A geomorphic history based on topographic map evidence

The eastern Grand River-Moreau River drainage divide area is located in northwest South Dakota, USA. Although detailed topographic maps of the eastern Grand River-Moreau 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 eastern Grand River-Moreau River drainage divide area 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 ended when headward erosion of the east and northeast-oriented Grand River valley captured the southeast-oriented flood flow.

Grand River-Moreau River location map

Figure 1: Grand River-Moreau River location map (select map and click for larger version). National Geographic Society map digitally presented using National Geographic Society TOPO software.

The east-oriented Grand River drainage basin is located primarily in northwest South Dakota, although the North Fork Grand River begins further northwest in southwest North Dakota. The east-oriented Moreau River drainage basin is located immediately south of the Grand River drainage basin and today both the Grand River and the Moreau River flow to the south-oriented Missouri River (Lake Oahe in figure 1). North of the Grand River drainage basin is the east-oriented Cannonball River drainage basin. South of the Moreau River drainage basin is the east-oriented Belle Fourche River-Cheyenne River drainage basin. West of the Belle Fourche, Moreau, Grand, and Cannonball River headwaters is the north-oriented Little Missouri River drainage basin (not shown in figure 1). A previous essay has illustrated and discussed South Fork Grand River-Moreau River drainage divide landform evidence (and can be found under Moreau River or SD Grand River on the sidebar category list) and this essay illustrates and discusses the Grand River-Moreau River drainage divide landform evidence in the area east of Thunder Butte Creek. Evidence presented here will demonstrate how eastern Grand River-Moreau River drainage divide landforms were formed during an immense southeast-oriented flood that was captured first by headward erosion of the deep east-oriented Moreau River valley and then by headward erosion of the deep east-oriented Grand River valley. Reasons why the deep Moreau and Grand River valleys eroded headward to capture southeast-oriented flood water cannot be determined from evidence in this essay, although headward erosion of the two deep valleys was probably related in some way to a significant lowering of base level that occurred as the southeast-oriented flood occurred. Perhaps this base level lowering occurred because the deep south-oriented Missouri River valley eroded north. Or, perhaps other events east of the figure 1 map area played a role as well. Nor can the flood water source be determined from evidence presented here, although the region is located along a recognized North American ice sheet margin and rapid ice sheet melting would be a logical water source.

Detailed eastern Grand River-Moreau River drainage divide area location map

Figure 2: Detailed eastern Grand River-Moreau River drainage divide area location map. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 2 illustrates the eastern Grand River-Moreau River drainage divide area this essay illustrates and discuss. The Grand River flows east through Corson County and the Moreau River flows east through Dewey County. The Grand River-Moreau River drainage divide roughly follows the west-east county line. This essay begins by looking at a detailed map of the Black Horse Creek drainage basin, which is the east and north-oriented Grand River tributary located in southwest Corson County. Second will be a detailed map of Beaver Creek (unnamed southeast-oriented Moreau River on figure 2) headwaters just south of Glad Valley. Proceeding slightly further east the third detailed map will look at the headwaters of the southeast-oriented Irish Creek, another Moreau River tributary. Continuing east along the Grand River-Moreau River drainage divide the fourth detailed map will show Firesteel Creek headwaters in the Isabel area. The fifth detailed map will feature the Firesteeel Creek-High Bank Creek-Little Moreau River drainage divide west of Timber Lake. Sixth will be a detailed map of the Grand River-Moreau River drainage divide east of Timber Lake. Seventh will be a detailed map of the northeast oriented Snake Creek headwaters along the Grand River-Moreau River drainage divide near Trail City. Finally, the eighth detailed map will illustrate the Grand River-Moreau River-Missouri River drainage divide. Note the many southeast-oriented tributaries flowing to the east-oriented Grand River and the east-oriented Moreau River. These multiple southeast-oriented tributaries are evidence both the Grand River valley and the Moreau River valley were eroded headward to capture an immense southeast-oriented flood, where flood waters were moving on multiple southeast-oriented flood flow routes, probably in some form of an ever-changing southeast-oriented anastomosing channel complex. Headward erosion of the deep Moreau River valley had to occur first and beheaded southeast-oriented flow going to drainage basins further south and east. Moreau River valley headward erosion was followed by Grand River valley headward erosion, which beheaded southeast-oriented flood flow routes carrying water to the southeast-oriented Moreau River tributaries and in the process created what is today the eastern Grand River-Moreau River drainage divide. Detailed evidence supports this interpretation.

Black Horse Butte Creek drainage basin area map

Figure 3: Black Horse Butte drainage basin area map. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 3 provides a detailed map of the upper Black Horse Creek drainage basin, which is located on the north (or Grand River side) of the Grand River-Moreau River drainage divide. The north-oriented Black Horse Creek segment located in the figure 3 northeast quadrant flows to the east-oriented Grand River, which is located north of the figure 3 map area. The Black Horse Creek valley eroded south, west, and southwest from a newly eroded deep Grand River valley to capture southeast-oriented flood flow to the Moreau River valley on yet to be beheaded (by Grand River valley headward erosion) flood flow routes. Evidence to support this interpretation is found in southeast-oriented Black Horse Creek segments and the many southeast-oriented tributaries flowing today to Black Horse Creek, especially in the headwaters area along the figure 3 west edge. Also note the presence of northwest-oriented tributaries to Black Horse Creek and northeast-oriented Black Horse Creek tributaries. The northwest-oriented tributaries are also relics of beheaded southeast-oriented flood flow routes, where flood water already on the northwest ends of beheaded flow routes reversed flow direction to move northwest to the newly eroded deep valley that had beheaded the flood flow routes. Rapid flood erosion of the Black Horse Creek drainage basin ceased when the deep Grand River valley eroded further west and captured the southeast-oriented flood waters that had been flowing into the Black Horse Creek drainage basin area. The Black Horse Creek drainage basin is a significant topographic feature and large amounts of southeast-oriented flood water were required to erode it. But, before the Black Horse Creek valley eroded south, west, and southwest the flood waters were flowing to the newly eroded east-oriented Moreau River valley on the south side of the present-day Grand River-Moreau River drainage divide. The next two figures and discussions will address what those southeast-oriented flood waters did on the Moreau River side of the drainage divide prior to being captured by Black Horse Creek valley headward erosion.

Beaver Creek headcut in the Glad Valley area

Figure 4: Beaver Creek headcut in the Glad Valley area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 4 illustrates the area just south of figure 3 and some of the figure 4 north edge overlaps some of the figure 3 south edge. South-oriented streams in the figure 4 south half flow to the east-oriented Moreau River while north-oriented streams in the figure 4 north half flow to the east-oriented Grand River. West of Glad Valley the red highway roughly follows the Grand River-Moreau River drainage divide. Just east of Glad Valley the drainage divide turns north-northeast and just south of the county line it turns east again. The semi-circular escarpment-surrounded basin immediately south of Glad Valley is now drained by East and West Beaver Creeks, which join to form southeast-oriented Beaver Creek and was formed as southeast-oriented flood waters eroded a large southeast-oriented headcut headward or northwest into the upland topographic surface. To do so the flood water had to flow over what is today the deep Grand River valley, meaning the Grand River valley and its associated Black Horse Creek valley did not exist at the time the Beaver Creek headcut eroded northwest. East of the Beaver Creek drainage basin is another escarpment-surrounded basin drained by southeast-oriented Irish Creek (figure 5 will better illustrate this Irish Creek basin). A shallow through valley can be seen linking the Irish Creek headwaters with headwaters of a northwest-oriented East Twin Buttes Creek tributary, which eventually drains to Black Horse Creek and the east-oriented Grand River. Northwest-oriented tributaries to northeast-oriented Twin Buttes Creek originated as reversals of flood flow on the northwest ends of southeast-oriented flood flow routes that had been eroding the Irish Creek headcut northwest when headward erosion of the northeast-oriented Twin Buttes Creek valley beheaded those flood flow routes.

Irish Creek headcut east of the Glad Valley area

Figure 5: Irish Creek headcut east of the Glad Valley area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 5 illustrates the Grand River-Moreau River drainage divide east of the figure 4 map area and provides a significant area of overlap with figure 4. The northeast-oriented drainage basin with major northwest-oriented tributaries in the northwest figure 5 quadrant is the Twin Buttes Creek drainage basin which drains to north-oriented Black Horse Creek and the east-oriented Grand River. The east-oriented drainage basin in the figure 5 northeast quadrant is the Firesteel Creek drainage basin and will be better illustrated in figure 6. The southeast-oriented Beaver Creek abandoned headcut is located just south of Glad Valley and the south-southeast-oriented Irish Creek abandoned headcut is located just to the east of that. Like the Beaver Creek abandoned headcut the south-southeast-oriented Irish Creek escarpment-surrounded basin or abandoned headcut was eroded north-northwest by large volumes of flood waters that had flowed south-southeast over a topographic surface that was removed when the deep east-oriented Grand River valley eroded west. The Grand River-Moreau River drainage divide was created in this region when the deep Grand River valley eroded west (north of the figure 5 map area) and northeast-oriented tributary valleys eroded southwest from that newly eroded deep valley to capture southeast-oriented flood waters still moving on yet to be beheaded (by Grand River valley headward erosion) southeast-oriented flood flow routes. Once the southeast-oriented flood flow routes were beheaded, flood waters on the northwest ends of those beheaded flood flow routes reversed flow direction to flow northwest into the newly eroded and deep northeast-oriented valleys leading to the newly eroded and deeper Grand River valley. Once all southeast-oriented flood flow routes crossing the figure 5 map area had been beheaded rapid erosion of the landscape ceased.

Firesteel Creek drainage basin in the Isabel area

Figure 6: Firesteel Creek drainage basin in the Isabel area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 6 illustrates the Grand River-Moreau River drainage divide east of the figure 5 map area. Cottonwood Creek can be seen in the figure 6 northwest corner flowing northeast to the east-oriented Grand River. The Firesteel Creek drainage basin, which drains north to the Grand River, occupies most of the figure 6 area, although southeast oriented streams in the figure 6 southeast quadrant are headwaters of the southeast oriented Little Moreau River, which flows to the east oriented Moreau River. Also, north of Brewer Butte along the figure 6 east edge, are northeast oriented High Bank Creek headwaters which drain to the Grand River. The Grand River-Moreau River drainage divide roughly follows the route of the abandoned railroad between Isabel and Firesteel and then jogs north around the southeast oriented Little Moreau River headwaters before jogging south again to continue in an east-northeast direction through the Brewer Butte area. Close study of the drainage divide reveals numerous shallow through valleys linking headwaters of north, northeast, and northwest-oriented Grand River tributaries with the headwaters of south and southeast-oriented Moreau River tributaries. These shallow through valleys are evidence of multiple flood flow routes, probably of the type developed in large-scale anastomosing channel complexes, and were carrying flood water south and southeast prior to being beheaded by headward erosion of the deep Grand River valley and its various tributary valleys. Note the many southeast- and northwest-oriented Firesteeel Creek tributaries. The northwest-southeast alignment of those tributaries is a relic of the southeast-oriented flood routes captured as the deep Firesteel Creek valley eroded headward from the newly formed Grand River valley to behead and capture flood flow on yet to be beheaded (by Grand River valley headward erosion) southeast-oriented flood flow routes. Northwest-oriented tributary valleys were eroded by reversals of flood flow on the northwest ends of the beheaded flood flow routes. Headward erosion of the Firesteel Creek valley beheaded and captured flood flow that was eroding the southeast-oriented Little Moreau River drainage basin. Subsequently headward erosion of the Cottonwood Creek valley beheaded and captured southeast-oriented flood flow that was eroding the Firesteel Creek drainage basin. Once the southeast-oriented flood flow had been captured and diverted to the newly eroded deep Grand River valley, rapid erosion of the figure 6 landscape ceased.

Firesteel Creek-High Bank Creek-Little Moreau River drainage divide

Figure 7: Firesteel Creek-High Bank Creek-Little Moreau River drainage divide. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 7 illustrates the Grand River-Moreau River drainage divide east of the figure 6 map area and the western third of figure 7 overlaps the eastern third of figure 6. The Firesteel Creek drainage basin occupies the figure 7 northwest area while the northeast-oriented High Bank Creek drainage basin occupies the northeast area and the southeast-oriented Little Moreau River drainage basin occupies most of the southeast area. Just north of Timber Lake are the east-oriented headwaters of Dog Butte Creek, which east of the figure 7 map area turns south and then southeast to flow to Whitehorse Creek and the east-oriented Moreau River. Figure 7 evidence is similar to figure 6 evidence and includes multiple shallow through valleys crossing the Grand River-Moreau River drainage divide and northwest and southeast-oriented tributaries to Firesteel Creek and High Bank Creek, the major northeast-oriented Grand River tributaries. Erosion events recorded in the figure 7 map evidence begin with southeast-oriented flood flow moving across the region to the southeast-oriented Little Moreau River drainage basin and headward erosion of the east-oriented Dog Butte Creek valley segment. Next headward erosion of the northeast-oriented High Bank Creek valley from what was then the actively eroding Grand River valley captured southeast-oriented flood flow routes to the Dog Butte Creek drainage basin, with reversals of flood flow on the northwest ends of beheaded flood flow routes being responsible for eroding the northwest-oriented tributary valleys. Headward erosion the Grand River valley and its tributary Firesteel Creek valley then beheaded southeast-oriented flood flow routes to the High Bank Creek drainage basin and subsequently headward erosion of the Firesteel Creek valley beheaded and captured all southeast-oriented flood flow routes to the southeast-oriented Little Moreau River drainage basin. Rapid erosion of the figure 7 map area ended when headward erosion of the east-oriented Grand River valley and tributary valleys beheaded and captured all southeast-oriented flood flow routes that had once crossed the figure 7 region.

Grand River-Moreau River drainage divide east of Timber Lake

Figure 8: Grand River-Moreau River drainage divide east of Timber Lake. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Continuing east along the Grand River-Moreau drainage divide we come to the Timber Lake region. Figure 8 shows a map area just east of the figure 7 map area and there is some overlap. National Geographic Society TOPO digital maps at the scale shown do not include contour lines for the figure 8 northeast quadrant. Northeast-oriented streams include High Bank Creek in the figure 8 northwest corner and Little Oak and Snake Creek headwaters further east. These northeast-oriented streams flow to the Grand River. Dog Butte Creek flows east through Timber Lake and then turns south and southeast to flow to the east-oriented Moreau River (not shown). In the figure 8 southeast corner are a series of southeast-oriented and interconnected escarpment-surrounded basins or abandoned headcuts that are today drained by southeast-oriented Moreau River tributaries. These especially well-preserved southeast-oriented escarpment-surrounded basins illustrate how southeast-oriented flood waters lowered the regional topographic surface by eroding broad headcuts headward along the routes of southeast-oriented flood flow routes. Southeast-oriented flood flow that had been eroding these southeast-oriented abandoned headcuts ended when headward erosion of the Grand River-Little Oak Creek valley headcut and in rapid succession the deeper Grand River-High Bank Creek valley headcut beheaded the flood flow routes. Again, note northwest-oriented tributaries to the northeast-oriented High Bank Creek. These northwest-oriented tributaries originated as reversals of flood flow on the northwest ends of beheaded southeast-oriented flood flow routes. Once southeast-oriented flood flow routes across the figure 8 map region had been beheaded rapid erosion of the figure 8 region landscape ceased.

Grand River- Moreau River drainage divide in the Trail City area

Figure 9: Grand River- Moreau River drainage divide in the Trail City area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 9 continues the journey east along the Grand River-Moreau River drainage divide and again overlaps some of the figure 8 map area. National Geographic Society TOPO digital maps at the figure 9 scale do not show contour lines for the figure 9 north half, although drainage patterns shown are adequate for the drainage pattern history discussed here. East-oriented drainage along the figure 9 center east edge flows directly to the south-oriented Missouri River. Northeast-oriented Snake Creek (northeast figure 9 quadrant) and Little Oak Creek (northwest figure 9 corner) drain to the Grand River. Between those two northeast-oriented streams are northeast-oriented headwaters of Deep Bank Creek (unnamed on figure 9), which also flows to the Grand River. The interconnected southeast-oriented escarpment-surrounded basins or abandoned headcuts previously discussed are better illustrated in figure 9. The large south-southeast-oriented escarpment-surrounded basin or abandoned headcut south of the word “RESERVATION” encompasses what is today the Du Charme Creek drainage basin. Again, these escarpment-surrounded basins or abandoned headcuts illustrate how flood waters eroded large headcuts headward along flood flow routes to strip the easily eroded bedrock surface and lower the topographic surface from whatever level had existed prior to the flood events to the topographic surface level we see today. The excellent preservation of these abandoned headcuts probably reflects the rapidity with which southeast-oriented flood flow routes eroding them were captured and beheaded by headward erosion of the deep Missouri River valley-Grand River valley-Snake Creek valley, and Little Oak valley headcuts to the north. Once southeast-oriented flood flow had been beheaded and captured rapid erosion of the figure 9 ceased and the region 9 landscape has not significantly changed since.

Grand River-Moreau River-Missouri River drainage divide

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

Figure 10 completes the journey east along the Grand River-Moreau River drainage divide. National Geographic TOPO maps for the figure 10 north half do not show contour lines at the scale shown, but shading gives a rough approximation of topography involved. Lake Oahe fills the Missouri River valley and is impounded behind Oahe Dam, which is located downstream on the south-oriented Missouri River. The northwest oriented Lake Oahe arm northwest of Mobridge, where the word “LAKE” is located, is the southeast-oriented Grand River valley. North of Mobridge the Missouri River valley is oriented in a southwest direction upstream from its confluence with the southeast-oriented Grand River and downstream the river flows in a southeast direction before turning south to reach the Moreau River-Missouri River confluence (not shown). The southeast-oriented Missouri River-Grand River valley segment was eroded headward along a southeast-oriented flood flow route, although additional map evidence is needed to fully work out Missouri River valley origin details. The northeast-oriented Missouri River tributary southwest of Mobridge is Deadman Creek and it has northwest and southeast-oriented tributaries indicating the Deadman Creek valley was eroded southwest from the Missouri River valley across multiple southeast-oriented flood flow routes. The Du Charme Creek escarpment-surrounded basin or abandoned headcut is located southeast of Trail City in the figure 10 southwest quadrant and east of it are several southeast-oriented Moreau River tributary valleys. Southeast-oriented flood flow to several of the southeast-oriented drainage basins east of the Du Charme Creek drainage basin ended when the Deadman Creek valley eroded southwest to behead and capture that southeast-oriented flood flow. Southeast-oriented flood flow to the Deadman Creek drainage basin ended when the Claymore Creek valley eroded south and southwest from what is today the Grand River mouth to behead and capture southeast-oriented flood flow routes to the Deadman Creek drainage basin. Shortly thereafter headward erosion of the Snake Creek valley southwest from the actively eroding Grand River valley beheaded and captured southeast-oriented flood flow to the Du Charme Creek drainage basin (and adjacent southeast-oriented drainage basins). Once all southeast-oriented flood flow across the figure 10 region was beheaded and captured rapid erosion of the figure 10 region ceased and the landscape has changed little since.

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 the detailed 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 were created using National Geographic 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.