide area seen in less detail in figure 5. The east-west continental divide follows the Beaverhead Mountains crest ridge and serves as the county line and extends from the north edge of figure 6 (west half) in a southeast direction to the south edge of figure 6 (east half). The northeast side of the Beaverhead Mountains crest ridge appears to have been eroded by valley glaciers with multiple cirques at the valley heads. Again the concern in this essay is with the valley origins and the valleys existed before the valley glaciers formed. There are notches (cols or through valleys) linking the northeast oriented valleys on the east side of the continental divide with the southwest oriented valleys on the west side of the continental divide. For example in the west half of section 34 there is a notch in the continental divide linking the northeast oriented Pioneer Creek valley with the southwest oriented West Fork Wimpey Creek valley. The map contour interval for figure 6 is 40 feet and the notch floor elevation at the continental divide is between 9400 and 9440 feet. In section 28 to the northwest the continental divide elevations rise to 10,062 feet while immediately to the southeast the continental divide rises 9789 feet suggesting the notch is more than 340 feet deep. While dwarfed by the surrounding mountain slopes the notch may be evidence of a former through valley that was eroded by southwest oriented flood flow. Perhaps even more intriguing are the notches just east of the continental divide that cross drainage divides between northeast oriented valleys. For example, in the northwest quadrant of section 19 (next to north edge of figure 6) there is a notch linking the Berry Lake valley with the Pioneer Creek valley. The floor of that notch at the drainage divide has an elevation of between 9360 and 9400 feet. The ridge to the northeast rise to 10,060 feet (north of figure 6) and the peak west of Timberline Lake rises to 10,049 feet suggesting the notch is approximately 650 feet deep. In section 29 (south and east of Pioneer Lake) there is a notch linking the Pioneer Creek valley with the Jahnke Creek valley. The floor of that notch at the drainage divide has an elevation of between 9280 and 9320 feet. The ridge to the northeast rises to 9626 feet and the continental divide to west rises to a comparable elevation suggesting the notch is approximately 300 feet deep. These notches are evidence of what was once a southeast oriented flood flow channel located on the alignment of the present day east-west continental divide. Uplift of the Beaverhead Mountains apparently occurred on the alignment of that early flood flow channel, which was dismembered first by headward erosion of deeper southwest oriented valleys from a deeper southeast oriented flood flow channel on the present day north oriented Lemhi River alignment. Continued uplift of the Beaverhead Mountains then resulted in reversals of flood flow that created the present day northeast oriented streams. Glaciation of the mountains occurred at a later time.

Big Hole River-Bloody Dick Creek drainage divide area

Figure 7: Big Hole River-Bloody Dick Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 7 illustrates the Big Hole River-Bloody Dick Creek drainage divide area south and east of figure 5 and includes an overlap area with figure 5. The continental divide follows the Beaverhead Mountains crest ridge and extends from the north edge of figure 7 (west half near Center Mountain) in a southeast direction to the south edge of figure 7 (east half). The northwest oriented Lemhi River can just barely be seen in the southwest corner of figure 7. Wimpey Creek is the southwest oriented stream joining the Lemhi River at the southwest corner of figure 7. Note other southwest oriented streams originating near the high Beaverhead Mountains crest ridge and flowing to the northwest oriented Lemhi River as barbed tributaries. East of the continental divide Darkhorse Creek originates near the center of figure 7 and flows in an east-northeast direction to Skinner Meadows where the north-northeast oriented Big Hole River begins and flows to north edge of figure 7 (east half). North of figure 7 the Big Hole River flows in a north direction along the west flank of the Pioneer Mountains before making a U-turn to flow in a south direction along the east flank of the Pioneer Mountains and then making a second U-turn to flow in a northeast direction to join the north-northeast oriented Beaverhead River and to form the Jefferson River (see figure 1). Immediately south of Darkhouse Creek are the northeast oriented headwaters of Bloody Dick Creek, which south of Skinner Meadows turn to flow in southeast direction to the southeast corner of figure 7. South and east of figure 7 Bloody Dick Creek flows in a southeast and east direction (south of the Pioneer Mountains) to join east oriented Horse Prairie Creek and then to enter the north-northeast oriented Beaverhead River. Note the well-defined through valley linking the north oriented Big Hole River valley with the southeast oriented Bloody Dick Creek valley. The map contour interval for figure 7 is 50 meters and the through valley floor elevation at the drainage divide is between 2300 and 2350 meters. Black Mountain to the east rises to 2801 meters while elevations north and east of Goldstone Pass to the west rise to more than 2900 meters. These elevations suggest the Big Hole River-Bloody Dick Creek through valley is at least 450 meters deep. The through valley is evidence of a major south oriented flood flow channel that was roughly parallel to and east of the present day continental divide. The Lemhi River valley is evidence of what was probably a deeper south oriented flood flow channel parallel to and west of the present day continental divide. These are the flood flow channels that captured the southeast oriented flood flow that once was flowing in a flood flow channel along the present day continental divide alignment (see figures 5 and 6 discussions) and which carved the present day east-west continental divide.

Detailed map of Big Hole River-Bloody Dick Creek drainage divide area

Figure 8: Big Hole River-Bloody Dick 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 Big Hole River-Bloody Dick Creek drainage divide area seen in less detail in figure 7. The east-west continental divide is located along the marked county line seen in the southwest quadrant of figure 8. Bloody Dick Creek originates in section 15 (in southwest quadrant of figure 8) and flows in a northeast direction to the corner of sections 1, 6, 7, and 12 where it turns to flow in a south-southeast direction to the southeast corner of figure 8. Park Creek is the northeast oriented stream flowing from the south center edge of figure 8 to join south-southeast oriented Bloody Dick Creek as a barbed tributary in section 7. Darkhorse Creek originates near the west center edge of figure 8 and flows in an east-northeast direction to Skinner Meadows in section 1 and 36 where it forms the north oriented Big Hole River. The deep through valley linking the north oriented Big Hole River valley with the south-southeast oriented Bloody Dick Creek valley is located in sections 1 and 6. The map contour interval for figure 8 is 40 feet and the through valley floor elevation at the drainage divide is between 7560 and 7600 feet. Black Mountain in section 5 to the east rises to 9190 feet while higher elevations can be found along the continental divide to the west. These elevations suggest the through valley is approximately 1600 feet deep. The through valley is a water-eroded feature and was eroded by south and southeast oriented flood flow moving adjacent to and parallel to the present day continental divide. Note also in the northwest corner of figure 8 a spring with the headwaters of a northeast oriented valley. South of that spring is a narrow ridge or arête and then a south oriented tributary to east-northeast oriented Darkhorse Creek. There are two deep notches (or cols) in the arête with the deepest having an elevation at the drainage divide of between 9080 and 9120 feet. The ridge to the east rises to 9788 feet while higher elevations can be found along the continental divide to the west (although not immediately to the west) suggesting the notches are approximately 600 feet deep. These notches provide evidence of a southeast oriented flood flow channel that once flowed along and adjacent to the present day continental divide. Continue south from those notches and the next notch of interest is south of Cowbone Lake and links the Darkhorse Creek valley (which is north of the continental divide) with the west oriented headwaters valley of southwest oriented Pratt Creek (on the south side of the continental divide). South and east of the Pratt Creek headwaters valley is Goldstone Pass, which links the Pratt Creek valley with the Bloody Dick valley. Apparently the flood flow channel that formed on the present day continental divide alignment flowed across the present day continental divide location into the present day Pratt Creek headwaters area and then flowed back across the continental divide location into the present day Bloody Dick Creek headwaters area. South of the Bloody Dick Creek headwaters in section 15 the flood flow channel can be followed through a notch in the ridge along the south edge of figure 8 into the Park Creek valley. That notch, which is only partially shown in figure 8, is approximately 500 feet deep.

Bloody Dick Creek-Lemhi River drainage divide area

Figure 9: Bloody Dick Creek-Lemhi River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 9 illustrates the Bloody Dick Creek-Lemhi River drainage divide area south and east of figure 7 and does not include an overlap area with figure 7. The east-west continental divide is shown with a dashed line following the meandering Beaverhead Mountains crest from near the north center edge of figure 9 to near the south center edge of figure 9. The Lemhi River flows in a north and north-northwest direction near the west edge of figure 9. Sandy Creek, Kenney Creek, Pattee Creek, and Agency Creek are named southwest oriented and barbed tributaries to the north oriented Lemhi River with Agency Creek turning to flow in a west direction before joining the Lemhi River. Agency Creek originates near Lemhi Pass in the southeast quadrant of figure 9. Bloody Dick Creek flows in a south and southeast direction from the north edge of figure 9 (east half) to the east center edge of figure 9. Trail Creek originates on the east side of Lemhi Pass and flows in an east-southeast direction to near the east edge of figure 9 where it is joined by north oriented Bear Creek and then flows in a northeast direction (east of figure 9) to join Bloody Dick Creek, which joins other streams to form east oriented Horse Prairie Creek, which then flows to the north-northeast oriented Beaverhead River. The map contour interval for figure 9 is 40 meters. The elevation at Lemhi Pass is shown as 2247 meters. North of Lemhi Pass the continental divide rises to at least 2657 meters and as seen in earlier figures rises even higher to the north of figure 9. South of Lemhi Pass the continental divide rises to more than 3000 meters suggesting Lemhi Pass is at least 400 meters deep. The Lemhi Pass origin is somewhat more difficult to interpret than some of the other through valleys seen in this essay, but was probably eroded by a southeast oriented flood flow channel diverging from the south oriented flood flow channel on the present day north oriented Lemhi River alignment. Floodwaters in this diverging flood flow channel were probably flowing to a south oriented flood flow channel of the present day north oriented Red Rock River and Beaverhead River alignments (see figure 1). An alternate interpretation is south oriented floodwaters from east of the continental divide flowed in a diverging flood flow channel to the deeper flood flow channel on the Lemhi River alignment, although that interpretation requires much more complex flood flow movements. Figure 10 below provides a detailed topographic map to better illustrate features in the Lemhi Pass region.

Detailed map of Agency Creek-Trail Creek drainage divide area

Figure 10: Detailed map of Agency Creek-Trail 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 Agency Creek-Trail Creek drainage divide area seen in less detail in figure 9. The east-west continental divide serves as the county and state border and extends from the north center edge of figure 10 in a south direction and then turns to extend in a west-southwest direction to near the southwest corner of figure 10. Lemhi Pass is near the center of figure 10. Trail Creek originates on the east side of Lemhi Pass and flows in an east-southeast direction to the east edge of figure 10 (south half). North Frying Pan Creek is the east-northeast stream flowing from near the continental divide in the southwest corner region of figure 10 to join Trail Creek near the Selway Ranch in section 24 (near east edge of figure 10). Agency Creek originates on the west side of Lemhi Pass and flows in a northwest, southwest, and northwest direction to the west edge of figure 10. Note the north-northwest oriented tributary flowing from near the continental divide to join Agency Creek in section 15. The map contour interval for figure 10 is 40 feet and the Lemhi Pass elevation is shown as being 7373 feet. The continental divide near the southwest corner of figure 10 rises 8447 feet and just north of figure 10 the continental divide reaches an elevation of 8370 feet. These elevations suggest Lemhi Pass is at least 1000 feet deep. Other deep passes across the continental divide can also be seen in figure 10. For example near the corner of sections 26 and 27 (north of continental divide) and section 20 (south of continental divide) in the southwest quadrant of figure 10 a deep pass links the north-northwest oriented Agency Creek tributary valley with the east-northeast oriented North Frying Pan Creek valley. The floor of that pass at the drainage divide has an elevation of between 7320 and 7360 feet, which is slightly lower than the Lemhi Pass elevation. These and other passes crossing the continental divide were probably eroded by southeast oriented flood flow moving from the south oriented flood flow channel on the present day north oriented Lemhi River alignment to the Trail Creek valley and then to a south oriented flood flow channel on the present day north oriented Red Rock River-Beaverhead River alignment. Beaverhead Mountains uplift occurred as floodwaters were flowing across the region. At first floodwaters eroded deep valleys into the rising mountain mass, but in time the crustal warping formed a barrier causing floodwaters to find new and deeper routes and eventually reversing the flood flow west of the present day continental divide. Flood flow movements in this region were complex and I will not attempt to fully decipher them, but the deep passes provide evidence that multiple flood flow channels once crossed what is today the continental divide.

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.