A geomorphic history based on topographic map evidence

Abstract:

This essay provides an overview of more detailed essays describing Moreau River drainage basin landform origins. The detailed essays can be found under Moreau River on this website’s sidebar category list. The Moreau River drainage basin is located in northwest South Dakota. To the west is the north oriented Little Missouri River drainage basin. To the north is the east oriented Grand River drainage basin. To the south is the southeast oriented Belle Fourche River drainage basin in the west and the northeast-oriented Cheyenne River drainage basin in the east. The Moreau River drains in a southeast, east, and northeast direction and joins the south-oriented Missouri River. Tributaries from the north are southeast oriented suggesting the Moreau River valley and major tributary valleys eroded headward across multiple southeast oriented flood flow channels, such as might be found in a large-scale southeast oriented anastomosing channel complex. Evidence supporting this southeast oriented flood interpretation includes northwest-southeast oriented through valleys eroded across drainage divides, large abandoned southeast-and east oriented headcuts, and southeast- and northwest-oriented tributaries to east and northeast-oriented Moreau River valley segments and tributaries (where the northwest-oriented tributary valleys were eroded by flood flow reversals on northwest ends of beheaded flood flow channels). Flood waters responsible for eroding the Moreau River drainage basin were derived from a rapidly melting North American ice sheet and flowed in a southeast direction along the decaying ice sheet’s southwest margin. Flood waters flowing across the Moreau River drainage basin were captured and then flowed in a northeast direction across the location of the present day south-oriented Missouri River to a breach in the ice sheet’s southwest margin to join a large melt water river, which had carved an immense ice-walled and bedrock-floored canyon into the ice sheet’s surface. Originally flow in the ice-walled and bedrock-floored canyon was south-oriented, although late in its history the flow direction was reversed. Missouri River valley headward erosion captured the east and northeast-oriented Moreau River drainage basin as the deep Missouri River valley eroded north along the decaying ice sheet’s southwest margin. Headward erosion of the Moreau River valley and its tributary valleys deeply eroded the Moreau River drainage basin area. It is impossible to determine how much erosion occurred because southeast-oriented flood waters flowed across what are today the Moreau River drainage basin’s highest elevations. Flood flow across the Moreau River drainage basin ended when the deep north and east-oriented Little Missouri River valley eroded headward from a deep ice sheet margin breach in central North Dakota and captured the southeast-oriented flood flow. Moreau River drainage basin landforms have changed little since flood flow routes to the Moreau River drainage basin were beheaded.

Figure 1: Location map for northwest South Dakota’s Moreau River drainage basin. National Geographic Society map digitally presented using National Geographic Society TOPO software.

Moreau River drainage basin drainage history

The Moreau River drainage basin is located in northwest South Dakota. The North and South Forks of the Moreau River originate west of Redig, South Dakota and initially flow in southeast directions. The South Fork Moreau River turns to flow in a northeast direction to join the southeast oriented North Fork and then the Moreau River flows in an east and northeast direction to join the south-oriented Missouri River. North of the Moreau River drainage basin is the east-oriented Grand River drainage basin. South of the Moreau River drainage basin is the southeast oriented Belle Fourche River drainage basin in the west and the northeast-oriented Cheyenne River drainage basin in the east. West of the Moreau River drainage basin is the north-oriented Little Missouri River drainage basin. In west-central North Dakota the north-oriented Little Missouri River turns to flow in an east direction to join the south-oriented Missouri River. The Moreau River is one of several southeast and east-oriented tributaries flowing to the south-oriented Missouri River, which are located east of the narrow north-oriented Little Missouri River drainage basin. Most of the southeast and east-oriented rivers in southwest North Dakota and northwest South Dakota, including the Moreau River, turn to flow in northeast directions before entering the south-oriented Missouri River as barbed tributaries, although the Moreau River does turn to flow in a southeast direction a short distance before joining the Missouri River.

• Why do rivers in southwest North Dakota and northwest South Dakota flow in southeast, east, and northeast directions to join the south-oriented Missouri River while just to the west of their headwaters is the north oriented Little Missouri River? A popular hypothesis is the north oriented Little Missouri River and the northeast-oriented Missouri River tributaries are relics of a north oriented pre-glacial drainage system. According to this hypothesis an ice sheet margin blocked north and northeast-oriented rivers and the south-oriented Missouri River valley was eroded along the ice sheet margin to drain the blocked drainage. Evidence supporting this pre-glacial drainage system hypothesis includes the presence of valleys east and north of the present day Missouri River valley, in which the pre-glacial north and northeast-oriented rivers are hypothesized to have flowed. While this commonly accepted hypothesis has been repeatedly published it fails to explain much of the regional topographic map evidence and it also defies common sense logic. For example, the pre-glacial north-oriented drainage system hypothesis does not explain the southeast-oriented headwaters of almost every northeast-oriented Missouri River tributary. Further, the hypothesis does not explain the origin of the asymmetric drainage divide between the north-oriented Little Missouri River and the various east-oriented Missouri River tributaries. In addition the hypothesis defies common sense logic by assuming valleys eroded into easily eroded bedrock could survive glacial erosion when located underneath a large ice sheet and could be also be preserved from destruction by melt water flooding when that ice sheet melted.

Figure 2: Reduced size maps of Little Missouri River-Moreau River drainage area near East and West Short Pine Hills and also Little Missouri River-Grand River and Grand River-Moreau River drainage divides north and east of East Short Pine Hills. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

If the pre-glacial north-oriented drainage system hypothesis is not correct then how and when was the Moreau River drainage basin eroded? Figure 2 provides important clues as to what really happened. The north-oriented Little Missouri River is located near the figure 2 west edge. Note how Little Missouri River tributaries from the east are all northwest-oriented. The larger tree covered (shown in green) upland area is the West Short Pine Hills and the smaller tree covered upland is the East Short Pine Hills. The north-facing escarpment north of the East Short Pine Hills is known as “The Jumpoff” and north and east of the escarpment is the South Fork Grand River drainage basin. The North Fork Moreau River originates near the crest of “The Jumpoff” escarpment just north of the East Short Pine Hills and flows in a southeast direction to the figure 2 east edge. Northwest-oriented drainage located along the “The Jumpoff” crest flows to the Little Missouri River.

• Note how the West and East Short Pine Hills are located on the margin of what appears to be a south oriented escarpment and south of the West Short Pine Hills there is what appears to be an east-facing escarpment. The south and east-facing escarpment crests mark the Little Missouri River-South Fork Moreau River drainage divide, with the southeast-oriented South Fork Moreau River originating in the resulting escarpment-sounded basin. Note how almost all streams in the South Fork Moreau River drainage basin area have southeast-oriented headwaters and the Little Missouri River tributaries south, west, and north of the West Short Pine Hills are northwest-oriented while Little Missouri River tributaries between the East and West Short Pine Hills are north-oriented. The escarpments bounding the escarpment-surrounded South Fork Moreau River headwaters drainage basin provide evidence of a giant southeast-oriented headcut, which was eroding headward along the route of a massive southeast-oriented flood. Headward erosion of the north-oriented Little Missouri River valley captured the southeast-oriented flood flow and diverted the flood waters to the north.

• What occurred to produce the figure 2 landscape seen today began with immense southeast-oriented floods flowing across a topographic surface at least as high as the tops of the East and West Short Pine Hills. Flood waters were flowing along the southwest margin of what was then a rapidly melting North American ice sheet and probably eroded the entire region between the decaying ice sheet’s southwest margin and the Black Hills.. The ice sheet had been thick, probably several kilometers thick, and had been located in a deep “hole”, which the ice sheet had formed by a combination of deep glacial erosion and crustal warping caused by the ice sheet weight. When it was at its maximum size the ice sheet had stood high above the pre-glacial surface, but also had roots that extended well below the pre-glacial surface. At the time flood waters eroded the Moreau River drainage basin most of the thick ice sheet had melted, and at least in North and South Dakota what was left were the decaying ice sheet roots, which had been located below the pre-glacial surface. Flood waters flowing in a southeast direction along the ice sheet’s southwest margin deeply eroded the pre-glacial surface and the northwest South Dakota surface seen today has no resemblance to that pre-glacial surface.

• Events on the decaying ice sheet’s surface played an important role in the Moreau River drainage basin evolution. Immense melt water rivers flowed across the ice sheet surface to the ice sheet’s southern margin and carved what became giant ice-walled and bedrock-floored canyons. A huge southeast and south-oriented ice-walled and bedrock-floored canyon was located east of the present day Missouri River valley in North and South Dakota and the Missouri Escarpment is what remains of that giant canyon’s west and southwest wall. The floor of that huge ice-walled and bedrock-floored canyon was significantly lower in elevation than the bedrock surface south and west of the decaying ice sheet and separating the immense ice marginal floods from the much deeper ice-walled and bedrock floored canyon floor was the decaying and detached ice sheet southwest margin or ice barrier. While not in the Moreau River drainage basin, the elevation difference still exists today and elevations at the Missouri Escarpment base are significantly lower than elevations of the surface into which the Missouri River valley has been eroded.

• In time the ice sheet’s southwest margin had decayed sufficiently that ice marginal floods could breach the ice barrier and flow into the giant ice-walled and bedrock-floored canyon. At about the same time south-oriented flood flow on the canyon’s floor was captured and diverted north and east as new and lower outlets to the present day Saint Lawrence drainage basin and later to what is now Hudson Bay opened up. Breaches in the decaying ice sheet southwest margin ice barrier triggered the headward erosion of deep northeast and north-oriented valleys which began to capture the southeast-oriented flood flow. The Little Missouri River valley eroded west from an ice margin breach in central North Dakota and then eroded south across the entire southeast-oriented flood flow route between the decaying ice sheet margin and the Black Hills. But previously deep northeast-oriented valleys had eroded headward from more southerly breaches and had captured selected segments of the southeast-oriented flood flow. Note how the Cheyenne and Moreau Rivers in South Dakota flow in northeast directions to reach the south-oriented Missouri River. Note also in North Dakota how the Cannonball, Heart, and Knife Rivers flow in northeast directions to reach the south-oriented Missouri River.

Figure 3: South Fork Grand River-Moreau River drainage divide at Reva Gap (Slim Buttes), South Dakota. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Within the Moreau River drainage basin today, evidence for massive southeast-oriented flood flow and deep flood erosion is located almost everywhere. Figure 3 illustrates the South Fork Grand River-Moreau River drainage divide area at Reva Gap and Slim Buttes. Northwest oriented Sioux Creek and north and northwest-oriented Squaw Creek are tributaries to the northeast and east oriented South Fork Grand River, which is located west and north of the figure 3 map area. Southeast-oriented streams east of the Slim Buttes upland are Moreau River tributaries. Reva Gap is actually a series of remarkable northwest-southeast oriented through valleys eroded across the north-south oriented Slim Buttes upland (located in the area where the west to east oriented highway crosses the Slim Buttes upland). Note how the southeast-oriented Moreau River tributary valleys are linked by through valleys in the Reva Gap region with northwest-oriented Sioux Creek tributary valleys. The drainage alignment and the through valleys are relics of multiple southeast-oriented flood flow channels which crossed the figure 3 map area prior to headward erosion of the deep east and northeast-oriented South Fork Grand River valley. Headward erosion of the Moreau River valley east and south of the figure 3 map area captured the southeast-oriented flood flow and the southeast-oriented flood waters eroded the southeast-oriented Moreau River tributary valleys. The Slim Buttes upland surface cap rock apparently prevented the southeast-oriented Moreau River tributary valleys from eroding deep valleys across Slim Buttes and the southeast-oriented flood flow channels to those actively eroding Moreau River tributary valleys were beheaded by headward erosion of the east and northeast-oriented South Fork Grand River valley (north and west of the figure 3 map area). Flood waters on northwest ends of beheaded flood flow channels reversed flow direction to erode the north- and northwest-oriented South Fork Grand River tributary valleys.

• How deeply did the flood water erode the Moreau River drainage basin? Slim Buttes in figure 3 and the East and West Short Pine Hills in figure 2 are among the highest points in the Moreau River drainage basin. Evidence in both figures 2 and 3 strongly suggests flood waters once flowed across a topographic surface at least as high as the tops of Slim Buttes and the East and West Short Pine Hills. The figure 3 map contour interval is 20 meters and the Slim Buttes upland surface rises to more 1100 meters. The East and West Short Pine Hills each rise to elevations greater than 1200 meters. West of the West Short Pine Hills and west of the north-oriented Little Missouri River are more isolated buttes and uplands also rising to elevations greater than 1200 meters. South of the Slim Buttes upland the North and South Forks of the Moreau River have eroded valleys into a topographic surface averaging between 850 and 900 meters in elevation. The floor of the South Fork Grand River, which is located directly north of the isolated Slim Buttes upland, has an elevation of about 820 meters. In other words, flood waters stripped more than 200 meters of material from much of northwest South Dakota and there is no way to determine how much material was removed from the region before flood waters lowered the regional surface to the level of the present day isolated upland surfaces.

Figure 4: Moreau River valley near Thunder Butte, South Dakota. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

What evidence shows east-oriented Moreau River valley eroded across massive southeast-oriented floods for its entire length? Figure 4 illustrates the east-northeast oriented Moreau River valley near its junction with Thunder Butte Creek. Figure 1 illustrates how this figure 4 map area is significantly east of the figures 2 and 3 map areas. Thunder Butte Creek is the larger southeast-oriented tributary joining the Moreau River at the town of Thunder Butte. Note how in figure 4 Moreau River tributaries from the north are southeast-oriented and tributaries from the south are northwest-oriented and join the east-oriented Moreau River as barbed tributaries. This remarkable aligned drainage pattern provides evidence the deep Moreau River valley eroded headward across massive southeast-oriented floods, which flowed across the entire figure 4 map area. The northwest-oriented tributary valleys were eroded by reversals of flood flow on northwest ends of beheaded southeast-oriented flood flow channels. Flood waters flowing across the figure 4 map area (and probably the entire northwest South Dakota area) were probably eroding southeast-oriented flood flow channels, which were components of an immense southeast-oriented anastomosing channel complex. Headward erosion of the deep east-northeast oriented Moreau River valley beheaded the southeast-oriented flood flow channels in sequence from east to west. Because the flood flow channels were beheaded one channel at a time, and because anastomosing flood flow channels are interconnected, reversed flood flow in a newly beheaded flood flow channel could capture yet to be beheaded flood flow from adjacent channels further to the west. Such captured flood water enabled the reversed flood flow to erode significant northwest-oriented valleys and also created a situation where east and south of the actively eroding Moreau River valley head flood waters were flowing in a northwest direction in newly beheaded flood flow channels while west of the actively eroding Moreau River valley head flood waters were flowing in a southeast direction in adjacent flood flow channels.

Figure 5: Reduce sized maps illustrating the region where Moreau River joins the Missouri River. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

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.