Milk River-Missouri River drainage divide area landform origins between Beaver Creek and Larb Creek, Montana, USA

· Milk River, Montana, MT Missouri River
Authors

A geomorphic history based on topographic map evidence

Abstract:

The Milk River-Missouri River drainage divide area between Beaver Creek and Larb Creek is located in Phillips and Valley Counties, Montana, USA. Although detailed topographic maps of the Milk River-Missouri River drainage divide area between Beaver Creek and Larb Creek have been available for more than fifty years detailed map evidence has not previously been used to interpret the region’s geomorphic history. The interpretation provided here is based entirely on topographic map evidence. Beaver Creek and Larb Creek are north oriented Milk River tributaries. The Milk River is an east oriented Missouri River tributary and is located north of the Missouri River. The Milk River-Missouri River drainage divide area between Beaver Creek and Larb Creek is interpreted to have been eroded during immense southeast-oriented flood events, the first of which flowed on a topographic surface at least as high as the highest points in the present-day drainage divide area. Flood erosion across the drainage divide ended when headward erosion of the Milk and Missouri River valleys captured all southeast-oriented flood flow.

Preface:

The following interpretation of detailed topographic map evidence is provided as evidence in the Missouri River drainage basin landform origins research project, which is compiling similar evidence for all major drainage divides contained within the Missouri River drainage basin and for all major drainage divides with and within certain adjacent drainage basins. The research project is interpreting evidence in the context of a previously unexplored geomorphology paradigm, which is briefly described in the introduction below. 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 Montana Milk River-Missouri River drainage divide area landform origins between Beaver Creek and Larb Creek. 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 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 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 similar essays is a thick North American ice sheet, comparable in thickness to the present day Antarctic ice sheet, occupied approximately the North American region usually recognized to have been glaciated and through its weight and erosive actions created a “deep” North American “hole”, 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 Milk River-Missouri River drainage divide area landform evidence between Beaver Creek and Larb Creek in Montana will be regarded as evidence supporting the “thick ice sheet that melted fast” paradigm.

Milk River-Missouri River drainage divide area between Beaver Creek and Larb Creek location map

Figure 1: Milk River-Missouri River drainage divide area between Beaver Creek and Larb Creek 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 Milk River-Missouri River drainage divide area between Beaver Creek and Larb Creek location map and illustrates a region in northeast Montana. The Missouri River flows in an east-southeast direction from the figure 1 west edge to Fort Peck Lake before joining the southeast-oriented Milk River and flowing to Brockton, Montana near the figure 1 east edge. The Milk River flows from the northwest corner area to Havre, Chinook, Zurich, Harlem, Dodson, Wagner, Malta, Saco, Hinsdale, Vandalia, Tampico, and Glasgow and joins the Missouri River east of Nashua, Montana. The Milk River-Missouri River drainage divide area discussed here is located between Beaver Creek and Larb Creek. Beaver Creek flows southeast from the Little Rocky Mountains and then northeast and southeast before turning north and finally turns southeast and east to join the Milk River near Hinsdale. Larb Creek as shown in figure 1 flows north to join the Milk River near Saco. The unnamed south-oriented stream flowing from the Larb Creek headwaters area to the Missouri River is Timber Creek, and also serves as eastern boundary for the drainage divide region discussed here. The western boundary of the drainage divide region discussed is north-oriented Beaver Creek valley segment and a through valley (not shown in figure 1) linking that north-oriented Beaver Creek valley segment with the Missouri River valley to the south. The Milk River is the northern boundary and the Missouri River is the southern boundary. The Milk River-Missouri River drainage divide area west of the area discussed is addressed in other essays by looking at drainage divides between additional Milk River and Missouri River tributaries (Beaver Creek, Peoples Creek, Birch Creek, and Big Sandy Creek, among others). Based on evidence from the hundreds of Missouri River drainage basin landform origins research project essays published on this website landform evidence illustrated here is interpreted in the context of an immense southeast-oriented flood flowing across the figure 1 map area and which was systematically captured and diverted northeast by headward erosion of deep valleys eroded into a topographic surface at least as high as the figure 1 region highest elevations today.

  • The east-oriented Missouri River valley eroded west to near the location of Nashua (where the Milk River joins the Missouri River) and the Milk River valley eroded northwest to capture southeast-oriented flood water and to divert flood flow to the newly eroded Missouri River valley east of Nashua. Northeast, east, and north-oriented tributary valleys eroded headward from the newly eroded north-oriented Milk River valley and one such tributary valley was the valley now used by the northeast-oriented Missouri River located southwest of Nashua. The north-oriented Larb Creek and Beaver Creek valleys were north-oriented Milk River tributary valleys that eroded south to capture southeast-oriented flood flow. The north-oriented Larb Creek valley eroded south along what is today the south-oriented Timber Creek valley to the present day Missouri River valley location, and perhaps southwest from there. The north-oriented Beaver Creek valley segment also eroded south along what is today a south-oriented Telegraph Creek valley segment and southwest and west from there (Telegraph Creek is not shown on figure 1). However, the present day northeast-oriented Missouri River valley, beheaded north-oriented flood flow to both the north-oriented Telegraph Creek-Beaver Creek valley and the north-oriented Timber Creek-Larb Creek valley. Flood waters on the south ends of the beheaded north-oriented flood flow routes reversed flow direction and eroded the present day south-oriented Telegraph Creek valley and the south-oriented Timber Creek valley segments. The Milk River-Missouri River drainage divide area east of Larb Creek essay, Missouri River-Big Dry Creek drainage divide area essay, the Musselshell River-Yellowstone River drainage divide area in Musselshell and Yellowstone Counties essay, the Musselshell River-Yellowstone River drainage divide area in Treasure and Rosebud Counties essay, and in the Big Dry Creek-Yellowstone River drainage divide area essay describe drainage divide areas located near the Milk River-Missouri River drainage divide area discussed here and can be found under appropriate river or creek names on the sidebar category list (use MT Missouri River for Missouri River in Montana-also shorter streams generally have shorter essay lists).

Milk River-Missouri River drainage divide area between Beaver Creek and Larb Creek detailed location map

Figure 2: Milk River-Missouri River drainage divide area between Beaver Creek and Larb Creek detailed location map. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 2 illustrates a somewhat more detailed map of the Milk River-Missouri River drainage divide area discussed in this essay. Phillips and Valley Counties are located in Montana. The Missouri River forms the county line in the figure 2 south half and flows northeast to join the southeast-oriented Milk River downstream from Fort Peck Lake in the figure 2 east center area. In Phillips County the Milk River flows east and northeast in the figure 2 north half and then turns southeast as it flows into Valley County to join the Missouri River downstream from Nashua. Larb Creek flows north in western Valley County (near the county line) and enters Phillips County in the Larb Hills area before joining east-oriented Beaver Creek near Beaverton. Beaver Creek flows southeast and northeast from the Little Rocky Mountains (near figure 2 west edge) to the 7th Auxiliary Guide Meridian and then turns southeast before turning north. Beaver Creek after flowing north turns southeast and east and joins the southeast-oriented Milk River near Hinsdale. Timber Creek flows south along the same alignment as that used by north-oriented Larb Creek and joins the Missouri River as a barbed tributary near Phillips-Valley County line. Telegraph Creek is a northwest, south-southwest and southeast-oriented Missouri River tributary located southeast of Sun Prairie in the figure 2 south center area. Telegraph Creek flows south in a large through valley that forms the western boundary for the drainage divide region discussed in this essay. Beaver Creek flows north in the same through valley and Flat Creek (near Sun Prairie) is a north-oriented Beaver Creek tributary that also drains the through valley. Note the large number of southeast-oriented and northwest-oriented Beaver Creek tributaries and other southeast-oriented and northwest-oriented drainage in the Beaver Creek-Larb Creek drainage divide area. The southeast-northwest drainage alignment is evidence major trunk stream valleys were eroded headward across multiple southeast-oriented flood flow channels such as might be found in a large southeast-oriented anastomosing channel complex. The northwest-oriented tributary valleys were eroded by reversals of flood water on the northwest ends of beheaded southeast-oriented flood flow channels. Because channels were anastomosing (or interconnected) and because trunk stream valleys eroded headward, the reversed flood flow in newly beheaded channels could usually capture flood waters from adjacent yet to be beheaded flood flow channels. With the help of such captured yet to be beheaded flood waters the reversed flood flow in newly beheaded flood flow channels could often erode significant northwest-oriented valleys. Detailed maps below better tell the story.

North end of Beaver Creek-Larb Creek drainage divide area

Figure 3: North end of Beaver Creek-Larb Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 3 illustrates the Milk River valley at the north end of the Beaver Creek-Larb Creek drainage divide area. The Milk River enters the figure 3 map area in the northwest corner and makes a loop to the north around a maze of abandoned valleys (one of which is the location of Nelson Reservoir) before turning east-southeast to flow to the figure 3 east edge. The maze of abandoned valleys is a large flood eroded anastomosing channel complex, which provides evidence the Milk River valley was eroded by a large southeast-oriented flood. Beaver Creek flows northeast along the west side of the Larb Hills and turns to flow southeast along the northeast side of the Larb Hills. Larb Creek has eroded a deep north-oriented valley into the Larb Hills (located in the figure 3 southeast quadrant) and joins Beaver Creek near Beaverton. Beaver Creek joins the southeast-oriented Milk River east of the figure 3 Map area. The north end of the Beaver Creek-Larb Creek drainage divide area discussed here is in the Larb Hills west of Larb Creek and east of Beaver Creek. Note southeast-oriented Larb Creek tributaries and northwest-oriented Beaver Creek tributaries. This southeast-northwest orientation of tributary valleys is evidence the Larb Hills were eroded by southeast-oriented flood waters moving on a topographic surface at least as high as the highest Larb Hills elevations today. At that time the deep Milk River and Beaver Creek valleys to the north and west did not exist. Headward erosion of the deep southeast-oriented Milk River and Beaver Creek valleys was into that high level topographic surface. The deep north-oriented Larb Creek valley eroded south from the newly eroded southeast-oriented Milk River-Beaver Creek valley to capture southeast-oriented flood flow and next the deep northeast-oriented Beaver Creek valley eroded southwest to capture southeast-oriented flood flow that was moving to what was then the newly eroded north-oriented Larb Creek valley. Northwest-oriented Beaver Creek tributary valleys were eroded by reversals of flood flow on the northwest ends of beheaded southeast-oriented flood flow routes. Subsequently, flood waters from west of figure 3 eroded the anastomosing maze of Milk River valley channels.

Beaver Creek-Larb Creek drainage divide area near Lost Coulee

Figure 4: Beaver Creek-Larb Creek drainage divide area near Lost Coulee. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 4 illustrates the Beaver Creek-Larb Creek drainage divide area south of the figure 3 map area and includes overlap areas with figure 3. North-oriented Beaver Creek is located in the figure 4 west half and north-oriented Larb Creek is located in the figure 4 east half. Note southeast-oriented Larb Creek tributaries and the northwest orientation of Beaver Creek tributaries or of at least segments of the Beaver Creek tributary valleys.  Southeast-oriented Lost Coulee located in the figure 4 southeast quadrant has eroded headward into a high level erosion surface. Figure 4 evidence can be explained by southeast-oriented flood waters flowing across the entire figure 4 map area and eroding what is today the high level erosion surface forming the present day Beaver Creek-Larb Creek drainage divide. How much material was removed prior to formation of that erosion surface cannot be determined from evidence presented here, although it is probable considerable overlying material was removed. Headward erosion of the deep north-oriented Larb Creek valley captured the southeast-oriented flood flow and diverted the flood water north into what was then the newly eroded southeast-oriented Beaver Creek-Milk River valley. Subsequently headward erosion of the north-oriented Beaver Creek valley captured the southeast-oriented flood flow and in doing so beheaded flood flow routes to the newly eroded north-oriented Larb Creek valley. Flood waters on the northwest ends of the beheaded southeast-oriented flood flow routes reversed flow to flow northwest and west to the newly eroded north-oriented Beaver Creek valley. Subsequently headward erosion of the Beaver Creek valley captured significant southeast-oriented flood waters further to the south (see figures below). Large volumes of flood waters moved north in the newly eroded Beaver Creek valley and eroded the large Beaver Creek valley seen today. As seen in figures below the Larb Creek valley and the north-oriented Beaver Creek valley are the north ends of well-defined north-south through valleys linking the present day Missouri River valley with the Milk River valley. It is possible that for a short period of time these two through valleys were components of an east-oriented anastomosing channel complex moving flood waters between the present day Missouri and Milk River valleys.

Beaver Creek-Larb Creek drainage divide area south of Moss Coulee

Figure 5: Beaver Creek-Larb Creek drainage divide area south of Moss Coulee. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 5 illustrates the Beaver Creek-Larb Creek drainage divide area south of the figure 4 map area and includes overlap areas with figure 4. North-oriented Beaver Creek is located in the figure 5 west half and north-northeast and north oriented Larb Creek is located in the figure 5 east half. Moss Coulee drains west to join Beaver Creek in the figure 5 northwest quadrant. Note southeast-oriented Larb Creek tributaries and northwest-oriented Beaver Creek tributaries. For example, Snowbank Coulee is a northwest-oriented Moss Coulee tributary and is linked by a through valley with southeast-oriented Box Elder Coulee. Continue south along the drainage divide and note other northwest-southeast oriented through valleys linking northwest-oriented Beaver Creek tributary valleys with southeast-oriented Larb Creek tributaries. These through valleys in addition to the northwest-southeast drainage alignment provide evidence southeast-oriented flood waters flowed across the present day Beaver Creek-Larb Creek drainage divide into what was then the newly eroded north-northeast and north oriented Larb Creek valley. The source of the southeast-oriented flood waters cannot be determined from evidence presented here. However, the hundreds of Missouri River drainage basin landform origins research project essays published on this website when taken as a group can be used to trace flood waters both up flood to source areas and down flood to see where flood waters were going. A logical flood water source would be rapid melting of a thick North American ice sheet located in a deep “hole” occupying approximately the North American location usually recognized to have been glaciated. The deep “hole” would have been created by deep glacial erosion and by crustal warping caused by the ice sheet weight. Such a flood water source would not only explain the immense southeast-oriented floods this essay series describes, but would also explain why deep valleys were eroding headward to capture the southeast-oriented flood waters and diverting the flood waters further and further to the northeast and north into space in the deep “hole” the rapidly melting thick ice sheet had once occupied.

Beaver Creek-Larb Creek drainage divide area at Larb Creek headwaters

Figure 6: Beaver Creek-Larb Creek drainage divide area at Larb Creek headwaters. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 6 illustrates the Beaver Creek-Larb Creek drainage divide area southeast of the figure 5 map area. Larb Creek flows southeast to the figure 6 southeast corner and then north-northeast along the figure 6 east edge. Beaver Creek flows northeast across the figure 6 northwest corner. The same pattern of northwest-southeast oriented through valleys linking headwaters northwest-oriented Beaver Creek tributaries and with headwaters of southeast-oriented Larb Creek tributaries provides evidence southeast flood waters once flowed across the present day Beaver Creek-Larb Creek drainage divide. The southeast-oriented flood waters were captured by headward erosion of the north-northeast oriented Larb Creek valley and diverted north to the southeast-oriented Milk River valley. Subsequently headward erosion of the northeast and north-oriented Beaver Creek valley captured the southeast-oriented flood waters and by doing so beheaded the southeast-oriented flood flow routes to the newly eroded north-northeast oriented Larb Creek valley. Named northwest-oriented Beaver Creek tributaries from north to south are Grove Coulee, Spring Creek, and Tallow Creek. Note how headwaters of northwest-oriented Grove Coulee area linked by a northwest-southeast oriented through valley with headwaters of a southeast-oriented Larb Creek tributary valley. Continuing south note how a well-defined northwest-southeast oriented through valley links the northwest-oriented Spring Creek valley with the southeast-oriented Larb Creek headwaters valley. Continuing southeast along the Larb Creek valley note how it enters a large north-oriented through valley that continues south of the Larb Creek elbow of capture. South of the Larb Creek elbow of capture the north-south through valley is drained by south-oriented Timber Creek. Figure 6a below illustrates the Larb Creek-Timber Creek drainage divide. In figure 6a the Larb Creek elbow of capture is located in the north center. Note how Timber Creek flows northwest from the figure 6a east center edge area and then west to enter the north-south oriented through valley and then turns south in the through valley to flow the figure 6a south edge. The northwest- and west-oriented Timber Creek valley segments were eroded by reversals of flood flow when the north-oriented Larb Creek valley eroded south and the southeast-oriented Larb Creek headwaters valley was eroded by southeast-oriented flood water flowing to the newly eroded north-oriented Larb Creek valley. The Larb Creek-Timber Creek drainage divide was created when headward erosion of the deep Missouri River valley to the south beheaded north-oriented flood flow in what was then a north-oriented flood flow channel. Flood waters on the south end of the beheaded north-oriented flood flow channel reversed direction and flowed south to the newly eroded and deeper Missouri River valley. The reversed flood waters eroded the south-oriented Timber Creek valley and created the Larb Creek-Timber Creek drainage divide. Note the relationship of the Larb Creek-Timber Creek drainage divide area to the Sag seen in the figure 6a southwest quadrant and in figure 7 below.

Figure 6a: Larb Creek-Timber Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Sun Prairie Flats-Beaver Creek elbow of Capture

Figure 7: Sun Prairie Flats-Beaver Creek elbow of capture. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 7 illustrates the Beaver Creek elbow of capture northwest of the figure 6 map area. A large northeast oriented valley extends from the figure 7 southwest corner to the figure 7 north edge. Beaver Creek flows southeast from the figure 7 northwest corner area into the large northeast oriented valley and flows then flows northeast in the northeast oriented valley. Northeast-oriented Flat Creek drains the Sun Prairie Flats valley southwest of the Beaver Creek elbow of capture. Northwest-oriented Spring Creek flows from the figure 7 east edge to join Flat Creek north of Content, Montana (see figure 6 for Spring Creek headwaters and relationship to southeast-oriented Larb Creek headwaters). Northwest-oriented Tallow Creek flows from the figure 7 south edge (east half). The Tallow Creek drainage divide with southeast-oriented Lone Tree Creek, which flows to south-oriented Timber Creek, is shown in figure 8a below and illustrates a northwest-southeast oriented through valley. Tributaries to the large northeast-oriented Sun Prairie Flats valley from the northwest (including Beaver Creek) are southeast-oriented and from the southeast are northwest-oriented, providing evidence the northeast-oriented Sun Prairie Flats valley was eroded headward across southeast-oriented flood flow routes. The southeast-oriented tributary valleys eroded headward along southeast-oriented flood flow routes from what was then the newly eroded north-oriented Sun Prairie Flats valley. The northwest-oriented tributary valleys were eroded by reversals of flood flow on the northwest ends of beheaded southeast-oriented flood flow routes. What makes the Sun Prairie Flats valley unusual is its size, which indicates large volumes of flood water moved through the valley. Figure 7a below illustrates the Sun Prairie Flats southwest of the figure 7 map area and again shows northwest-oriented tributaries from the southeast and southeast-oriented tributaries from northwest. The northwest-oriented Sage Creek valley in the figure 7a southeast quadrant is an unusually large and deep northwest-southeast oriented through valley crossing the present day Flat Creek-Timber Creek drainage divide. Note how northwest-oriented Sage Creek flows to north-northeast-oriented Flat Creek, which as seen in figure 7 flow to northeast- and north-oriented Beaver Creek. Figure 8 below, which depicts an area south and east of the figure 7a map area, illustrates the Sage Creek-Timber Creek drainage divide in that northwest-southeast oriented through valley. Timber Creek flows south to join the Missouri River.

Figure 7a: Sun Prairie Flats and Sage Creek valley. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Sage Creek-Timber Creek drainage divide

Figure 8: Sage Creek-Timber Creek drainage divide. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 8 illustrates the Sage Creek-Timber Creek drainage divide in the large and deep northwest-southeast oriented through valley linking the north-oriented Flat Creek-Beaver Creek valley with the south-oriented Timber Creek valley. Figures 6 and 6a illustrated the Timber Creek-Larb Creek through valley and the drainage divide in that valley north of the figure 8 map area. Figure 7a illustrated where west and northwest-oriented Sage Creek, which flows along the floor of the deep through valley and then into the northeast oriented Sun Prairie Flats valley, joins northeast oriented Flat Creek in flowing to northeast and north oriented Beaver Creek. South-oriented Timber Creek is located in the figure 8 east half and is flowing south in the south end of the north-south oriented Larb Creek-Timber Creek through valley. Note southeast oriented Lone Tree Creek flowing from the figure 8 north center area into the northwest-southeast oriented Sage Creek through valley and draining the east end of that through valley. Figure 8a below illustrates how Lone Tree Creek headwaters are linked to northwest-oriented Tallow Creek headwaters by a northwest-southeast oriented through valley, which provides evidence the Sage Creek-Lone Tree Creek through valley was not the only deep through valley crossing the drainage divide. The multiple deep through valleys suggest there were multiple channels of southeast-oriented flood flow that were reversed to flow northwest and north to the north-oriented Beaver Creek valley. Figure 8 evidence shows predominantly southeast-oriented tributary valleys to the Timber Creek valley, which provide evidence the Timber Creek valley eroded headward to capture multiple southeast-oriented flood flow routes. Based on figure 8 evidence alone one could argue the Timber Creek valley initially eroded north from the deep Missouri River valley. However, it is the south end of the north-south oriented Larb Creek-Timber Creek through valley and probably was eroded headward from the Milk River valley in the north and subsequently reversed on the south end. Probably the Larb Creek-Timber Creek drainage divide developed while large volumes of southeast-oriented flood flow were still moving into what is now the south-oriented Timber Creek drainage basin (note how Timber Creek has eroded a deep south-oriented valley). But, flood flow changes in the west apparently caused southeast-oriented flood waters to reverse flow direction and to flow west and northwest to the north-oriented Beaver Creek valley.

Figure 8a: Tallow Creek-Lone Tree Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Sun Prairie Flats through valley

Figure 9: Sun Prairie Flats through valley. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 9 illustrates the Sun Prairie Flats through valley located south of the figure 7a map area and includes overlap areas with figure 7a. West and northwest-oriented Sage Creek is located in the figure 9 northeast corner area. Flat Creek flows southeast in the Big Sag located along the figure 9 north edge to the lake in the figure 9 north center area and then flows northeast to join northeast and north-oriented Beaver Creek as shown in previous figures. First Creek flows southeast from the figure 9 northwest corner area to the First Creek Reservoir and then into the Sun Prairie Flats area where on figure 9 its course is then lost. Northwest-oriented Telegraph Creek flows from the figure 9 southeast corner to enter the north-northeast oriented Sun Prairie Flats through valley and then turns southwest and south to flow to Missouri River. Second Creek flows southeast in the figure 9 southwest quadrant and appears to join south-oriented Telegraph Creek. While the Sun Prairie Flats valley, the Big Sag valley, and the Sage Creek valley are unusually large the same pattern of northwest-oriented tributaries from the east and southeast-oriented tributaries from the northwest prevails. This pattern provides evidence the north-northeast oriented Sun Prairie Flats valley was eroded headward across southeast-oriented flood flow that originally flowed on a topographic surface at least as high as the highest figure 9 elevations today. For reasons previously stated (and also based on evidence seen in figure 10 below) I believe the Sun Prairie Flats valley was originally eroded as a north-oriented valley and the drainage divide between north-oriented Beaver Creek (Flat Creek) and south-oriented Telegraph Creek developed when headward erosion of the deep Missouri River valley captured the north-oriented food flow route supplying flood waters to the Sun Prairie Flats valley.

Telegraph Creek-Missouri River drainage divide area

Figure 10: Telegraph Creek-Missouri River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 10 illustrates the south end of the Sun Prairie Flats valley south of the figure 9 map area and includes overlap areas with figure 9. The flooded Missouri River valley (flooded behind Fort Peck Dam) are located in the figure 10 south half. The north-northeast oriented Sun Prairie Flats valley is located in the figure 10 west half and Telegraph Creek flows south-southwest and then southeast and east into the deep Missouri River valley. Note how the through valley continues southwest to the figure 10 southwest corner from where Telegraph Creek turns to flow southeast and is now drained by a northeast-oriented tributary to southeast-oriented Telegraph Creek. This evidence suggests the north-oriented flood waters that eroded the large north-oriented Sun Prairie Flats-Beaver Creek through valley came from west of the figure 10 map area and that headward erosion of the deep Missouri River valley from the northeast and east (aided by headward erosion of the southeast-oriented Telegraph Creek valley segment) captured the northeast-oriented flood flow and also reversed flood waters on the south end of the present day Telegraph Creek-Beaver Creek through valley. While outside of the region covered by this essay, the northeast-oriented flood flow to the Sun Prairie Flats valley must have been beheaded further to the west at about the same time it was captured by headward erosion of the southeast-oriented Telegraph Creek valley segment. Other figure 10 evidence shows northwest-oriented Telegraph Creek tributaries flowing from the Larb Hills area towards the figure 10 north edge (center and east half) and multiple southeast-oriented Missouri River tributaries flowing in the opposite direction. A close look at the evidence reveals through valleys linking headwaters of the northwest-oriented Telegraph Creek tributaries and the headwaters of the southeast-oriented Missouri River tributaries. The northwest-southeast drainage orientation and the through valleys provide evidence that southeast-oriented flood waters once flowed on a topographic surface at least as high as the present day Larb Hills. Flood waters were first captured by headward erosion of the deep north-oriented Missouri River valley (located in the figure 10 southeast corner). However, headward erosion of the northeast and north-oriented Sun Prairie Flats-Beaver Creek valley beheaded the southeast-oriented flood flow shortly thereafter (because the southeast-oriented Missouri River valley tributaries are short). The large Sun Prairie Flats valley was eroded during the time period it took the Missouri River valley (and southeast-oriented Telegraph Creek valley segment) to erode headward from the figure 10 southeast corner to capture northeast-oriented flood flow in the Sun Prairie Flats valley.

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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