Abstract:
This essay uses topographic map evidence to interpret Dearborn River-South Fork Dearborn River drainage divide area landform origins in Lewis and Clark County, Montana. The Dearborn River originates in the high Lewis and Clark Mountain Range along the east-west continental divide and flows in a southeast direction with some northeast-oriented segments to join a north-northeast oriented Missouri River segment. The South Fork Dearborn River also originates along the continental divide and flows in a north and north-northeast direction to join the southeast-oriented Dearborn River. The Dearborn River-South Fork Dearborn River drainage divide area investigated here is located north and east of the continental divide, south of the Dearborn River, and west of the South Fork Dearborn River. Mountain ridges and other drainage divides in the study region are crossed by deep through valleys providing evidence of former diverging and converging southeast-oriented flood flow channels which were systematically captured by headward erosion of the much deeper Dearborn River valley and its deep tributary valleys. Topographic map evidence suggests the presence of a large-scale anastomosing channel complex that was initially eroded into a high level erosion surface equivalent in elevation to the highest study region mountain ridges today, including the present day east-west continental divide. Based on evidence from essays written about other similar drainage divide areas flood waters are interpreted to have been derived from a rapidly melting thick North American ice sheet located in a deep “hole.” Mountain ranges forming the east-west continental divide to the north and in the study region are interpreted to have been uplifted by ice sheet related crustal warping which among other things formed the deep “hole’s” west and southwest rim. Immense south and southeast-oriented ice-marginal melt water floods flowed from Canada into Montana along this deep “hole” rim and were captured by headward erosion of deep valleys, such as the deep Dearborn River valley and its tributary valleys, as the mountain ranges were being uplifted.
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 available at this site may be found by selecting desired Missouri River tributaries and/or states from this essay’s sidebar category list.
Introduction
- The purpose of this essay is to use topographic map interpretation methods to explore the Dearborn River-South Fork Dearborn River drainage divide area landform origins in Lewis and Clark County, Montana, USA. Map interpretation methods can be used to unravel many geomorphic 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 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 essays in the Missouri River drainage basin landform origins research project 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 Dearborn River-South Fork Dearborn River drainage divide area landform evidence in Lewis and Clark County, Montana will be regarded as evidence supporting the “thick ice sheet that melted fast” paradigm (see essay available at header). This essay is included in the Missouri River drainage basin landform origins research project essay collection.
Dearborn River-South Fork Dearborn River drainage divide area location map
Figure 1: Dearborn River-South Fork Dearborn 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 Dearborn River-South Fork Dearborn River drainage basin location map and illustrates a large region in west-central Montana with a small area in Idaho located in the southwest corner. The shaded green area straddling the north edge of the figure 1 northwest quadrant is the Glacier National Park south half. The east-west continental divide extends in a south-southeast direction through Glacier National Park to Marias Pass and then along or near the Lewis and Clark Range crest to the figure 1 south edge (west of Helena). Figure 1 areas east of the continental divide drain to the Missouri River with water eventually reaching the Gulf of Mexico. Montana figure 1 regions west of the continental divide drain to north and northwest-oriented Clark Fork, which flows to the figure 1 west center edge, with water eventually reaching the Pacific Ocean. The Missouri River flows in a north-northwest direction along west side of the Big Belt Mountains in the figure 1 southeast quadrant from the figure 1 south edge to Wolf Creek. At Wolf Creek the Missouri River turns to flow in a northeast direction to Great Falls, Fort Benton, Loma, and the figure 1 east edge (north half). Tributaries to the northeast-oriented Missouri River segment from the south include the north-northwest oriented Smith River and other unnamed northwest and north-northwest oriented streams. Named tributaries to the northeast-oriented Missouri River segment from the west include (from south to north) the southeast, northeast, southeast, east, and southeast oriented Dearborn River; the south-southeast and east oriented Sun River; the east-oriented Teton River; and the east-southeast and south oriented Marias River. Each of these rivers has headwaters originating along the east-west continental divide. The Dearborn River drains the southern Lewis and Clark Range east of the continental divide while the south-southeast and west oriented Blackfoot River drains the southern Lewis and Clark Range west of the continental divide. The South Fork Dearborn River is not shown in figure 1, but is a north-northeast oriented tributary originating along the continental divide directly west of the town of Wolf Creek and flowing parallel to and southeast from highway 200 to join the Dearborn River. The Dearborn River-South Dearborn River drainage divide area investigated in this essay is located west and north of the South Fork Dearborn River, north and east of the continental divide, and south and west of the Dearborn River. Other Dearborn River drainage divide area essays, including an essay describing the Dearborn River-Blackfoot River drainage divide segment of the continental divide can be found under the Dearborn River category listed on the sidebar.
- Today the Dearborn River-South Dearborn River drainage divide area is a region of high mountains and deep valleys, although topographic map evidence suggests at one time immense south and southeast-oriented floods flowed across the region on a high level erosion surface at least as high as the high mountain ridges (including the continental divide) today. Based on hundreds of previously written essays in the Missouri River drainage basin landform origins research project the flood waters are interpreted to have been derived from the western margin of a rapidly melting North American ice sheet. The ice sheet was located in a deep “hole” and initially stood above its western margin. The Missouri River drainage basin in Montana and northern Wyoming is the deep “hole’s” deeply eroded southwest wall while the Canadian Rocky Mountains in Alberta and British Columbia represent the deep “hole’s” deeply eroded west rim. Huge ice-marginal meltwater floods flowed in south and southeast directions along the deep “hole’s” west rim from Canada into western Montana and then across the figure 1 area. At that time present day topography did not exist and flood waters flowed freely along routes corresponding with crests of present day mountain ranges, including routes parallel to and crossing the modern-day east-west continental divide. These immense floods initially continued in south and southeast directions into Wyoming, Colorado, and even New Mexico until uplift of the Rocky Mountains (from south to north) diverted flood waters to both the east and the west. Diversion of flood waters to the east and west was aided by headward erosion of deep valleys from both directions to capture the massive south and southeast-oriented flood flow. Headward erosion of these deep valleys combined with mountain uplift, which occurred as flood waters flowed across them, systematically carved the east-west continental divide from south to north. Mountain uplift was probably the result of crustal warping caused by the ice sheet’s great weight and may have been further aided by deep flood water erosion of bedrock from the rising mountain crests. This pattern of continental divide formation proceeded until mountain uplift and ice sheet melting reached a stage where the south and southeast-oriented ice-marginal melt water floods flowing from Canada across Montana and northern Wyoming were flowing on a bedrock surface higher in elevation than at least some regions on the decaying ice sheet’s surface. West of the present day east-west continental divide headward erosion of the deep Columbia River-Snake River valley also initiated a different erosion pattern.
- The deep “hole” did not exist when the ice sheet first formed, but developed over time as the ice sheet existed and proceeded to melt. Probably for much of its history the ice sheet stood high above the surrounding non glaciated regions, which probably included much of Montana and adjacent northern Wyoming. In time however, the ice sheet surface was lowered and crustal warping raised the deep “hole” rim in the form of mountain ranges and the bedrock surface south and west of the decaying ice sheet’s southwest margin was no long significantly lower than the ice sheet surface (at least near the southwest margin). Lowest points on the decaying ice sheet surface were floors of giant south oriented ice-walled canyons being carved into the ice sheet surface by huge south-oriented supra-glacial melt water rivers supplying vast quantities of water to the evolving Mississippi River and Missouri River valley systems. Of particular importance to Montana was a giant southeast and south-oriented ice-walled canyon in present day Saskatchewan, North Dakota, and South Dakota that in time became an ice-walled and bedrock-floored canyon which detached the ice sheet’s southwest margin. Today the Missouri Escarpment in Saskatchewan, North Dakota, and South Dakota is what remains of that giant ice-walled canyon’s southwest and west wall. Deep northeast and east-oriented valleys eroded headward from that giant ice-walled canyon into Montana and northern Wyoming to capture the immense south- and southeast-oriented melt water floods south and west of the ice sheet southwest margin. These valleys eroded headward in sequence with headward erosion of each valley beheading flood flow routes to the newly eroded valley immediately to the southeast or south. Flood waters on northwest and north ends of beheaded flood flow channels often reversed flow direction to erode northwest and north-oriented tributary valleys. In the figure 1 map area the north-northwest oriented Missouri River valley segment and Smith River valley along with valleys of other northwest and north-oriented Missouri River tributaries were eroded by such reversals of flood flow on north ends of what had been south-oriented flood flow channels. This pattern is seen on a smaller scale in the Dearborn River drainage basin topographic maps illustrated below.
Detailed location map for Dearborn River-South Fork Dearborn River drainage divide area
Figure 2: Detailed location map for Dearborn River-South Fork Dearborn 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 Dearborn River-South Fork Dearborn River drainage divide area in Lewis and Clark County, Montana. County boundaries are shown and county names are given. Lewis and Clark County is located in the figure 2 center, Powell County in the southwest quadrant, and Cascade County in the northeast corner. Green areas are National Forest lands and generally represent mountainous regions. The Missouri River flows in a north-northwest direction from the figure 2 southeast corner to Holter Lake and Holter Dam and then turns to flow in a north-northeast direction to the figure 2 east center edge. The east-west continental divide is marked with a dashed line, is labeled in the figure 2 south center region and can be traced in a north-northwest direction to the figure 2 north edge (west half). The Dearborn River originates near Scapegoat Mountain (just north of the words “Scapegoat Wilderness”) and flows in an east direction almost to Welcome Pass where it then turns to flow in a southeast direction to south of Steamboat Mountain. South of Steamboat Mountain the Dearborn River turns to flow for a short distance in a northeast direction and then flows in a southeast direction across southwest-northeast oriented highway 200 to meet the north-northeast oriented South Fork Dearborn River. After being joined by the South Fork Dearborn River the Dearborn River makes a northeast jog and then resumes its southeast direction to flow to the north-northeast oriented Missouri River along the Lewis and Clark-Cascade County line (near the figure 2 east center edge). The South Fork Dearborn River originates just east of the continental divide (west of the town of Wolf Creek) and flows in a north and north-northeast direction to join the Dearborn River. West of the north-northeast oriented South Fork Dearborn River is the north-northeast oriented Middle Fork Dearborn River, which originates at Rogers Pass. Rogers Pass is an important pass across the continental divide and links the north-northeast oriented Middle Fork Dearborn River valley with the southwest and west oriented Blackfoot River valley. Alice Creek is a south-oriented Blackfoot River tributary west of Rogers Pass and is aligned with a an unnamed north-oriented Dearborn River tributary (East Fork Falls Creek) north of the continental divide. Landers Fork is a southeast oriented Blackfoot River tributary west of Alice Creek and flows parallel to the southeast-oriented Dearborn River, although on the west side of the continental divide. Topographic maps illustrated below show evidence for southeast- and south-oriented flood flow channels that crossed the region prior to headward erosion of present day deep valleys.
Middle Fork Dearborn River-South Fork Dearborn River drainage divide area
Figure 3: Middle Fork Dearborn River-South Fork Dearborn River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.
- Figure 3 illustrates the Middle Fork Dearborn River-South Fork Dearborn River drainage divide area. The east-west continental divide is marked with a dashed line and is labeled and extends in a south-southeast and southeast direction from the figure 3 northwest corner to the figure 3 south edge (west of center). The South Fork Dearborn River originates south of the figure 3 south center edge (just east of the continental divide) and flows in a north and north-northeast direction from the figure 3 south center edge to the north edge (east half). Highway 200 is the highway going from near the figure 3 southwest corner to the figure 3 north center edge and crosses the continental divide at Rogers Pass. North of Rogers Pass highway 200 is located adjacent to the north-northeast oriented Middle Fork Dearborn River, which flows to the figure 3 north center edge. South of Rogers Pass highway 200 follows south-southwest oriented Pass Creek to the west and southwest-oriented Blackfoot River valley (seen in the figure 3 southwest corner area). Just west and north of Rogers Pass is Cadotte Pass, which links a northeast-oriented Middle Fork Dearborn River tributary valley with the south-southwest oriented Cadotte Creek valley. South of Red Mountain (in the figure 3 northwest corner) is Lewis and Clark Pass, which links the east-northeast oriented Green Creek valley (a Middle Fork Dearborn River tributary) with the south-southwest and south-southeast oriented Alice Creek valley (not seen in figure 3, but a Blackfoot River tributary). These and other unnamed passes across the east-west continental divide are evidence of south-oriented flood flow channels that once crossed the figure 3 map area.
- Flood flow across the present day continental divide at Rogers Pass and Cadotte Pass was beheaded by headward erosion of the deep Dearborn River valley north of the figure 3 map area. Flood waters on the north end of beheaded flood flow channels reversed flow direction to erode the north-northeast oriented Middle Fork Dearborn River valley. The north and north-northeast oriented South Fork Dearborn River valley was eroded by a similar reversal of a south-oriented flood flow channel. Headward erosion of the deep Dearborn River valley beheaded the south-oriented flood flow channels in sequence from east to west. These flood flow reversals occurred at a time when areas to the west of the reversed flood flow channels had not yet been deeply eroded and flood waters were still flowing on a high level erosion surface. These yet to be captured south-oriented flood waters were captured by the newly reversed flood flow and helped erode the newly reversed and deep north-oriented valleys. Note the area between the Middle Fork Dearborn River and the South Fork Dearborn River north and east of Sunset Mountain. Note how several northwest-southeast oriented through valleys cross the drainage divide. For example note the northwest-southeast oriented through valley north of the figure 3 center linking the southeast-oriented Camp Creek valley with the north-northeast oriented Middle Fork Dearborn River valley. These northwest-southeast oriented through valleys were eroded by southeast-oriented flood flow channels prior to reversal of what had been the south-oriented Middle Fork Dearborn River flood flow channel. Reversal of the Middle Fork Dearborn River flood flow channel to erode the much deeper north-northeast oriented Middle Fork Dearborn River valley beheaded the southeast-oriented flood flow channels. Flood waters on northwest ends of the beheaded southeast-oriented flood flow channels also reversed flow direction to erode northwest-oriented Middle Fork Dearborn River tributary valleys.
Detailed map of Middle Fork Dearborn River-Camp Creek drainage divide area
Figure 4: Detailed map of Middle Fork Dearborn River-Camp 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 Middle Fork Dearborn River-Camp Creek drainage divide area seen in less detail in figure 3. The north-northeast oriented Middle Fork Dearborn River flows from the figure 4 west center edge to the figure 4 north edge (west half). The South Fork Dearborn River flows from the figure 4 south edge (east half) to the figure 4 east center edge. Camp Creek originates a short distance south of figure 4 and flows in a northeast direction to join a southeast-oriented tributary in section 25 and then to flow in a southeast direction to join the north-northeast oriented South Fork Dearborn River as a barbed tributary near the figure 4 south edge. The southeast-oriented Camp Creek tributary is linked by a northwest-southeast oriented through valley in the section 25 northwest quadrant with a north and north-northwest oriented Middle Fork Dearborn River tributary valley and also less directly with a northwest-oriented South Fork Dearborn River tributary valley. The section 25 through valley floor elevation at the drainage divide is shown by a spot elevation of 5072 feet (the figure 4 map contour interval is 40 feet). The hill-top in the section 24 southeast quadrant (to the northeast of the through valley) has an elevation of between 5640 and 5680 feet (Joes Mountain further to the northeast rises to 5921 feet). Sunset Mountain in section 26 (southwest of the through valley) rises to 6375 feet. In other words the northwest-southeast oriented through valley in section 25 is approximately 600 feet deep and may be even deeper based on the Joes Mountain elevation. This 600 plus foot deep through valley is a water eroded feature and was eroded by water from two converging flood flow channels prior to headward erosion of the deep north-northeast oriented Middle Fork Dearborn River valley. The two converging flood flow channels were beheaded and reversed by headward erosion of the much deeper north-northeast oriented Middle Fork Dearborn River valley to erode the deeper north and north-northwest and northwest-oriented Middle Fork Dearborn River tributary valleys seen in figure 4. The converging flood flow channels provide evidence of a southeast-oriented anastomosing channel complex, which eroded deep flood flow channel valleys headward from the deep north-northeast oriented South Fork Dearborn River valley prior to headward erosion of the deep Middle Fork Dearborn River valley. Probably prior to headward erosion of the deep South Fork Dearborn River valley flood waters were flowing across the figure 4 map area on an erosion surface as high or higher than the highest figure 4 elevations today.
Detailed map of Middle Fork Dearborn River-South Fork Dearborn River drainage divide area
Figure 5: Detailed map of Middle Fork Dearborn River-South Fork Dearborn River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.
- Figure 5 provides a detailed topographic map of the Middle Fork Dearborn River-South Fork Dearborn River drainage divide area seen in less detail in figure 3 and located north and east of the figure 4 map area. The Middle Fork Dearborn River flows in a north-northeast direction from the figure 5 southwest corner to the figure 5 north edge (west half) and joins the southeast-oriented Dearborn River north of the figure 5 map area. The Dearborn River flows in a southeast direction from the figure 5 north center edge to the figure 5 east edge (north half) where it is joined by the north-northeast oriented South Fork Dearborn River and makes a jog to the northeast. Johnson Mountain is the high forested area straddling the figure 5 south edge (just west of center) and reaches an elevation of 5910 feet (the figure 5 map contour interval is 40 feet). The hill in the section 32 northeast corner, which is probably an eroded hogback ridge, reaches an elevation of 4707 feet. The highway in section 32 makes use of a northwest-southeast oriented through valley eroded between the two above high points. The through valley today links the north-oriented Spring Gulch valley, which drains to the north-northeast oriented Middle Fork Dearborn River valley with an east and northeast-oriented Dearborn River tributary valley, but in a larger picture links the north-northeast oriented Middle Fork Dearborn River valley with the north-northeast oriented South Fork Dearborn River valley. The through valley is a water eroded feature and was eroded by southeast-oriented flood flow moving to what was then the newly eroded and deep north-northeast oriented South Fork Dearborn River valley. Note northwest-oriented South Dearborn River tributaries, which flow in valleys eroded by reversals of flood flow on northwest ends of beheaded southeast-oriented flood flow channels. Headward erosion of the deep north-northeast oriented Middle Fork Dearborn River valley beheaded the southeast-oriented flood flow to the newly eroded South Fork Dearborn River valley and the Dearborn River valley (as shown by the unnamed Dearborn River tributary flowing from section 32 through sections 33, 34, and 35 to join the Dearborn River in section 26). The figure 5 map area is located along the mountain front and probably is underlain by steeply dipping beds, which have influenced how the region was eroded. However, the valleys are water eroded valleys and can best be explained in the context of massive southeast-oriented flood flow across the entire region.
East Fork Falls Creek-Middle Fork Dearborn River drainage divide area
Figure 6: East Fork Falls Creek-Middle Fork Dearborn River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.
- Figure 6 illustrates the East Fork Falls Creek-Middle Fork Dearborn River drainage divide area north and west of the figure 3 map area and includes overlap areas with figure 3. The east-west continental divide is marked by a dashed line along the high ridge in the figure 6 southwest quadrant and connects Burnt Point, Red Mountain and Lewis and Clark Pass. The South Fork Dearborn River flows in a north-northeast direction across the figure 6 southeast corner. The Dearborn River flows in a southeast direction from the figure 6 north edge (east of center) to the figure 6 east edge (north of center). The Middle Fork Dearborn River flows in a north-northeast direction from the figure 6 south edge (east of center, where highway 200 crosses the figure 6 south edge) and flows to the Dearborn River near the figure 6 east edge. The East Fork Falls Creek originates just north of the continental divide, between Burnt Point and Red Mountain, and flows in a north and west-northwest direction into the figure 6 northwest quadrant to join the northeast-oriented West Fork Falls Creek and to form north-oriented Falls Creek, which flows to the figure 6 north edge (west half) and which joins the Dearborn River north of the figure 6 map area. Middle Fork Dearborn River tributaries from the west include (from south to north) Bedrock Creek, Big Skunk Creek, and Cuniff Creek. Note how these three east and southeast-oriented Middle Fork Dearborn River are linked by high level through valleys with the north-oriented East Fork Falls Creek or north-oriented Falls Creek valleys. The high level through valley linking the east and southeast-oriented Bedrock Creek headwaters with the north-oriented East Fork Falls Creek valley is located north of Red Mountain and south of (Falls) Creek Ridge. The through valley appears as a saddle on a narrow north-south oriented ridge. The figure 6 contour interval is 50 meters and low point on the saddle (or the through valley floor) elevation is between 1900 and 1950 meters. (Falls) Creek Ridge to the north reaches an elevation of 2089 meters while Red Mountain to the south rises to 2218 meters. Based on the (Falls) Creek Ridge elevation the through valley linking the East Fork Falls Creek valley with the Bedrock Creek valley is at least 300 meters deep. What appears to be a saddle eroded into a narrow ridge provides evidence of an east- or southeast-oriented flood flow channel which once moved flood water to what was then the actively eroding Bedrock Creek valley. The through valley linking the East Fork Falls Creek valley with the east-southeast oriented Big Skunk Creek valley is illustrated in figure 7 while the through valley linking the north-oriented Falls Creek valley with the east-southeast oriented Cuniff Creek valley is illustrated in figures 8 and 9 below.
Detailed map of East Fork Falls Creek-Big Skunk Creek drainage divide area
Figure 7: Detailed map of East Fork Falls Creek-Big Skunk Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.
- Figure 7 provides a detailed topographic map of the East Fork Falls Creek-Big Skunk Creek drainage divide area seen in less detail in figure 6 above. The East Fork Falls Creek flows in a north and west-northwest direction from the figure 7 south center edge to the figure 7 west center edge and west of figure 7 joins the West Fork Falls Creek to form north-oriented Falls Creek. A southwest-dipping erosion resistant rock unit appears to be capping Table Mountain and west- and southwest-dipping rock units also appear to influencing topography in the figure 7 southwest quadrant. The west-northwest oriented East Fork Falls Creek valley segment has eroded a deep water gap across these dipping erosion resistant rock layers. Big Skunk Creek originates near the corner of sections 7, 12, 13, and 18 and flows in an east, southeast, and east direction to the figure 7 east edge (south of center) and east of the figure 7 map area to the north-northeast oriented Middle Fork Dearborn River. A southwest-oriented East Fork Falls Creek tributary valley in section 13 is linked by a through valley with the east-oriented Big Skunk Creek headwaters valley. The figure 7 map contour interval is 40 feet and the through valley floor elevation at the drainage divide is between 6280 and 6320 feet. Falls Creek Ridge to the south rises to 6783 feet (6886 feet south of the figure 7 map area) while Table Mountain to the north rises to 7122 feet (and to 7163 feet north of figure 7). The through valley is at least 500 feet deep and is a water eroded feature. Probably the west-northwest oriented East Falls Creek valley segment was initiated as western extension of the east-southeast oriented Big Skunk Creek valley, which was initiated as a flood flow channel to what was then the newly eroded and deep north-northeast oriented Middle Fork Dearborn River valley. At that time the deep Dearborn River valley was eroding headward north of the figure 7 map area and in time it beheaded south-oriented flood flow routes to developing and deep east-southeast oriented flood channels eroding headward from the newly eroded Middle Fork Dearborn River valley. Flood waters on north ends of the beheaded flood flow routes reversed flow direction to erode the much deeper north-oriented Falls Creek valley and to reverse east-southeast oriented flood flow to erode a much deeper west-northwest oriented East Fork Falls Creek valley. The much deeper west-northwest-oriented East Fork Falls Creek valley beheaded south-oriented flood flow on what is now the north-oriented East Fork Falls Creek alignment. Flood waters on the north end of the beheaded flood flow channel reversed flow direction to erode a much deeper north-oriented East Fork Falls Creek valley. The flood flow reversals and erosion of the much deeper valleys was probably aided by mountain uplift that occurred at the same time.
Dearborn River-Middle Fork Dearborn River drainage divide area
Figure 8: Dearborn River-Middle Fork Dearborn River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.
- Figure 8 illustrates the Dearborn River-Middle Fork Dearborn River drainage divide area north and west of the figure 6 map area and includes significant overlap areas with figure 6. The Dearborn River flows in a southeast direction in Devils Glen (near figure 8 west center edge) and then turns to flow in a northeast direction almost to Bean Lake (in figure 8 north center). Just before reaching Bean Lake the Dearborn River turns to flow in a southeast direction and with a small jog to the northeast continues to flow in a southeast direction to the figure 8 east edge (south half). The north-northeast oriented Middle Fork Dearborn River flows across the figure 8 southeast corner and joins the Dearborn River east of the figure 8 map area. Southeast-oriented Auchard Creek in the figure 8 northeast quadrant is a Dearborn River tributary as are northeast oriented headwaters of Flat Creek (unlabeled in figure 8) flowing to the figure 8 north center edge. The East Fork Falls Creek flows in a north direction from the figure 8 south center edge (between Blowout Mountain and Creek Ridge) and then turns to flow in a west-northwest direction to join northeast-oriented West Fork Falls Creek and to form north-oriented Falls Creek, which then flows to the northeast-oriented Dearborn River. Joslin Creek is the northwest-oriented tributary flowing from the Joslin Basin to join Falls Creek near the point where it joins the Dearborn River. South and east of Joslin Basin is Cuniff Basin where Cuniff Creek originates. Cuniff Creek flows in a northeast direction toward the southeast oriented Dearborn River, but then turns to flow in a southeast direction parallel to the adjacent Dearborn River before finally joining the Dearborn River near the figure 8 east edge. Note how southeast-oriented Cuniff Creek headwaters are linked by a northwest-southeast oriented through valley with northwest-oriented Joslin Creek. Figure 9 provides a detailed topographic map to better illustrate the through valley, although the through valley is evidence of another southeast-oriented flood flow channel that existed prior to headward erosion of the deep northeast-oriented Dearborn River valley and its deep north-oriented Falls Creek tributary valley. A north-south oriented through valley just east of the mountain front linking the southeast- and northeast-oriented Dearborn River valley (at the elbow of capture) with the northeast- and southeast-oriented Cuniff Creek valley (at its elbow of capture) provides evidence of what were once converging and diverging flood flow channels. Flood waters were coming from north and west of the figure 8 map area, which as seen in earlier figures is located near the east-west continental divide. When the deep Dearborn River valley and its deep tributary valleys eroded headward into the figure 8 map area flood waters were initially flowing on a high level erosion surface at least as high as the high mountain ridges seen in figure 8. Through valleys (or saddles) can be seen eroded into those high mountain ridges and provide evidence of the flood flow channels that were captured by headward erosion of the deep Dearborn River valley and its deep tributary valleys (probably as the mountains were rising).
Detailed map of Joslin Creek-Cuniff Creek drainage divide area
Figure 9: Detailed map of Joslin Creek-Cuniff Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.
- Figure 9 illustrates a detailed topographic map of the Joslin Creek-Cuniff Creek drainage divide area seen in less detail in figure 8 above. The Dearborn River flows in a northeast direction across the figure 9 northwest corner and then flows in a southeast and east-northeast direction in the figure 9 northeast corner. Falls Creek is the north-oriented tributary flowing along the figure 9 west margin to join the northeast-oriented Dearborn River. Joslin Creek is the northwest-oriented stream originating in Joslin Basin (near figure 9 center) to join the northeast-oriented Dearborn River. Cuniff Creek originates in the Cuniff Basin (in figure 9 southeast quadrant) and flows in a northeast and east-northeast direction to the figure 9 east center edge. Note in the section PB 37 northwest corner a trail crossing the drainage divide between the Joslin Basin to the northwest and the Cuniff Basin to the southeast. The trail crosses the drainage divide at a pass (or through valley) which provides evidence of a northwest-southeast oriented flood flow channel that once crossed the region. At that time the deep northeast-oriented Dearborn River valley and its deep north-oriented Falls Creek valley did not exist and the deep Joslin Basin had yet to be eroded. Flood waters could freely flow from north and west of the figure 9 map area to what was then the actively eroding Cuniff Basin. Today the elevation where the trail crosses the drainage divide is between 5920 and 5940 feet (the figure 9 map contour interval is 20 feet, except along the south margin where the contour interval is 40 feet). The high ridge in section 36 to the north reaches an elevation of more than 6400 feet while Table Mountain to the south reaches an elevation of 7163 feet. The northwest-southeast oriented through valley linking the Joslin Basin and the Cuniff Basin is approximately 500 feet deep based on the ridge elevation to the north, but may have been much deeper when originally eroded. The northwest-oriented Joslin Creek valley, including Joslin Basin, was eroded by a reversal of flood flow when the much deeper northeast-oriented Dearborn River valley beheaded the southeast oriented flood flow channel. Probably yet to be beheaded flood flow from west of the actively eroding Dearborn River valley head was able to reach the Joslin Basin and to help erode the northwest-oriented valley and basin. At that time flood waters west of the actively eroding Cuniff Basin were flowing on a high level erosion surface equivalent in elevation to high ridges seen in figure 9 (if not higher), although mountain uplift was probably occurring at that time and then continued after flood flow ended. Headward erosion of the deep north-oriented Falls Creek valley captured southeast- and east-oriented flood waters from west of the actively eroding Dearborn River valley head and ended all flood flow to the Joslin Basin, which has changed little since.
Detailed map of Dearborn River-Cuniff Creek drainage divide area
Figure 10: Detailed map of Dearborn River-Cuniff 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 Dearborn River-Cuniff Creek drainage divide area seen in less detail in figure 8 and is located east of the figure 9 map area (and includes overlap areas with figure 9). The Dearborn River flows from the figure 10 west edge (north half) in a south, east, and northeast direction almost to the figure 10 north edge (in section 28) and then in section 27 turns to flow in a southeast direction to the figure 10 east edge (south half). Cuniff Creek flows in a northeast direction from the figure 10 west edge (south half) into section 32 and then flows in an east direction across the section 32 south half. Cuniff Creek next flows in a southeast direction across the section 33 southwest corner before flowing into section 4 where it turns to flow in an east direction to section 3. In section 3 Cuniff Creek turns again to flow in a southeast direction to the figure 10 south edge (near southeast corner). Note how a northwest-southeast oriented through valley extends from the Dearborn River valley in the section 29 southeast quadrant across section 13 to the Cuniff Creek valley in section 3 (a road is located on the floor of this through valley). This through valley is drained from section 13 to the southeast by a southeast-oriented Cuniff Creek tributary. Also note along the figure 10 west center edge a north-south oriented through valley linking the Dearborn River valley with the Cuniff Creek valley. In the figure 10 northeast quadrant, north and east of the Dearborn River valley there are additional northwest-southeast oriented through valleys with streamlined erosional residuals separating them. The figure 10 map evidence documents what was once a large-scale southeast-oriented anastomosing channel complex, where diverging and converging flood flow channels were being eroded into the regional landscape. Headward erosion of the deep Dearborn River valley eventually beheaded the diverging flood flow channels and became the surviving main drainage route. This anastomosing channel complex was eroded late in the erosion history of the region. As seen in earlier figures flood waters flowed across what are today much higher elevations west and north of the figure 10 map area. Probably those higher elevation regions were being uplifted as flood waters flowed across them (and have probably been uplifted further since), however it is also probable flood waters also deeply eroded regions east of the mountain front (such as the figure 10 map area). How much erosion took place in the figure 10 map area before the anastomosing channel complex was eroded into the figure 10 landscape cannot be determined from figure 10 evidence, however it is possible great thicknesses of bedrock materials were removed.
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.
Missouri River drainage basin landform origins research project 
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