Abstract:

This essay uses topographic map evidence to interpret landform origins between the Jefferson River and the South Boulder River in the Tobacco Root Mountains, Madison County, Montana. The Jefferson River is formed west of the Tobacco Root Mountains at the confluence of the south and northeast oriented Big Hole River, the north-northeast oriented Beaverhead River, and the north and northwest oriented Ruby River and then flows in north-northeast, east, east-southeast direction around the Tobacco Root Mountains north end before turning to flow in a northeast direction to join the north oriented Madison and Gallatin Rivers and to form the north oriented Missouri River. The South Boulder River originates in the high Tobacco Root Mountains and flows in a north-northeast direction to join the east-southeast oriented Jefferson River near the point where the south oriented Boulder River also joins the Jefferson River. Before joining the Jefferson River the north oriented South Boulder River crosses a large southeast oriented through valley linking the south oriented Boulder River valley with the north oriented Madison River valley. Headward erosion of the deeper east-southeast oriented Jefferson River valley captured the south oriented flood flow to a south oriented flood flow channel on the present north oriented Madison River alignment and caused a reversal of flood flow on the north end of what is today the north oriented South Boulder River valley. The through valley provides evidence of a major south and southeast oriented flood flow channel that existed prior to a massive flood flow reversal that created the north oriented Missouri River and tributary river drainage systems seen today. Further south in the high Tobacco Root Mountains through valleys or mountain passes link north and northwest oriented valleys with southeast or south oriented valleys and provide evidence of flood flow channels that once crossed what are today some of the highest ridges in Tobacco Root Mountains. Floodwaters are interpreted to have initially flowed across the region prior to formation of the present day Jefferson River valley west and north of the Tobacco Root Mountains. Floodwaters are interpreted to have been derived from a melting thick North American ice sheet’s western margin and were flowing from western Canada. Tobacco Root Mountains uplift occurred as floodwaters flowed across the region and was probably caused by the thick ice sheet presence and/or by the melt water flood erosion of the mountain core and/or deposition flood eroded debris in adjacent valleys.

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 Jefferson River-South Boulder River drainage divide area landform origins in the Tobacco Root Mountains, Madison County, Montana 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 Jefferson River-South Boulder River drainage divide area landform evidence in the Tobacco Root Mountains, Madison County, Montana will be regarded as evidence supporting the “thick ice sheet that melted fast” paradigm.

Jefferson River-South Boulder River drainage divide area location map

Figure 1: Jefferson River-South Boulder 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 Jefferson River-South Boulder River drainage divide in the Tobacco Root Mountains, Madison County, Montana and primarily illustrates a region in southwest and south central Montana with the northwest corner of Wyoming and Yellowstone National Park in the southeast corner of figure 1 and an area of Idaho located in the southwest corner of figure 1. The Tobacco Root Mountains are labeled and are located near the center of figure 1. The Jefferson River is formed near Twin Bridges (west side of the Tobacco Root Mountains) at the confluence of the north-northeast oriented Beaverhead River and the north, southeast, south, and northeast oriented Big Hole River. From the Twin Bridges area the Jefferson River flows in a north-northeast, east, and northeast direction to near Three Forks, where it joins the north oriented Madison River and the north and northwest oriented Gallatin River to form the north and north-northwest oriented Missouri River. North of figure 1 the Missouri River turns to flow in a northeast and east direction and eventually reaches North Dakota where it turns to flow in a southeast and south direction. The unlabeled north and northwest oriented drainage route through the towns of Alder and Laurin, which joins the Beaverhead River near Twin Bridges, is the Ruby River. The unlabeled north-northeast oriented drainage route in the Tobacco Root Mountains and which joins the Jefferson River near Cardwell is the South Boulder River. The unlabeled north, northeast, southeast, and south oriented drainage route also joining the Jefferson River near Cardwell is the Boulder River (which is why the river in the Tobacco Root Mountains has the name South Boulder River). The Jefferson River-South Boulder River drainage divide area investigated in this essay is generally located east and south of the Jefferson River, west of the South Boulder River, and north of the northwest oriented Ruby River segment.

A brief look at the big picture erosion history will help understand discussions related to 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 melting thick North American ice sheet and were flowing in a south and southeast direction from western Canada to and across the figure 1 region. North oriented rivers in figure 1, including the north oriented Missouri, Madison, and Gallatin Rivers are flowing in valleys that originated as south oriented flood flow channels. When floodwaters first flowed across the region the present day mountain ranges did not exist and floodwaters could freely flow in south and southeast directions across the region. The mountain ranges emerged as floodwaters flowed across the region and initially floodwaters flowed across what are today high mountains including the Tobacco Root Mountains. More successful, or deeper, flood flow channels moving floodwaters across the mountain ranges first captured floodwaters from adjacent less successful flood flow channels and later floodwaters were captured by flood flow channels moving floodwaters between the emerging mountain ranges. These captures of the south and southeast oriented flood flow channels initially were made by the headward erosion of south-southwest and southwest oriented valleys from the more successful, or deeper, south oriented or southeast oriented flood flow channels. Through valleys or mountain passes crossing present-day drainage divides today provide evidence of the former flood flow channels. Segments of the present day north oriented Jefferson River and South Boulder River valleys probably were initiated by the headward erosion of south-southwest oriented and/or southwest oriented valleys across south and southeast oriented flood flow routes. However, as the regional mountain ranges emerged a massive flood flow reversal occurred, which systematically reversed flood flow in most, but not all of the south oriented flood flow channels to create the north oriented drainage routes seen today.

The flood flow reversals that created the present day north-northeast, east, and northeast oriented Jefferson River and north-northeast oriented South Boulder River were probably indirectly caused by crustal warping that occurred as melt water floods flowed across the region with the crustal warping being related to thick ice sheet presence north and east of figure 1, although the direct cause of the massive flood flow reversal was headward erosion of a deep northeast oriented valley across Montana (the northeast oriented Missouri River valley north of figure 1), which beheaded south oriented flood flow channels supplying floodwaters to a south oriented Missouri River flood flow channel on the present-day north and north-northwest oriented Missouri River alignment. At Three Forks, Montana this south oriented flood flow channel split into several diverging south oriented flood flow channels. The deep northeast oriented valley (now northeast oriented Missouri River valley north of figure 1 was eroding headward from space in the deep “hole” the melting ice sheet had occupied and was capturing the south and southeast oriented ice-marginal melt water floods and diverting the captured floodwaters into space being opened up in the deep “hole” where the melting the ice sheet had been located. This northeast oriented valley was much deeper than the beheaded south oriented flood flow channels and floodwaters on north ends of the beheaded flood flow channels reversed flow direction to create the north oriented Missouri River drainage route seen north of Three Forks in figure 1 and the north oriented Missouri River tributary drainage routes south of Three Forks.The reversal of flow in the Missouri River flood flow channel also reversed flood flow in flood flow channels on the Gallatin and Madison River alignments to create the north oriented Madison and Gallatin River drainage routes seen today. Headward erosion of the deep northeast Jefferson River valley from this reversed Missouri River flood flow channel next beheaded flood flow channels further to the west and floodwaters on north ends of those beheaded flood flow channels reversed flow direction to create north oriented drainage routes, including segments of the north-northeast oriented South Boulder River, although as seen in more detailed maps illustrated below the South Boulder River history was much more complex than a one time flood flow reversal.

Detailed location map for Jefferson River-South Boulder River drainage divide area

Figure 2: Detailed location map Jefferson River-South Boulder 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 Jefferson River-South Boulder River drainage divide area in Madison County, Montana and shows drainage routes not seen in figure 1. Green shaded areas are National Forest lands, which generally are located in mountainous regions. County boundaries are shown and Madison County is labeled. Jefferson County is located north of Madison County and Butte is located in Silver Bow County in the northwest corner of figure 2.  The Tobacco Root Mountains are located in Madison County. The Jefferson River is formed at the confluence of the northeast oriented Big Hole River, north-northeast oriented Beaverhead River, and northwest oriented Ruby River near the town of Twin Bridges (in southwest quadrant of figure 2). From Twin Bridges the Jefferson River flows in a north-northeast, east, southeast, and northeast direction to near the town of Three Forks (in northeast quadrant of figure 2) where it joins the north oriented Madison River and northwest oriented Gallatin River to form the north oriented Missouri River (the short segment seen in figure 2 north of Three Forks is oriented in a northeast direction). The Madison River flows in a north direction from the south edge of figure 2 (east half, south of Ennis Lake) to join the Jefferson River near Three Forks. The South Boulder River originates in the Tobacco Root Mountains and flows in a north-northeast direction to join the Jefferson River near the town of Cardwell (west of Lewis and Clark Cavern State Park). Antelope Creek is a northeast oriented Jefferson River tributary located east of the South Boulder River and North Willow Creek is a northeast oriented Jefferson River tributary located east of Antelope Creek. Note unlabeled northwest oriented Jefferson River tributaries originating west of the South Boulder River and southeast oriented (and barbed) tributaries flowing to the north-northeast oriented Jefferson River from the west. The southeast and northwest oriented tributary orientations suggest the Jefferson River valley eroded headward across southeast oriented flood flow. If so the Tobacco Root Mountains at that time did not form a massive topographic barrier to southeast oriented flood flow as they do today. Also note southwest oriented (and barbed) Ruby River tributaries originating in the Tobacco Root Mountains south of the north-northeast oriented South Boulder River headwaters. The southwest oriented (and barbed) Ruby River tributaries originating in the Tobacco Root Mountains and seen in figure 2 are roughly aligned with northeast and north-northeast Jefferson River tributaries originating in the Tobacco Root Mountains. This rough alignment suggests the alignments may be related to southwest, and south-southwest oriented flood flow channels that once crossed the region where the Tobacco Root Mountains now stand. If so the floodwaters flowed across the region prior to the emergence of the Tobacco Root Mountains as a high mountain range. Assuming the present day tributary valleys are located along alignments of former flood flow channels emergence of the Tobacco Root Mountains as a high mountain range occurred as floodwaters flowed across the region, which probably means the emergence was related in some way to the thick ice sheet presence north of east of figure 2 and/or to the melt water flood flow regional erosion and deposition.

Jefferson River-Antelope Creek drainage divide area

Figure 3: Jefferson River-Antelope 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 Jefferson River-Antelope Creek drainage divide area along the Tobacco Root Mountains north margin. The Jefferson River serves as the Jefferson-Madison County line across much of the north half of figure 3 and flows in an east direction from the west edge of figure 3 (near northwest corner) to Cardwell and then turns to flow tin a southeast direction through a deep water gap eroded into the London Hills upland before turning to flow to the east edge of figure 3 (north of center). East of figure 3 the Jefferson River turns to flow in a northeast direction to flow to the north oriented Missouri River. The northern margin of the Tobacco Root Mountains can be seen in the southwest quadrant of figure 3. The South Boulder River flows in a north-northeast and north direction from the south edge of figure 3 (west of center) to join the southeast oriented Jefferson River as a barbed tributary just south of La Hood Park. Antelope Creek flows in a north-northeast direction from the south edge of figure 3 (east half) to join the Jefferson River near the east center edge of figure 3. Summit Valley is today a major northwest-to-southeast oriented through valley linking the north oriented South Boulder River valley with the north-northeast oriented Antelope Creek valley. While not seen in figure 3 the Boulder River flows in a south direction on the same alignment as the north oriented South Boulder River valley and joins the Jefferson River near Cardwell (see figure 1). Also not seen in figure 3 is how the Summit Valley through valley continues in a southeast direction to the northeast oriented North Willow Creek valley located east of figure 3 and then to the north oriented Madison River valley. North Willow Creek flows in a northeast direction to join the northeast oriented Jefferson River (see figure 2). The Summit Valley through valley was eroded by a major south and southeast oriented flood flow channel prior to headward erosion of the present day Jefferson River valley and was moving south oriented floodwaters from the Boulder River alignment to the north to a south oriented flood flow channel on the present day north oriented Madison River alignment to the east and south. Headward erosion of the deeper northeast oriented Jefferson River-Willow Creek valley from the newly reversed flood flow channel on the present day north oriented Missouri River alignment captured the south and southeast oriented flood flow channel and diverted floodwaters in a northeast and north direction to the newly reversed Missouri River valley. Next headward erosion of deeper Jefferson River-Antelope Creek valley captured the south and southeast oriented flood flow channel and soon thereafter Jefferson River valley headward beheaded the south oriented flood flow channel. Floodwaters on the north end of the beheaded flood flow channel reversed flow direction to create the north oriented South Boulder River drainage route north and west of Summit Valley. Probably the north oriented South Boulder River drainage route in the Tobacco Root Mountains had been beheaded and reversed at an earlier time, although at one time it had been a south oriented flood flow route. Headward erosion of the deep southeast oriented Summit Valley through valley from the deep south oriented flood flow channel on the present day north oriented Madison River alignment probably beheaded and reversed the south oriented flood flow channel on the present day north oriented South Boulder River alignment south of figure 3.

Detailed map of South Boulder River-Antelope Creek drainage divide area

Figure 4: Detailed map of South Boulder River-Antelope 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 South Boulder River-Antelope Creek drainage divide area seen is less detail in figure 3. The South Boulder River flows in a north-northeast direction across the northwest corner of figure 4. Antelope Creek flows in a north-northeast direction from the south edge of figure 4 (east half) to the east edge of figure 4 (north of center). Hogbacks along the Tobacco Root Mountains north margin can be seen in the southwest quadrant of figure 4 while the upland in the northeast quadrant of figure 4 is the London Hills. Between the Tobacco Root Mountains and the London Hills is the northwest-to-southeast oriented Summit Valley through valley. Note southwest and northwest oriented South Boulder River tributaries which make U-turns in sections 36, 31, and 32 and in the north half of section 6 (near north edge of northwest quadrant of figure 4). Also note southeast and east oriented Dogtown Sewer, which drains to Antelope Creek. The drainage divide between the northwest oriented South Boulder River tributaries and Dogtown Sewer is located in section 6. The map contour interval for figure 4 is 40 feet and the Summit Valley drainage divide elevation is between 4840 and 4880 feet. Elevations to the northeast in the London Hills rise to 6180 feet while elevations in section 14 to the southwest rise to 6480 feet and much higher elevations are found further to the southwest. These elevations suggest the Summit Valley through valley is at least 1300 feet deep. The Summit Valley through valley is related to the underlying geologic structures, but it is also a water-eroded feature and at one time served as a major southeast oriented flood flow channel moving south oriented floodwaters from the south oriented Boulder River valley (north of figure 4 and north of the Jefferson River) to a south oriented flood flow channel on the present day north oriented Madison River alignment (east of figure 4). Antelope Creek appears to have deposited a large alluvial fan on the through valley floor near the southeast corner of figure 4 and then eroded valleys into that alluvial fan, suggesting changing flow conditions in the Antelope Creek headwaters area after southeast oriented flood flow in the Summit Valley through valley ceased.

Mill Creek-South Boulder River drainage divide area

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

Figure 5 illustrates the Mill Creek-South Boulder River drainage divide area south and west of figure 3 and includes an overlap area with figure 3. The Jefferson River flows in a north-northeast direction across the northwest corner of figure 5. The South Boulder River flows in a north-northeast direction from the south edge of figure 5 (east half) to the north edge of figure 5 (near northeast corner). The south-southwest to north-northeast oriented Tobacco Root Mountains crest ridge is located between the north-northeast oriented Jefferson River and the north-northeast oriented South Boulder River. Jefferson River tributaries originating the Tobacco Root Mountains are generally oriented in northwest directions, although some in the Tobacco Root Mountains are oriented southwest directions. Mill Canyon for example has a significant southwest oriented segment and also has south oriented tributaries in addition to its northwest oriented headwaters and tributaries. One of the south oriented tributary valleys is linked by a north-to-south oriented through valley with a north oriented Perry Canyon tributary valley. The through valley may be evidence of a former south oriented flood flow channel captured by headward erosion of the much deeper southwest oriented Mill Canyon valley and subsequently captured by headward erosion of the much deeper Perry Canyon valley. A more significant through valley links the north-northwest oriented Mill Canyon valley with an east-northeast oriented South Boulder River tributary valley. The map contour interval for figure 5 is 50 meters and the through valley elevation at the drainage divide is between 2350 and 2400 meters. Brownback Mountain to the north rises to more than 2800 meters while an unnamed mountain to the south rises to more than 3000 meters suggesting the through valley may be as much as 400 meters deep. The through valley is a water eroded feature and was probably eroded by a southeast flood flow channel moving floodwaters to a south oriented flood flow channel on the present day north oriented South Boulder River alignment. At that time the Tobacco Root Mountains were just beginning to emerge and the deep Jefferson River valley to the west and northwest did not exist. Floodwaters eroded a deep valley into the emerging Tobacco Root Mountains before headward erosion of the deeper southwest oriented Mill Creek valley captured the south or southeast oriented flood flow and diverted the floodwaters to a deeper south oriented flood flow channel being eroded into what is today the north oriented Jefferson River valley.

Detailed map of Mill Creek-South Boulder River drainage divide area

Figure 6: Detailed map of Mill Creek-South Boulder River 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 Mill Creek-South Boulder River drainage divide area seen in less detail in figure 5. Mammoth is the small community near the southeast corner of figure 6 and the South Boulder River flows in a north-northeast direction from the south edge of figure 6 (near Mammoth) to the east edge of figure 6 (north of center). Rock Creek originates in section 11 and flows in a northeast direction to the northwest corner of section 6 where it turns to flow in southeast direction to join the north-northeast oriented South Boulder River as a barbed tributary. Rock Creek Lake in section 2 and Hells Hole in section 11 appear to be located in basins that may have once been filled with small alpine glaciers. If so the alpine glaciers were formed after the flood events that eroded valleys seen in figure 6, although the alpine glaciers probably modified the basins in which they were located. Mill Creek originates in section 35 and flows in a north-northwest, west, and northwest direction in sections 35 and 34 before turning to flow in a southwest direction across the northwest corner of figure 6. Note the trail in section 2 leading from Rock Creek Lake to the Mill Creek valley. The map contour interval for figure 6 is 40 feet and the elevation where the trail crosses the Rock Creek-Mill Creek drainage divide is shown as being 7787 feet. While Rock Creek Lake is only 80 to 120 feet lower in elevation than the drainage divide, elevations drop very fast in the Mill Creek valley and are less than 6000 feet in the southwest oriented Mill Creek Canyon at the west edge of figure 6. Elevations in the South Boulder River valley are also less than 6000 feet at the east edge of figure 6 (north half). Dead Mountain in section 36 north of the Mill Creek-Rock Creek through valley or pass reaches an elevation of 8893 feet and Brownback Mountain just north of figure 6 reaches an elevation of 9283 feet. The unnamed high point in the southeast corner of section 10 to the southwest has an elevation of 9439 feet and Manhead Mountain just south of figure 6 reaches 9966 feet. These elevations suggest the Mill Creek-Rock Creek through valley or pass may be as much as 1100 feet deep. This 1100-foot deep northwest-to-southeast oriented through valley or pass was probably initially eroded as a southeast oriented flood channel at a time when the deep Jefferson River valley to the northwest did not exist. It is probable, however, that there was a reversal of flood flow in the through valley after headward erosion of the much deeper southwest oriented Mill Creek valley beheaded the southeast flood flow channel. That reversal of flood flow moved south oriented flood flow on the present day north oriented South Boulder River alignment to the much deeper southwest oriented Mill Creek valley and probably was responsible for eroding the deep Mill Creek Canyon seen today. Subsequently a reversal of flood flow in the South Boulder River valley ended all flood flow to the deeper Mill Creek valley and created the Mill Creek-Rock Creek drainage divide seen in figure 6.

Beall Creek-South Boulder River drainage divide area

Figure 7. Beall Creek-South Boulder River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 7 illustrates the Beall Creek-South Boulder River drainage divide area south and slightly west of figure 5 and includes a significant overlap area with figure 5. The Jefferson River flows in a north-northeast direction across the northwest corner of figure 7. Beall Creek originates at Beall Lake in the high Tobacco Root Mountains (near center of figure 7) and flows in a north-northwest and northwest direction to the north edge of figure 7 (west of center) and joins the Jefferson River north of figure 7. Note other north-northwest and northwest oriented Jefferson River tributaries originating in the high Tobacco Root Mountains. The north-northwest oriented Jefferson River tributary valley alignments were probably initiated by south-southeast oriented flood flow channels, which once crossed the region. At that time the Tobacco Root Mountains did not stand 1500 meters higher than the surrounding region as they do today and the deep Jefferson River valley to the northwest did not exist. Flood flow in the south-southeast oriented flood flow channels was beheaded and reversed by headward erosion of a much deeper south-oriented flood flow channel west of the emerging Tobacco Root Mountains to create the northwest and north-northwest drainage routes seen today. The south oriented flood flow channel west of the Tobacco Root Mountains was subsequently reversed to create the north-northeast oriented Jefferson River drainage route. The South Boulder River originates near Sailor Lake (south of Beall Lake) in the high Tobacco Root Mountains and flows in an east, northeast, and north-northeast direction to the north edge of figure 7 (near northeast corner) and joins the Jefferson River north of figure 7. Curly Creek is a northeast oriented South Boulder River tributary located between the Beall Creek headwaters and the northeast oriented South Boulder River valley. South Willow Creek is the northeast and east-southeast oriented stream located in the southeast corner of figure 7 and east of figure 7 turns to flow in a northeast direction and the water eventually reaches the Jefferson River. The east-southeast oriented South Willow Creek alignment is the same as the alignment of southeast oriented North Meadow Creek (shown but not labeled in figure 2), which east and south of figure 7 flows to the north oriented Madison River as a barbed tributary. Today the region between the north-northwest oriented Beall Creek headwaters and the northeast oriented South Boulder River (and the east-southeast oriented South Willow Creek valley segment) to the south includes high mountain ridges 1500 meters higher than the Jefferson River valley to the northwest and is probably the last place one would think to look for evidence of former flood flow channels. Yet a close look at the drainage divides reveals several passes or through valleys linking the north-northwest oriented Beall Creek valley with northeast oriented Curly Creek valley and also linking the Curly Creek valley with the northeast oriented South Boulder River valley. Some of these through valleys (or passes) are more than 250 meters deep and provide evidence of a former southeast oriented flood flow channel from the present day north-northwest oriented Beall Creek valley to the South Willow Creek valley and then to North Meadow Creek valley, which would have moved floodwaters across what is today the high Tobacco Root Mountains to a south oriented flood flow channel on the present day north oriented Madison River alignment. Obviously at that time the Tobacco Root Mountains did not stand high above surrounding region as they do today.

Detailed map of Beall Creek-South Boulder River drainage divide area

Figure 8: Detailed map of Beall Creek-South Boulder River 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 Beall Creek-South Boulder River drainage divide area seen in less detail in figure 7. The South Boulder River flows in a northeast and north-northeast direction from the south center edge of figure 8 to the north edge of figure 8 (near northeast corner). Curly Creek flows in a northeast direction across section 34, the southeast corner of section 27, and section 26 before turning to flow in an east direction to join the north-northeast oriented South Boulder River in the northeast corner of section 25. Beall Creek originates at Beall Lake (near south edge of section 28) and flows in a north direction to the north edge of figure 8 (near northwest corner). The map contour interval for figure 8 is 40 feet and elevations of mountain peaks in figure 8 exceed 10,000 feet. At first glance high narrow ridges serve as drainage divides between the Beall Creek valley and the Curly Creek valley and also between the Curly Creek valley and the South Boulder River valley. But a close look at those high narrow ridges reveals deep gaps or notches, which provide evidence of former valleys that existed prior to the erosion of the present day northeast oriented Curly Creek and South Boulder River valleys. For example in section 27 the elevation on the drainage divide between a north-northwest oriented Beall Creek tributary valley and the northeast oriented Curly Creek valley drops to between 9000 and 9040 feet. The mountain to the northeast in the southeast corner of section 22 rises to 10,059 feet and the mountain to the southwest near the south edge of section 33 rises to 10,428 feet suggesting the notch in section 27 is at least 1000 feet deep. This 1000-foot deep notch is what is left of a former southeast oriented flood flow channel that once crossed what is today the high Tobacco Root Mountains. Follow the southeast alignment to Westphal Parks in section 35 and there is another deep gap or notch in the Curly Creek-South Boulder River drainage divide. The floor of that notch at the drainage divide has an elevation of between 8600 and 8640 feet. The mountain to the northeast (near north edge of section 35) rises to 9281 feet and the high point in section 34 to the southwest rises to 9958 feet suggesting the notch is at least 600 feet deep. Other similar notches can be used to trace what were probably diverging and converging flood flow channels at a time when the Tobacco Root Mountains did not stand high above surrounding regions. Evidence in figure 8 suggests alpine glaciers once formed in some of the valleys and further modified the landscape. The alpine glaciers were formed long after the flood flow ended and filled preexisting valleys, which had been eroded by the earlier floodwaters.

Boulder Creek-Wisconsin Creek drainage divide area

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

Figure 9 illustrates the Boulder Creek-Wisconsin Creek drainage divide area south and west of figure 7 and includes a significant overlap area with figure 7. The Jefferson River is formed at the confluence of the northeast oriented Big Hole River, north-northeast oriented Beaverhead River, and northwest oriented Ruby River near the town of Twin Bridges and then flows in a north direction to the north edge of figure 9 (near northwest corner). The South Boulder River originates in the high Tobacco Root Mountains (northeast quadrant of figure 9) and flows in a northeast and north direction to the northeast corner of figure 9. Wisconsin Creek originates at Crystal Lake in the high Tobacco Root Mountains (south of Mount Jackson) and flows in a south-southwest, southwest, west, and southwest direction to join the northwest oriented Ruby River as a barbed tributary (south of figure 9). Note other south and southwest oriented Ruby River tributaries also originating in the Tobacco Root Mountains. While the Tobacco Root Mountains drainage pattern seen in figure 9 could be considered a radial drainage pattern, there are linkages between north oriented valleys and south oriented valleys, such as those already seen. For example, north of Crystal Lake a gap or notch links the south oriented Wisconsin Creek valley with a north-northwest oriented tributary (named Dry Boulder Creek on more detailed maps) to west-northwest oriented Boulder Creek. The map contour interval for figure 9 is 50 meters and the notch elevation (at the drainage divide) is between 2950 and 3000 meters. Elevations on both sides of the notch rise to more than 3150 meters suggesting the notch is at least 150 meters deep. Much deeper notches, passes, or through valleys can be seen a short distance to the southwest on the ridge linking Mount Jackson with Old Baldy Mountain and link northwest oriented tributary valleys to an unnamed west oriented Jefferson River tributary with southeast oriented Wisconsin Creek tributary valleys. Floor elevations of these two through valleys or passes at the drainage divide are between 2750 and 2800 meters. Elevations on Mount Jackson rise to more than 3150 meters and on Old Baldy Mountain elevations rise to more than 3000 meters suggesting these through valleys or passes are at least 200 meters deep. These through valleys, notches, or passes provide evidence of multiple southeast and south-southeast oriented flood flow channels that once crossed the present day high ridge linking Mount Jackson with Old Baldy Mountain. At the time floodwaters eroded those southeast oriented valleys the large and deep Jefferson River valley to the west did not exist and the Tobacco Root Mountains did not stand high above the surrounding region as they do today. The Tobacco Root Mountains were uplifted as floodwaters flowed across them suggesting crustal warping responsible for the Tobacco Mountains uplift was directly or indirectly related to the thick ice sheet presence north and east of the study region and/or to melt water flood caused erosion and depositions taking place in the region.

Detailed map of Dry Boulder Creek-Wisconsin Creek drainage divide area

Figure 10:Detailed map of Dry Boulder Creek-Wisconsin 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 Dry Boulder Creek-Wisconsin Creek drainage divide area seen in less detail in figure 9. The region seen in figure 10 is in the high Tobacco Root Mountains and there is evidence of alpine glaciation, although our concern in this essay is with the origin of the drainage divides and valleys between those drainage divides. The alpine glaciation occurred after the drainage divides and valleys were formed and while the glaciation did modify the valleys and drainage divides the glaciation did not form the valleys and drainage divides. Crystal Lake is located in the south center region of figure 10 and Wisconsin Creek flows in a south direction from Crystal Lake to the south center edge of figure 10. Upper Boulder Lake is located in the north center region of figure 10 and Dry Boulder Creek flows in north-northwest direction from Upper Boulder Lake to a lake straddling the north edge of figure 10 (just west of center). Between Upper Boulder Lake and Crystal Lake is a high narrow ridge or arête linking APA Mountain with Lakeshore Mountain. The map contour interval for figure 10 is 40 feet and APA Mountain reaches an elevation of 10,400 feet while Lakeshore Mountain reaches an elevation of 10,451 feet. The ridge between the two mountains drops to an elevation of between 9640 and 9680 feet at its lowest point or at least 720 feet lower than the adjacent mountains. This notch is what remains of a south oriented flood flow channel that once crossed the region. Perhaps more convincing is Low Pass seen in the southwest corner of section 8 (southwest from APA Mountain). Low Pass has an elevation at the drainage divide of between 9240 and 9280 feet. Old Baldy Mountain, which is located just south of figure 10 reaches an elevation of 9907 feet suggesting Low Pass is at least 720 feet deep. Another northwest-to-southeast oriented pass or through valley is located south and west of Low Pass and has an elevation at the drainage divide of between 9080 and 9120 feet, suggesting the unnamed pass or through valley may be as much as 800 feet deep. While a case could be made for glacial erosion of the notch between APA Mountain and Lakeshore Mountain the two deeper passes or through valleys to the southwest are not in the glaciated region, which supports a non-glacial origin for the notch or pass between APA Mountain and Lakeshore Mountain. These passes or through valleys are remnants of converging south and southeast flood flow channels that once crossed the region and which were captured by headward erosion of a much deeper south-southwest oriented flood flow channel on the present day Wisconsin Creek alignment.

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.