Montana Dearborn River drainage basin landform origins overview essay

Authors

Abstract:

This is essay is an introduction to a collection of essays describing drainage divide areas in and surrounding the Montana Dearborn River drainage basin. This essay contains an overview of more detailed information contained in the five much more detailed essays, which can be found under the Dearborn River category (see sidebar). The Dearborn River is a southeast, northeast, and southeast-oriented Missouri River tributary originating in high mountains along the east-west continental divide with water eventually reaching the Gulf of Mexico. The southeast-oriented Dearborn River flows to a north-northeast oriented Missouri River segment as a barbed tributary. Deep through valleys link southeast-oriented Dearborn River tributaries and headwaters with north-oriented Sun River tributaries (with the Sun River being an east-oriented Missouri River tributary north of the Dearborn River). Further north- and east-oriented Dearborn River tributaries are linked by deep through valleys (or mountain passes) across the continental divide with south and west-oriented Blackfoot River tributaries, with Blackfoot River water eventually reaching the Pacific Ocean. Topographic map evidence illustrated in this introduction and much better illustrated in the more detailed essay demonstrates the Dearborn River basin was eroded by immense south and southeast-oriented floods. Flood waters originally flowed on an erosion surface equivalent to the elevation of the east-west continental divide elevation today, although the region was probably uplifted as flood waters flowed across it. Flood waters were captured by deep east- and west-oriented valleys that eroded headward into the region to carve the east-west continental divide. North-oriented valleys were eroded by massive flood flow reversals triggered when headward erosion of much deeper valleys beheaded south-oriented flood flow channels. Floods are interpreted to have been derived from a rapidly melting North American ice sheet, which was located in a deep “hole” and were flowing in a south and southeast direction from western Canada along the deep “hole’s” western and southwestern rim.

Introduction

  • This essay provides an introduction to the Montana Dearborn River drainage basin landform origins essay collection, which consists of five much more detailed essays illustrating and describing specific drainage divide areas within and surrounding the Dearborn River drainage basin. Access to the five more detailed essays can be obtained by selecting the Dearborn River category (see sidebar or heading under this essay’s title). All interpretations in this introduction and in the five more detailed essays are based entirely on topographic map evidence and no effort has been made to introduce evidence from other sources. Topographic map evidence from the Missouri River drainage basin landform origins research project, which is preparing detailed essays illustrating and describing all drainage divide areas within and surrounding the much larger Missouri River drainage basin, have been used in determining flood water sources and routes outside of the Dearborn River drainage basin region. The Missouri River drainage basin landform origins research project page provides in addition to a project introduction links to knoll collections related to specific Missouri River tributaries and segments. The project is underway as this introduction is being written and is expected to be completed in late 2012 or early 2013.
  • Topographic map evidence illustrated in the four topographic maps included in this introduction and the 40 topographic maps illustrated in the five more detailed individual essays in the collection demonstrates the Dearborn River drainage basin was eroded by massive south and southeast-oriented floods, which initially flowed on an erosion surface as high as the highest Dearborn River drainage basin elevations today (which include the present day east-west continental divide). Flood waters were captured by headward erosion of deep valleys from both east and west of the continental divide as deep south and east-oriented oriented anastomosing complexes of diverging and converging flood flow channels eroded headward from these deep valleys. Continued headward erosion of deep valleys into the region subsequently dismembered the east and south oriented flood flow channels leaving a record of through valleys across present day drainage divides, including the continental divide, and of elbows of capture and barbed tributaries. North-oriented valleys were eroded by flood waters on north ends of south oriented flood flow channels which had been beheaded and reversed by headward erosion of much deeper east- or northeast-oriented valleys. Based on evidence from other Missouri River drainage basin landform origins research project essays flood waters were derived from a rapidly melting thick North American ice sheet located in a deep “hole.” The Dearborn River drainage basin is located south of the deep “hole’s” western rim, which is today the Canadian Rocky Mountains in Alberta and British Columbia, and immense south and southeast oriented ice marginal floods flowed from Canada in south and southeast directions into Montana and across the Dearborn River drainage basin. Uplift of the Lewis and Clark Mountain Range, where the Dearborn River drainage basin is located, was probably caused by crustal warping related to the ice sheet’s tremendous weight and probably occurred as flood waters flowed across the region. While this interpretation is fundamentally different from previously published interpretations the topographic map evidence on which this flood origin interpretation is based has never been explained by previous investigators and is being interpreted for the first time in these essays.

Dearborn River drainage basin location map

Figure 1: Dearborn River drainage basin location map (select and click on maps to enlarge). National Geographic Society map digitally presented using National Geographic Society TOPO software.

 

  • Before proceeding further we need to locate the Montana Dearborn River drainage basin. Figure 1 is a location map showing a large region in central and western Montana. Great Falls is the major city located near the figure 1 center. The shaded green area in the figure 1 northwest corner is the southeast end of Glacier National Park. The east-west continental divide extends in a south-southeast direction from the figure 1 northwest corner to Marias Pass (along Glacier National Park south boundary) and then along or near the Lewis and Clark Range crest to the figure 1 south edge (west of Helena). The Missouri River flows in a north-northwest direction from the figure 1 south center edge (east of Helena) to near Wolf Creek and then turns to flow in a northeast direction to Great Falls, Fort Benton and Loma. South of the town of Big Sandy the Missouri River abruptly turns to flow in a south-southeast and east direction to the figure 1 east edge (north half) with water eventually reaching the Gulf of Mexico. Several Missouri River tributaries from the south tend to be oriented in north and north-northwest directions and in addition to the Missouri River segment upstream from the Wolf Creek area include the north-northwest oriented Smith River. These north-oriented tributary and headwaters valleys were eroded by reversals of flood flow on north ends of south-oriented flood flow channels beheaded by headward erosion of the much deeper Missouri River valley. Missouri River tributaries from the west tend to be oriented in more of an east direction and from south to north these include the Dearborn River, Sun River, Teton River, and Marias River. These major east-oriented Missouri River tributaries originate on the east side of the east-west continental divide and were eroded in sequence with headward erosion of each east-oriented valley beheading south-oriented flood flow channels to what was then the newly eroded east-oriented valley immediately to the south. West of the continental divide are the west-oriented Blackfoot River, north-northwest oriented South Fork Flathead River, and northwest-oriented Middle Fork Flathead River with water in those drainage systems eventually reaching the Pacific Ocean. The north-northwest oriented South Fork Flathead River and northwest-oriented Middle Fork Flathead River valleys were eroded by reversals of flood flow on north and northwest ends of south- and southeast-oriented flood flow channels which were beheaded by headward erosion of the much deeper south-oriented Flathead River valley (west of figure 1). The southeast-oriented Dearborn River drainage basin is located immediately east of the of the southwest and west-oriented Blackfoot River drainage basin and headward erosion of deep valleys in both drainage basins competed with each other in capturing flood flow channels supplying flood water to actively eroding valleys in the other drainage basin.

Dearborn River-North Fork Blackfoot River drainage divide area

Figure 2: Dearborn River-North Fork Blackfoot River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

 

  • Figure 2, which illustrates the Dearborn River-North Fork Blackfoot River drainage area, is an example of topographic map evidence supporting the deep flood erosion interpretation. The east-west continental divide is marked with a dashed line and is labeled near the south edge of the figure 2 southeast quadrant and extends in a northwest direction to the figure 2 west edge (north half). North and east of the continental divide are the Dearborn and Sun River drainage basins with water flowing to the Missouri River and eventually reaching the Gulf of Mexico. South and west of the continental divide is the North Fork Blackfoot River drainage basin with water eventually reaching the Pacific Ocean. Straight Creek Pass (just south of the figure 2 north center edge) is located on the Dearborn River-Sun River drainage divide. Straight Creek is a northwest-oriented Sun River tributary while Welcome Creek is a southeast-oriented Dearborn River tributary, which joins the northeast-oriented Dearborn River near Welcome Pass. After being joined by southeast-oriented Welcome Creek the Dearborn River abruptly turns to flow in a southeast direction to the figure 2 east edge (south half). Welcome Pass, which is located directly northeast of the Dearborn River elbow of capture, links the Dearborn River valley with the north-northeast oriented Moudess Creek valley, which drains to northeast-oriented Smith Creek which then flows to the east-oriented Sun River.
  • Smith Creek originates as a northwest oriented stream at Elk Pass (in figure 2 northeast quadrant west of Morse Mountain) and turns to flow in a north and northeast direction near the figure 2 north edge. The stream originating on the southeast side of Elk Pass is Elk Creek, which flows in a southeast direction to the figure east edge (south of Morse Mountain) and then which turns to flow in a northeast direction to join the east-oriented Sun River. The northwest-southeast oriented through valleys (at Elk Pass and Straight Creek Pass, among others) were eroded by southeast-oriented flood flow channels that were captured by headward erosion of deep valleys from the east. The Elk Pass flood flow channel was first captured by headward erosion of the much deeper northeast-oriented Elk Creek valley and was beheaded by headward erosion of the still deeper northeast-oriented Smith Creek valley. Flood waters on the northwest end of the beheaded flood flow channel reversed flow direction to erode the northwest- and north-oriented Smith Creek headwaters valley and to create the Smith Creek-Elk Creek drainage divide at Elk Pass. The same process occurred at Straight Creek Pass where headward erosion of the deep Dearborn River valley first captured the southeast-oriented flood flow channel. Headward erosion of the deeper east-oriented Sun River valley north of the figure 2 map area beheaded the flood flow channel and flood waters on the northwest end of the beheaded flood flow channel reversed flow direction to erode the northwest-oriented Straight Creek valley and to create the Straight Creek-Welcome Creek drainage divide at Straight Creek Pass.
  • South and west of the continental divide the west-southwest oriented stream flowing in Tobacco Valley is the North Fork Blackfoot River which gradually turns to flow in a southwest and south-southwest direction to the figure 2 south edge (west half) and south of the figure 2 map area flows in a south direction to the west-oriented Blackfoot River. Note how the North Fork Blackfoot River has north-oriented headwaters and how just east of the elbow of capture (where the north-oriented headwaters turn to flow in a west-southwest direction) there is an unlabeled pass across the continental divide (a pack trail crosses the continental divide at that location). The unnamed pass was eroded by an east and southeast-oriented flood flow channel which eroded headward from the deep southeast-oriented Dearborn River valley (or flood flow channel). In other words headward erosion of the deep southeast-oriented Dearborn River flood flow channel was capturing flood waters from west of the present day continental divide and the captured flood flow was moving to the Dearborn River flood flow channel along a route now used by the west-southwest oriented North Fork Blackfoot River segment in Tobacco Valley and then east to the Whitetail Creek alignment. Headward erosion of this deep east-oriented flood flow channel beheaded south-oriented flood flow on the present day north-oriented North Fork Blackfoot River headwaters alignment and flood waters on the north end of the beheaded flood flow channel reversed flow to erode the north-oriented North Fork Blackfoot River headwaters valley. Next headward erosion of the deeper south-oriented North Fork Blackfoot River valley (from what was then the actively eroding west-oriented Blackfoot River valley-south of figure 2) beheaded the east-oriented flood flow channel to the Dearborn River flood flow channel. Flood waters on the west end of the beheaded flood flow reversed flow direction to erode the west-southwest oriented North Fork Blackfoot River valley (Tobacco Valley), which captured the north-oriented North Fork Blackfoot River headwaters valley and which created the drainage divide (continental divide) between the North Fork Blackfoot River and Dearborn River.

Middle Fork Dearborn River-Blackfoot River drainage divide area

Figure 3: Middle Fork Dearborn River-Blackfoot River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

 

  • Figure 3 illustrates another example of topographic map evidence this time in the Middle Fork Dearborn River-Blackfoot River drainage divide area south and east of the figure 2 map area. The east-west continental divide is again marked with a dashed line and is labeled in the figure 3 center region and extends in south-southeast direction from the figure 3 north edge (west half) to the figure 3 south center edge. Highway 200 is the red highway extending from the figure 3 south edge (west half) to the figure 3 north edge (just east of center) and crosses the continental divide at Rogers Pass. The north-northeast oriented stream following highway 200 from Rogers Pass to the figure 3 north edge is the Middle Fork Dearborn River, which joins the Dearborn River north of the figure 3 map area. The north and north-northeast oriented stream originating just east of continental divide and north of the figure 3 south edge and flowing to the figure 3 north edge (east half) is the South Fork Dearborn River. The south-southwest oriented stream following highway 200 south of Rogers Pass is Pass Creek, which joins the west and southwest-oriented Blackfoot River, which then flows to the figure 3 south edge (west half). The south-southeast oriented Blackfoot River tributary flowing across the figure 3 southwest corner is Landers Fork and the south-southwest oriented Landers Fork tributary flowing from the Alice Creek Basin (near figure 3 northwest corner) is Alice Creek.
  • Rogers Pass is a deep through valley eroded across the continental divide and was eroded by a south-oriented flood flow channel to what was then the actively eroding Blackfoot River valley. A short distance north and west of Rogers Pass is Cadotte Pass, another deep pass or through valley eroded across the continental divide. North of Cadotte Pass is a northeast-oriented Middle Fork Dearborn River tributary valley. South of Cadotte Pass is the south-southwest oriented Cadotte Creek valley, which drains to the southwest-oriented Blackfoot River. Cadotte Pass was eroded by south-oriented flood flow channel that diverged from the Rogers Pass flood flow channel and then converged with it again in the Blackfoot River valley. The diverging and converging flood flow channels provide evidence of what was once a south-oriented anastomosing channel complex that crossed the east-west continental divide. Headward erosion of the much deeper Dearborn River valley north of the figure 3 map area beheaded the south-oriented flood flow channel and flood waters on the north end of the flood flow channel reversed flow direction to erode a deeper north-northeast oriented Middle Fork Dearborn River valley and its northeast-oriented tributary valley and to create the east-west continental divide at Rogers Pass and Cadotte Pass. The reversal in flood flow may have been aided by ice sheet caused crustal warping in the Lewis and Clark Range as flood waters flowed across the region.

Deadman Creek-Dog Creek drainage divide area

Figure 4 Deadman Creek-Dog Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

 

  • Moving north and east from the figure 3 map area figure 4 illustrates the Deadman Creek-Dog Creek drainage divide area and includes an overlap area with figure 3 (the southwest corner of figure 4 and northeast corner of figure 3 overlap). The north-northeast oriented Middle Fork Dearborn River joins the southeast-oriented Dearborn River near the figure 4 northwest corner and the Dearborn River continues to flow in a southeast direction until it is joined by the north-northeast oriented South Fork Dearborn River, at which point the Dearborn River turns to flow in a northeast direction before turning again near the figure 4 north center to meander in an east-southeast direction to the figure 4 east edge (north half) and then flow to the north-northeast oriented Missouri River (east of the figure 4 map area). The north-northeast oriented South Fork Dearborn River flows from the figure 4 southwest corner to join the Dearborn River. Note north of Limekiln Mountain (in figure 4 southwest quadrant) northwest-oriented Pruden Creek, which flows to the north-northeast oriented South Fork Dearborn River. Northwest-oriented Pruden Creek is linked by a northwest-southeast oriented through valley with southeast-oriented Rock Creek, which flows to the figure 4 south center edge and then to the Missouri River. Rock Creek flows along the southwest edge of The Reef, which appears to be a plunging anticline, and the through valley is probably related to the underlying geologic structure. However, the through valley is a water eroded feature and was eroded by southeast-oriented flood flow prior to headward erosion of the deep north-northeast-oriented South Fork Dearborn River valley, which beheaded the southeast-oriented flood flow. Flood waters on the northwest end of the beheaded flood flow channel reversed flow direction to erode the northwest-oriented Pruden Creek valley.
  • North and east of northwest-oriented Pruden Creek is the Nicholas Basin, which appears to be between the plunging anticline “arms”. The Nicholas Basin is drained by northwest-oriented tributaries to the north-northeast oriented South Fork Dearborn River. These northwest-oriented streams are flowing in valleys eroded by reversals of flood flow on northwest ends of beheaded flood flow routes. Flowing in a northeast direction across the plunging anticline “arms” is Deadman Creek which flows to the meandering east-southeast oriented Dearborn River. Note how Deadman Creek has eroded a well-defined water gap across the northeast “arm” of the plunging anticline and also has a northwest-oriented tributary and headwaters. Also note how a northwest-southeast oriented through valley links the southeast-oriented Dearborn River valley with the northeast-oriented Deadman Creek valley. Follow that northwest-southeast oriented through valley further south and east from the northeast-oriented Deadman Creek valley to the southeast-oriented Dog Creek valley, with Dog Creek being a southeast-oriented stream flowing to the figure 4 south edge (near southeast corner) and then to the Missouri River. The northwest-southeast oriented through valley is evidence of a large southeast-oriented flood flow channel that was systematically dismembered and captured by headward erosion of the deeper northeast-oriented Deadman Creek valley and then captured by headward erosion of the northeast-oriented Dearborn River valley segment. More through valleys in the figure 4 map area can be found and provide further evidence of southeast-oriented flood flow channels beheaded by headward erosion of deeper northeast-oriented valleys. Detailed topographic maps, some of which are illustrated in the more detailed essays, better illustrate the through valleys, elbows of capture, and barbed tributaries, all of which can best be explained in the context of the massive south and southeast-oriented flood flow.

South Fork Dearborn River-Little Prickly Pear Creek drainage divide area

Figure 5: South Fork Dearborn River-Little Prickly Pear drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

 

  • This overview of the Dearborn River drainage basin landform origins essay collection (found under Dearborn River on sidebar category list) concludes with a look at the South Fork Dearborn River-Little Prickly Pear Creek drainage divide area south of the figure 4 map area (and includes overlap areas with figure 4). Little Prickly Pear Creek flows in a north and northeast direction in the figure 5 southeast corner and joins the north-northwest and north-northeast oriented Missouri River at its elbow of capture a short distance east of the figure 5 map area. The north-northeast oriented Little Prickly Pear Creek valley can be considered a south-southwest extension of the north-northeast oriented Missouri River valley segment (see figure 1) while the north-northwest oriented Missouri River valley segment can be considered to be a very successful tributary valley eroded by a massive reversal of flood flow on the north end of a beheaded south- and/or southeast-oriented flood flow channel. The east-west continental divide is marked with a dashed line and extends from the figure 5 west edge (north of center) in a southeast and south direction to the figure 5 south edge (near southwest corner). The south-oriented stream originating just east of the continental divide is Canyon Creek, which is a Little Prickly Pear Creek tributary (the water makes a large U-turn to return to the figure 5 southeast corner). The north and north-northeast oriented stream originating just east of the continental divide and north of south-oriented Canyon Creek is the South Fork Dearborn River and flows to the figure 5 north edge (west half). The southeast-oriented stream originating just east of the South Fork Dearborn River headwaters and flowing to the figure 5 south center edge is the North Fork Lyons Creek, which south of the figure 5 map area joins Lyons Creek to flow to Little Prickly Pear Creek. The southeast-oriented stream originating between Roberts Mountain and Denton Mountain (just east of the South Fork Dearborn River) and joining Little Prickly Pear Creek at the town of Wolf Creek is Wolf Creek. Rock Creek can be seen flowing in a southeast direction along the southwest margin of the Reef and joins the Missouri River just downstream from the point where Little Prickly Pear Creek joins the Missouri River.
  • Note how the southeast-oriented Wolf Creek valley is linked to the north-oriented South Fork Dearborn River valley by a well-defined through valley between Roberts Mountain and Denton Mountain. The through valley is evidence of a former southeast-oriented flood flow channel that was beheaded by headward erosion of the much deeper north and north-northeast oriented Dearborn River valley. A similar northwest-southeast oriented through valley links the north-oriented South Fork Dearborn River headwaters valley with the southeast-oriented North Fork Lyons Creek valley. This northwest-southeast oriented through valley is also evidence of a southeast-oriented flood flow channel beheaded by headward erosion of the much deeper north-oriented South Fork Dearborn River valley. A less obvious north-south oriented through valley links the north-oriented South Fork Dearborn River valley with the south-oriented Canyon Creek valley, which is also linked by a shallow northwest-southeast oriented through valley across the continental divide with a northwest-oriented Blackfoot River headwaters valley. What is most remarkable about some of these shallower through valleys (or mountain passes) is their elevation. Today some of these through valleys are little more than shallow saddles notched into some of the highest mountain ridges seen on the figure 5 map. What this means is flood waters initially flowed on an erosion surface as high or higher than the highest figure 5 elevations today (although at that time the mountains were probably still being uplifted). The topographic map evidence strongly suggests the present day rugged mountain topography was produced by a combination of deep flood water erosion as deep valleys eroded headward into the region combined with mountain uplift which occurred as flood waters flowed across the region. If correctly interpreted the mountain uplift was caused by delayed crustal warping triggered by the thick ice sheet’s tremendous weight and perhaps aided by deep flood water erosion of great thickness of bedrock from the rising mountain ranges. Many more examples of the topographic map evidence supporting this flood origin interpretation are provided in the more detailed essays.

Additional information and sources of maps studied

This essay has provided only a sample of the detailed topographic map evidence supporting the flood erosion interpretation. Many additional illustrations could be provided. Readers are encouraged to look at mosaics of detailed topographic maps to see the abundance of available data. Maps used in this study were created and published by the United States Geologic Survey and can be obtained directly from the United States Geological Survey and/or from dealers offering United States Geological Survey maps. Hard copy maps can also be observed at United States Geological Survey map depositories, which are located throughout the United States and elsewhere. Illustrations used here were created using National Geographic Society TOPO software and digital map data. TOPO software and map data can be obtained from the National Geographic Society and/or dealers offering National Geographic Society digital map data.

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