A geomorphic history based on topographic map evidence

Boxelder Creek is a northeast-oriented tributary to the north and northeast-oriented Little Missouri River. Landforms located along the Boxelder Creek-Little Missouri River drainage divide provide important clues to the origin of eastern Montana and western South Dakota landforms. Although detailed topographic maps of the Boxelder Creek-Little Missouri River drainage divide area 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. Based on the topographic map evidence the Boxelder Creek-Little Missouri River drainage divide area is interpreted to have been eroded during immense 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 southeast of the Boxelder Creek-Little Missouri River drainage divide ended when headward erosion of the northeast-oriented Boxelder Creek valley captured the 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 Boxelder Creek-Little Missouri River drainage divide area landform origins. 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 Boxelder Creek-Little Missouri River drainage divide area landform evidence will be regarded as evidence supporting the “thick ice sheet that melted fast” paradigm.

Boxelder Creek-Little Missouri River location map

Figure 1: Boxelder Creek-Little Missouri River location map (select and click on maps to enlarge). United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

One of southeast Montana’s two major Little Missouri River tributaries is the northeast-oriented Boxelder Creek. Boxelder Creek begins west of the Little Missouri River near Hammond, Montana and flows in a northeast-oriented direction to join the Little Missouri near the point where North Dakota, South Dakota, and Montana meet (Harding County is the extreme northwest corner of South Dakota with North Dakota to the north and Carter County, Montana to the west). West of the Boxelder Creek drainage basin is the deeper Powder River drainage basin, with the Powder River flowing north-northeast and north-northwest to join the northeast-oriented Yellowstone River. South and southeast of the Boxelder Creek drainage basin are drainage basins of southeast-oriented Little Missouri River tributaries, with the Little Missouri River flowing northeast and north, sometimes parallel to Boxelder Creek. East of the Little Missouri River are east and southeast-oriented headwaters of the east-oriented Grand and Moreau Rivers. North of Boxelder Creek are headwaters of northwest-oriented streams flowing to the northeast-oriented Little Beaver Creek.

• Present-day major drainage routes as illustrated in this big picture view (figure 1) can be explained in the context of deep headcuts eroding headward to capture immense southeast-oriented floods. Initially the deep east–oriented Grand and Moreau River valley headcuts and their tributary valley headcuts eroded west and northwest to capture southeast-oriented floods. Southeast-oriented flood flow to these east-oriented headcuts was beheaded and captured and the present day Little Missouri River-Grand River and Little Missouri River-Moreau River drainage divides were created when the north and northeast-oriented Little Missouri River valley eroded south and southwest and diverted flood flow to the north. Headward erosion of the northeast-oriented Boxelder Creek headcut, also west of the newly eroded Little Missouri River valley captured southeast-oriented flood flow moving to the newly formed Little Missouri River valley.

Boxelder Creek confluence with Little Missouri River

Figure 2: Boxelder Creek confluence with Little Missouri River. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 2 illustrates the region where the northeast-oriented Boxelder Creek joins the north oriented Little Missouri River. Note the northwest-southeast alignment of the tributary drainage basins. Southeast-oriented Little Missouri River and Boxelder Creek tributaries are flowing southeast from a drainage divide with the Little Beaver Creek drainage basin to the northwest and are linked by through valleys to northwest-oriented Little Beaver Creek tributaries (not shown in figure 2). Northwest-oriented Little Missouri River tributaries are linked by through valleys with headwaters of the east-oriented Grand River. Note northwest-oriented Boxelder Creek tributaries and southeast-oriented Little Missouri River tributaries south of the Boxelder Creek-Little Missouri River confluence. This northwest-southeast alignment of tributary valleys is strong evidence multiple southeast-oriented flow routes, typical of those that would have been created by an immense southeast-oriented flood, predated headward erosion of the north oriented Little Missouri River valley and of the northeast-oriented Boxelder Creek valley. The Little Missouri River valley was eroded as a north oriented headcut face eroded south to systemically capture the southeast-oriented flood flow routes. Following headward erosion of the Little Missouri River valley headcut south of the present-day Boxelder Creek-Little Missouri confluence, although probably not much after, the Boxelder Creek valley headcut face began to erode southwest from the newly formed Little Missouri River valley and began to capture southeast-oriented flood flow that was moving to the newly formed Little Missouri River valley. Flood flow on the northwest ends of the various beheaded flood flow routes reversed flow direction to flow northwest into the newly formed and deeper north- and northeast-oriented Little Missouri River and Boxelder Creek valleys. These flow reversals often captured southeast-oriented flood flow from more southerly flow routes that had not yet been beheaded and were responsible for eroding the modern-day northwest-oriented tributary valleys.

Shaw Creek-Little Missouri River confluence area

Figure 3: Shaw Creek-Little Missouri River confluence area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 3 illustrates the region just southwest of the Boxelder Creek-Little Missouri River confluence area. Again the most striking feature is the northwest-southeast alignment of tributary valleys indicating the north-oriented Little Missouri River valley headcut and the northeast-oriented Boxelder Creek valley headcut eroded headward across multiple southeast-oriented flood flow channels to capture the flood flow and divert it north and northeast. Shaw Creek in the southeast quadrant of figure 3 illustrates how another northeast-oriented headcut began to erode headward from the newly formed Little Missouri River valley to capture flood flow that was moving to the Little Missouri River valley and South Fork Grand River drainage basin further to the southeast. As will be seen in subsequent maps the Boxelder Creek headcut was able to erode southwest faster than the Shaw Creek headcut and as a result was able to capture the southeast-oriented flood flow that had been feeding the Shaw Creek valley headcut headward erosion.

Shaw Creek headwaters area

Figure 4: Shaw Creek headwaters area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Continuing slightly to the southwest from figure 3, the figure 4 map illustrates the Shaw Creek headwaters area. Note southeast-oriented tributaries to northeast-oriented Shaw Creek, which provide evidence the Shaw Creek headcut eroded southwest to capture multiple southeast-oriented flood flow routes. Also note how  Shaw Creek headwaters are linked to headwaters of northwest-oriented Humbolt Creek, which flows to the northeast-oriented Boxelder Creek. The Humbolt Creek valley was probably initiated by southeast-oriented flow that Shaw Creek valley headward erosion captured. Subsequently headward erosion of the Boxelder Creek headcut beheaded southeast flood flow on the Humbolt Creek route and reversed the direction of flood flow in what is today the Humbolt Creek valley. The flow reversal captured southeast-oriented flood flow that had not yet been beheaded on the Spring Creek flow route and that water made a U-turn from being southeast-oriented to being northwest-oriented in the present-day Humbolt Creek headwaters area. Evidence for that U-turn can be observed in the elbow of capture where the northeast-oriented Humbolt Creek headwaters segment turns to become the northwest-oriented Humbolt Creek segment. Figure 5 will look at this evidence with a more detailed map. Note also in figure 4 how southeast-oriented Little Missouri River tributaries almost reach to the Shaw Creek headwaters area. Note also how Spring Creek and perhaps Humbolt Creek have eroded deep valleys or water gaps across an upland ridge. This evidence suggests southeast-oriented floodwaters first flowed on a topographic surface that was at least as high as the highest elevations in the figure 4 map region.

Shaw Creek-Humbolt Creek drainage divide detail map

Figure 5: Shaw Creek-Humbolt Creek drainage divide detail map. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 5 provides a detailed map of the Shaw Creek-Humbolt Creek drainage divide area and of the Humbolt Creek elbow of capture and its northeast-oriented valley segment. Southeast-oriented flood water flowing in what is today the northwest-oriented Humbolt Creek valley north of the figure 5 area and in the Spring Creek valley just west of the figure 5 map area was probably initially captured and diverted northeast by Shaw Creek. Then probably in fairly rapid succession Boxelder Creek beheaded the southeast-oriented flood flow in the Humbolt Creek valley, causing that flood flow to reverse flow direction and flow northwest to the newly eroded northeast-oriented Boxelder Creek valley and reversed flow on the Humbolt Creek route captured some of the southeast-oriented flood flow on the Spring Creek route causing that flow to make a U-turn. The net result of this sequence of events was flood flow responsible for eroding the northeast-oriented Squaw Creek valley headcut southwest was captured and diverted northwest to the northeast-oriented Boxelder Creek valley .

Spring Creek headwaters area

Figure 6: Spring Creek headwaters area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 6 illustrates more of the deeply upland region (unnamed on the map). Boxelder Creek flows in a northeast direction across the northwest corner of figure 6. Note how the upland forms an arc facing southeast and defines what appears to be an escarpment-surrounded basin. Note also how northwest and north-oriented Boxelder Creek tributaries have cut northwest-oriented valleys or water gaps through that upland arc to reach the northeast-oriented Boxelder Creek valley. The southeast-facing basin defined by the upland arc is an abandoned flood eroded headcut that was eroded headward into what must have a resistant bedrock mass by southeast-oriented floodwaters which were flowing across a topographic surface at least as high as the highest points on the upland ridge. The northwest-oriented Boxelder Creek tributary valleys were initially eroded as southeast-oriented flood water became concentrated in narrower channels, which captured more and more of the southeast-oriented flood flow. Headward erosion of the deep northeast-oriented Boxelder Creek headcut next systematically beheaded and captured those southeast-oriented flow routes with Humbolt Creek being beheaded first, Spring Creek beheaded next, and Snow Creek being beheaded next. As each of southeast-oriented flood flow route was beheaded, water already flowing on that route reversed flow direction to flow northwest to the newly formed and deeper Boxelder Creek valley. Apparently enough flood flow was still flowing on the yet to be beheaded southeast-oriented flow routes that reversed flow routes were able to capture some of that southeast-oriented flood flow. The case of the reversed flow on the Humbolt Creek route capturing southeast-oriented from the Spring Creek route was discussed in the figure 5 discussion. Reversed flow on the Spring Creek route captured southeast-oriented flow from the Snow Creek route and the east-oriented Spring Creek headwaters segment provides evidence of that capture. Perhaps even more revealing is the northeast-oriented headwaters segment of Snow Creek, which begins at the top of upland ridge just north of Rustler Divide. That northeast-oriented Snow Creek segment provides evidence reversed flow on the Snow Creek route, moving to the newly eroded northeast-oriented Boxelder Creek valley, was capturing yet to be beheaded southeast-oriented flood water that was moving on a topographic surface at least as high as the topographic surface defined by the Rustler Divide surface. In other words, the Boxelder Creek valley headcut  was at least as deep as the elevations of the uplands above the present-day Boxelder Creek floor. This evidence provides a hint of the magnitude of flood erosion and topographic change that occurred.

Area southeast of the Spring Creek escarpment-surrounded basin

Figure 7: Area southeast of the Spring Creek escarpment-surrounded basin. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 7 illustrates the area southeast of figure 6 and shows the relationship of the southeast-oriented Spring Creek escarpment-surrounded basin that is today drained to the northwest to Boxelder Creek. Wagon Creek, Ebert Creek, and North Slick Creek are common features shown on both figures 6 and 7 and are located in the northwest quadrant of figure 7 and the southeast quadrant of figure 6. Note how the north-oriented Little Missouri River is not flowing in deep valley, which would be expected if headward erosion of the Little Missouri River valley headcut had been into the topographic surface that exists today. Instead, headward erosion of the Little Missouri River valley headcut was into a topographic surface defined by the tops of the surrounding uplands, such as the upland into which the southeast-oriented Spring Creek escarpment-surrounded basin was eroded. The southeast-oriented Spring Creek escarpment-surrounded basin (or headcut) was eroded headward along a southeast-oriented flood route from the wall of newly formed north-oriented Little Missouri River valley. Erosion of the southeast-oriented Spring Creek escarpment-surrounded basin or headcut ceased when headward erosion of the northeast-oriented Boxelder Creek headcut captured southeast-oriented flood flow in the sequence of events partially described above (figure 6). Note in the southeast corner of figure 7 the Jumpoff, which is an escarpment at the Little Missouri River-South Fork Grand River drainage divide. The east-oriented South Fork Grand River headwaters are located in the large escarpment-surrounded basin defined by the Jumpoff escarpment. The magnitude of the Jumpoff escarpment-surrounded basin in width is probably similar to the magnitude (breadth) of the Little Missouri River and Boxelder Creek valley headcuts that initially eroded south and southwest and the South Fork Grand River valley headcut that had previously eroded west. Floodwaters flowing to and in the deep valleys carved by those headcuts were responsible for additional erosion to produce the landscape features we see today.  The Jumpoff escarpment-surrounded basin or headcut is further illustrated and described in the Little Missouri-South Fork Grand River drainage divide essay (found under either Little Missouri River or SD Grand River on sidebar category list).

Gergen Creek-Tie Creek drainage basin

Figure 8: Gergen Creek-Tie Creek drainage basin. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 8 illustrates the Gergen Creek-Tie Creek drainage a short distance southwest of the area illustrated in figure 6. Tie Creek flows to the northeast-oriented Little Missouri River which is located just southeast of the figure 8 southeast corner and is illustrated in figure 9 below. Note how northwest-oriented Boxelder Creek tributaries seen along the west edge of figure 8 have not eroded water gaps through the upland ridge and instead in this region the Boxelder Creek-Little Missouri River drainage divide follows the upland ridge crest. The southeast-oriented Gergen Creek-Tie Creek drainage basin is an escarpment-surrounded basin or abandoned headcut and was formed as a large southeast-oriented headcut eroded northwest along a southeast-oriented flood flow route. The flood flow was moving across a topographic surface that was at least as high as the upland crest and the southeast-facing headcut was being eroded into that topographic surface.

A measure of the Little Missouri River valley headcut width and depth

Figure 9: Little Missouri River valley in the Camp Crook area providing a measure of the Little Missouri River valley headcut width and depth. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 9 illustrates the Little Missouri River valley in the Camp Crook area. The upland in the southeast corner is the West Short Pine Hills and the upland in the northwest corner is unnamed on the maps and shall remain unnamed in this essay. Note how the Little Missouri River is flowing in a north-northeast direction between the two uplands. As described previously the deep Little Missouri River valley headcut eroded south to capture floodwaters that were flowing southeast across the region (in figure 9) to the southeast and east-oriented North Fork Moreau River. Initially the floodwaters were flowing on a topographic surface at least as high as the topographic surface defined by the highest elevations in the West Short Pine Hills and the unnamed upland to the northwest. It was into this surface that the large North Fork Moreau River valley headcut eroded west and northwest, the large Little Missouri River valley headcut eroded south, the large Boxelder Creek valley headcut eroded southwest, the Spring Creek escarpment-surrounded basin and the Gergen Creek-Tie Creek basin headcuts eroded northwest. The distance between the West Short Pine Hills northwest face and the unnamed upland (in figure 9) southeast face probably represents the approximate width of the deep north-oriented Little Missouri River valley headcut that was being eroded south and the elevation of the West Short Pine Hills and the unnamed upland to the northwest above the present-day Little Missouri River valley floor probably represents the Little Missouri River headcut depth. Headward erosion of the large headcuts along ever-changing flood flow routes systematically lowered the regional landscape leaving isolated buttes and uplands, such as the West Short Pine Hills and the unnamed upland to the northwest.

Hackberry Creek headcut

Figure 10: Hackberry Creek headcut. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

The Hackberry Creek headcut shown in figure 10 provides further evidence southeast-oriented floodwaters originally flowed on a topographic surface at least as high as the tops of the present day regional buttes and that deep southeast-oriented headcuts eroded northwest from the newly eroded deep Little Missouri River valley headcut. The deep Prairie Dog Creek-Cottonwood Creek through valley (southwest corner figure 10) apparently captured southeast-oriented flood flow that had been eroding the Hackberry Creek headcut, resulting in abandonment of the Hackberry Creek flood flow route.

Through valley linking Prairie Dog Creek and Cottonwood Creek

Figure 11: Through valley linking Prairie Dog Creek and Cottonwood Creek. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

One of the largest and best-defined northwest-to-southeast oriented through valleys linking northeast-oriented Boxelder Creek with the north-oriented Little Missouri River is the through valley occupied today by northwest-oriented Prairie Dog Creek and southeast-oriented Cottonwood Creek. Prairie Dog Creek flows to Boxelder Creek while Cottonwood Creek flows to the Little Missouri River. The through valley, like many other less spectacular through valleys crossing the present day Boxelder Creek-Little Missouri River drainage divide exists, it is scientific evidence and cannot be ignored. The width and depth of the through valley indicate large volumes of water eroded it. That water either flowed southeast or northwest. Figure 11 suggests the southeast direction is more likely because the southeast-oriented Cottonwood Creek headwaters are at the northwest end of the large water gap. In other words, southeast-oriented flood flow appears to have dominated with northwest-oriented flow being a reversal of flow on the northwest end of a beheaded flow route.

• The best explanation for the Prairie Dog Creek-Cottonwood Creek through valley origin is it was eroded by an immense southeast-oriented flood that originally flowed on a topographic surface at least as high as the tops of the present-day valley walls. Headward erosion of the deep northeast-oriented Little Missouri River valley headcut first captured southeast-oriented flood water and diverted the water north and northeast. The deep Little Missouri River valley headcut also established a new and lower base level and floodwaters eroded numerous deep headcuts including the deep Cottonwood Creek valley headcut northwest along southeast-oriented flood flow routes. At about the same time the northeast-oriented Boxelder Creek valley headcut eroded southwest and captured southeast-oriented flood flow route using in the newly eroded Cottonwood Creek valley. Floodwaters on the northwest end of the beheaded southeast-oriented flood flow route reversed direction and flowed northwest to the slightly deeper northeast-oriented Boxelder Creek valley, eroded the northwest-oriented Prairie Dog Creek valley, and created the present day Prairie Dog Creek-Cottonwood Creek drainage divide.

Blacktail Creek abandoned headcut

Figure 12: Blacktail Creek abandoned headcut. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Immediately southwest of the Prairie Dog Creek-Cottonwood Creek through valley is the southeast-oriented Blacktail Creek abandoned headcut, with Blacktail Creek flowing to the northeast-oriented Little Missouri River. The Blacktail Creek headcut is very similar to the Hackberry Creek headcut and provides further evidence the southeast-oriented floodwaters originally flowed on a topographic surface at least as high as the tops of the present day regional buttes and deep southeast-oriented headcuts eroded northwest into a high level topographic surface from the newly eroded Little Missouri River valley headcut. The Prairie Dog Creek-Cottonwood Creek through valley captured southeast-oriented flood flow that had been eroding the Blacktail Creek headcut, resulting in abandonment of the Blacktail Creek flood flow route. Boxelder Creek valley headward erosion subsequently captured southeast-oriented flood flow responsible for through valleys crossing the Boxelder Creek-Little Missouri River drainage divide southwest of the Blacktail Creek abandoned headcut. Today Blacktail Creek begins in a large southeast-oriented escarpment-surrounded basin that is an abandoned flood-formed headcut.

Boxelder Creek-Little Missouri River drainage divide, southwest end.

Note northwest-oriented tributaries to Boxelder Creek and southeast-oriented streams flowing to the  north oriented Little Missouri River east of the figure 13 area. Also note northwest-southeast oriented through valleys crossing the Boxelder Creek-Little Missouri River drainage divide. The through valleys are evidence of multiple northwest-southeast oriented flood flow routes that crossed the region and that were captured first by headward erosion of the north oriented Little Missouri River and then by headward erosion of the northeast-oriented Boxelder Creek valley. Southeast-oriented tributaries to the north and northeast-oriented trunk streams are barbed tributaries and are evidence of repeated capture events. The northwest-oriented tributaries can best be explained by repeated flood flow reversals on the northwest ends of beheaded southeast-oriented flood flow routes. The parallel southeast-oriented and northwest-oriented tributaries are evidence the north and northeast-oriented trunk stream valleys were eroded in sequence. First the north-oriented Little Missouri River valley headcut eroded south to capture the southeast-oriented floodwaters and to divert that flood water north. Next the Boxelder Creek valley headcut eroded southwest from the newly eroded Little Missouri River valley to capture southeast-oriented flood water and to divert the water northeast.

Boxelder Creek headwaters

Figure 14: Boxelder Creek headwaters area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 14 illustrates the Boxelder Creek headwaters area and the Boxelder Creek drainage divide with the north-oriented Powder River drainage basin to the west. Generally the upper topographic surface in  the figure 14 map east half is drained by northeast oriented Boxelder Creek to the Little Missouri River and the west half of the map area is drained by northwest oriented Powder River tributaries that flow to the north-northeast and north-northwest oriented Powder River, which flows to the northeast oriented Yellowstone River. One exception is in the north center of figure 14 where north-oriented Crow Creek (unnamed in figure 14) eroded a north-oriented valley south from the deep Powder River valley to capture southeast-oriented flood water that was flowing to the Boxelder Creek drainage basin. North of the figure 14 area Crow Creek turns northwest and eventually joins the Powder River. The other exception is in the figure 14 southeast quadrant. Note how Little and Big Groat Creeks in the southeast quadrant of figure 14 flow northwest and north to Boxelder Creek and also the flood eroded northwest-southeast streamlined residual just east of Little Groat Creek. Also note headwaters of two southeast-oriented streams in the figure 14 southeast corner. Those southeast-oriented streams flow to southeast-oriented Willow Creek, which flows to the northeast and north-oriented Little Missouri River, so the Boxelder Creek-Little Missouri River drainage divide passes between the headwaters of those two southeast oriented streams and headwaters of northwest- and north-oriented Little and Big Groat Creeks. Note also how West Fork Boxelder Creek is flowing northeast along the crest of the escarpment located at the Powder River-Little Missouri River drainage divide. This asymmetric drainage divide is evidence the deeper Powder River valley eroded into what must have been a topographic surface defined by Little Missouri River and Boxelder Creek erosion events. The figure 14 sequence of events can be explained in the context of immense southeast-oriented floods.