A geomorphic history based on topographic map evidence

The Niobrara River-Elkhorn River drainage divide area discussed here is located in northeastern Nebraska, USA. Although detailed topographic maps of the Niobrara River-Elkhorn 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. The Niobrara River-Elkhorn 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-oriented Niobrara River valley 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 Nebraska Niobrara River-Elkhorn 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 Nebraska Niobrara River-Elkhorn River drainage divide area landform evidence will be regarded as evidence supporting the “thick ice sheet that melted fast” paradigm.

Niobrara River-Elkhorn River drainage divide area general location map

Figure 1: Niobrara River-Elkhorn River drainage divide area general 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 location map for the Niobrara River-Elkhorn River drainage divide area. Nebraska is the state south of the west-to-east yellow boundary line and South Dakota is north of the west-to-east red boundary line. The east, southeast, and north-oriented Niobrara River flows from the figure 1 west edge through northern Nebraska to reach the Missouri River near Niobrara, Nebraska. The Elkhorn River originates south of Bassett, Nebraska and flows southeast and then northeast to Stuart before turning southeast to flow through Atkinson, O’Neill, Ewing, Clearwater, Neligh, Oakdale, Tilden, Meadow Grove, and Norfolk. Unnamed Niobrara River tributaries shown on figure 1 includes the north-oriented Verdigre River, which flows north from Royal through Verdigre to join the Niobrara River at its elbow of capture, where it changes from flowing southeast to flowing north to the Missouri River. North of the Niobrara River-Elkhorn River drainage divide area is the Keya Paha River-Niobrara River drainage divide area and the Ponca Creek-Niobrara River drainage divide area, where Ponca Creek is the unnamed Missouri River tributary flowing southeast from Colome, South Dakota (located in figure 1 northwest corner) to join the Missouri River near Verdal, Nebraska. These essays can be found under Niobrara River on the sidebar category list. Landform evidence illustrated here is interpreted in the context of immense southeast oriented floods flowing across the entire figure 1 map area and which were systematically captured and diverted east and perhaps even northeast by headward erosion of deep valleys eroded into a topographic surface at least as high as the figure 1 region highest elevations today. In the figure 1 map region headward erosion of the Elkhorn River valley first captured the southeast oriented flood flow and then the southeast-oriented Niobrara River valley eroded headward along a southeast-oriented flood flow route as a tributary to the actively eroding north-oriented Verdigre Creek valley (which may have continued north into South Dakota before being captured by headward erosion of the east and southeast-oriented Missouri River valley). Headward erosion of the much deeper southeast- and east-oriented Niobrara River valley captured flood flow moving to the newly eroded Elkhorn River valley and to the north-oriented Verdigre Creek valley (and subsequently to other north-oriented Niobrara River tributary valleys). Detailed maps will provide evidence supporting this interpretation.

Niobrara River-Elkhorn River drainage divide area detailed location map

Figure 2: Niobrara River-Elkhorn River drainage divide area detailed location map. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 2 provides a detailed location map for the Niobrara River-Elkhorn River drainage divide area. Keya Paha and Boyd Counties are located in Nebraska and areas north of them and/or of the Missouri River are located in South Dakota. The Niobrara River forms the boundary between Keya Paha County and Rock County and between Boyd County and Holt County before flowing southeast into Knox County and then north to the Missouri River. Note how the Niobrara River, Ponca Creek and the Missouri River area all flowing in parallel southeast-oriented valleys and how there are other southeast-oriented streams located throughout the figure 2 region south and west of the Missouri River. These parallel southeast-oriented valleys were probably initiated as interconnected channels in a large southeast-oriented anastomosing channel complex, evidence of which is further discussed in the Ponca Creek-Niobrara River drainage divide area essay. The southeast-oriented Elkhorn River valley may also have been initiated as a channel in that southeast-oriented anastomosing channel complex. Note how the Niobrara River flows northeast in eastern Keya Paha County and western Boyd County, immediately to the northwest of the southeast-oriented Elkhorn River valley segment. Note how northwest-oriented Ash Creek flows to the northeast-oriented Niobrara River using approximately the same alignment as the southeast-oriented Elkhorn River valley segment. That northeast-oriented Niobrara River valley segment may have been eroded across southeast-oriented flood flow moving to the southeast-oriented Elkhorn River valley segment. The Ash Creek valley would have been eroded by reversed flood flow on the northwest end of the southeast-oriented flood flow route carrying flood waters to what was then the actively eroding Elkhorn River valley. The northeast-oriented Elkhorn River valley segment probably was eroded southwest to capture yet to be beheaded (by headward erosion of the Niobrara River valley) southeast-oriented flood flow further to the south and west. The southeast-oriented Elkhorn River headwaters support this interpretation. This essay begins by illustrating detailed map evidence of the Verdigre Creek valley and tributaries to document the Verdigre Creek valley eroded headward across southeast-oriented flood flow. Next the essay looks at the northeast-oriented Eagle Creek valley to document the Eagle Creek valley also eroded across southeast-oriented flood flow. Finally the essay concludes by looking at evidence the northeast-oriented Niobrara River valley segment eroded headward across southeast-oriented flood flow that had been eroding the southeast-oriented Elkhorn River valley segment.

Niobrara River-North Branch Verdigre Creek drainage divide area

Figure 3: Niobrara River-North Branch Verdigre Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 4 illustrates the Niobrara River-North Branch Verdigre Creek drainage divide area. Verdigre Creek is the north-oriented stream flowing past Verdigre, Nebaska. The Niobrara River flows southeast from the figure 3 north center and turns north to join north-oriented Verdigre Creek and flow to the east and southeast oriented Missouri River, which is located north of the figure 3 map area. The Middle Branch Verdigre Creek is located along the figure 3 south edge and is better seen in figure 5 below. The northeast and southeast oriented North Branch Verdigre Creek is located south of the Niobrara River. Whenever it was formed the north-oriented Verdigre Creek valley eroded headward across multiple southeast oriented flood flow routes, such as might be found in a southeast-oriented anastomosing channel complex. Flood waters coming from the northwest eroded deep southeast-oriented valleys into the newly eroded Verdigre Creek west wall and reversed flood flow on the northwest ends of beheaded flood flow routes eroded northwest-oriented Verdigre Creek tributary valleys into the newly eroded Verdigre Creek valley east wall. It is possible the north-oriented Verdigre Creek valley orientation was established prior to headward erosion of the east- and southeast-oriented Missouri River valley. If so, it is also possible the north-oriented Verdigre Creek valley captured southeast-oriented flood flow moving along the present day Niobrara River valley and North Branch Verdigre Creek valley alignments prior to Missouri River valley capture of the north-oriented Verdigre Creek drainage system. However, once captured by the deep Missouri River valley a deep knick point was able to erode headward on the Niobrara River valley route, which ultimately captured all southeast-oriented flood flow to the Verdigre Creek drainage basin area. Note how multiple northeast and north-oriented tributaries have eroded south from the southeast-oriented Niobrara River valley. These tributary valleys were probably eroded in sequence from east to west to capture southeast-oriented flood flow south of the Niobrara River valley and to divert the flood waters into the Niobrara River valley. The relatively short southeast- and south-oriented North Branch Verdigre tributaries suggest Niobrara River valley headward erosion and headward erosion of Niobrara River tributary valleys captured the southeast-oriented flood flow before there was time to erode lengthy North Branch tributary valleys.

Detailed map of Niobrara River-North Branch Verdigre Creek drainage divide area

Figure 4: Detailed map of Niobrara River-North Branch Verdigre Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 4 illustrates a detailed map of the Niobrara River-North Branch Verdigre Creek drainage divide area west of Verdigre, Nebraska. Verdigre Creek flows northeast from the figure 4 south edge to Verdigre and flows north to the figure 4 north edge. Note northwest-oriented Verdigre Creek tributary valleys from the west (e.g. in northeast corner of figure 4). The North Branch Verdigre Creek flows southeast from the figure 4 west edge (south half) to join Verdigre Creek near the figure 4 south edge. Northwest-oriented drainage in section 26 (north of Bohemia Township) flows to northeast and north oriented Schindler Creek, which flows to the southeast-oriented Niobrara River. Northwest-oriented drainage to an independent and unnamed northeast oriented Niobrara River tributary is located in section 28 in the figure 4 northwest corner. What has happened to produce the figure 4 drainage features began with headward erosion of the north oriented Verdigre Creek valley headward across southeast-oriented flood flow. Reversals of flood flow on the northwest ends of beheaded southeast-oriented flood flow routes eroded the northwest-oriented Verdigre Creek tributary valleys. Southeast-oriented flood flow eroded southeast-oriented tributary valleys into the Verdigre Creek west wall and several such valleys are present including the southeast-oriented North Branch Verdigre Creek valley. South-oriented North Branch tributary valleys eroded north from the newly eroded North Branch valley in sequence from east to west to capture southeast-oriented flood flow north of the actively eroding North Branch valley head. At the same time the southeast-oriented Niobrara River valley was eroding northwest to the north of the figure 4 map area and north and northeast oriented tributary valleys were eroding headward in sequence from east to west from its south wall to capture southeast-oriented flood flow south of the actively eroding Niobrara River valley head. Where northeast-oriented Niobrara River tributary valleys beheaded southeast-oriented flood flow routes there frequently were reversals of flood flow on the northwest ends of the beheaded flood flow routes to erode northwest-oriented tributary valleys to the northeast-oriented-oriented Niobrara River tributary valleys. Because the southeast-oriented flood flow was moving in an anastomosing channel complex (which means the channels were interconnected), reversed flow on beheaded flood flow routes frequently captured flood waters from yet to be beheaded flood flow routes, a process which often enabled reversed flow to erode significant northwest-oriented tributary valleys.

Middle Branch Verdigre River-Verdigre Creek drainage divide area

Figure 5: Middle Branch Verdigre River-Verdigre Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 5 illustrates the Middle Branch Verdigre Creek-Verdigre Creek drainage divide area south of the figure 3 map area and includes overlap areas with figure 3. Verdigre Creek flows north-northeast from the figure 5 south edge to the figure 5 northeast corner. The Middle Branch Verdigre Creek flows northeast from the figure 5 west edge (south half) to the figure 5 north center and then east to join Verdigre Creek. Northeast-oriented North Branch Verdigre Creek is located in the figure 5 northwest corner. Note how almost all tributaries to  northeast and east-oriented Middle Branch Verdigre Creek from the west are southeast oriented. Also note the large number of northwest-oriented Verdigre Creek tributaries from the east along the figure 5 east edge. These southeast and northwest-oriented tributaries provide evidence the Verdigre Creek valley and the Middle Branch Verdigre Creek valley eroded headward across multiple southeast-oriented flood flow routes. Again the northwest-oriented tributary valleys were eroded by reversals of flood flow on the northwest ends of beheaded southeast-oriented flood flow routes (remember flood waters were flowing in interconnecting or anastomosing channels and reversed flow often was able to capture water from yet to be beheaded flood flow routes). The Middle Branch Verdigre Creek-Verdigre Creek drainage divide area is somewhat more complicated. There are southeast-oriented Verdigre Creek tributaries and there are northwest-oriented Middle Branch Verdigre Creek tributary valleys, but many Middle Branch Verdigre Creek tributaries are north-oriented and some are even northeast-oriented. Also, there are some northeast-oriented Verdigre Creek tributaries as well. The northeast-oriented Verdigre Creek tributary valleys eroded southwest to capture southeast-oriented flood flow routes yet to be beheaded by headward erosion of the Middle Branch Verdgre Creek valley. Subsequently these southeast-oriented flood flow routes were beheaded by headward erosion of north- and northeast-oriented Middle Branch Verdigre Creek tributary valleys. North and northeast-oriented Middle Branch Verdigre Creek tributary valleys eroded south and southwest in sequence from east to west also to capture yet to be beheaded southeast-oriented flood flow routes. Headward erosion of the deeper Middle Branch Verdigre Creek valley subsequently captured all of these southeast-oriented flood flow routes as well. Shortly thereafter headward erosion of the northeast-oriented North Branch Verdigre Creek valley further to the northwest captured all of the southeast-oriented flood flow routes to the Middle Branch Verdigre Creek valley.

Detailed map of Middle Branch Verdigre Creek-Verdigre Creek drainage divide area

Figure 6: Detailed map of Middle Branch Verdigre Creek-Verdigre Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 6 illustrates a detailed map of the Middle Branch Verdigre Creek-Verdigre Creek drainage divide area seen in less detail in figure 5 above. Verdigre Creek flows north-northeast from the figure 6 south edge (east half) to the figure 6 northeast corner. The Middle Branch Verdigre Creek flows across the figure 6 north half to join Verdigre Creek. A north- and northeast-oriented Middle Branch Verdigre Creek tributary flows from the figure 6 south edge to join Middle Branch Verdigre Creek in section 29 in the figure 6 northwest quadrant. Note how Middle Branch Verdigre Creek tributaries from the north are southeast-oriented. Also note how Middle Branch tributaries from the south are either northwest-oriented or northeast-oriented and how the northeast-oriented tributaries have northwest-oriented and sometimes southeast-oriented tributaries. In the case of Verdigre Creek note how tributaries from the east are northwest-oriented and tributaries from the west are either southeast-oriented or northeast-oriented, and tributaries to the northeast-oriented Verdigre Creek tributaries are generally southeast-oriented. This northwest-southeast tributary orientation is evidence the deep Verdigre Creek valley and subsequently the deep Middle Branch Verdigre Creek valley eroded headward across multiple southeast-oriented flood flow routes. The southeast-oriented tributary valleys were eroded directly by southeast-oriented flood flow routes entering newly eroded valleys. The northwest-oriented tributary valleys were eroded by reversals of flood flow on the northwest ends of beheaded flood flow routes. Because the southeast-oriented flood flow was moving in anastomosing (or interconnected) channels reversed flow on beheaded flood flow routes frequently captured flood flow from yet to be beheaded flood flow routes. With the aid of this captured flood water the reversed flood flow was often able to erode significant northwest-oriented tributary valleys. The source of the flood water cannot be determined from evidence presented here. However, rapid melting of a thick North American ice sheet located in a deep “hole” (occupying approximately the North American area generally recognized to have been glaciated) would be a logical flood water source and would also explain why deep valleys were eroding headward to capture southeast-oriented flood water and to divert the water further and further to the northeast (see the entire Missouri River drainage basin landform origin research project essay collection published on this website for complete documentation of these diversions).

South Branch Verdigre Creek-Elkhorn River drainage divide area

Figure 7: South Branch Verdigre Creek-Elkhorn River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 7 illustrates the South Branch Verdigre Creek-Elkhorn River drainage divide area. The Elkhorn River flows southeast across the figure 7 southwest corner. The South Branch Verdigre Creek flows southeast (northeast of Page, Nebraska) and then turns northeast, east and northeast to flow to the figure 7 northeast corner. Big Springs Creek is the northeast-oriented South Branch tributary joining the South Branch where it changes from flowing east to flowing northeast. The East Branch Verdigre Creek flows north-northwest to join the South Branch near the figure 7 corner. Headwaters of the northeast-oriented Middle Branch Verdigre Creek can be seen along the figure 7 north edge (north and slightly east of Page). Note the predominance of southeast-oriented tributaries from the north to northeast-oriented South Branch Verdigre Creek and of northwest-oriented tributaries from the south to both the South Branch and Big Springs Creek. This predominance of southeast-northeast oriented tributaries is evidence the South Branch valley and the Big Springs Creek valley eroded headward across multiple southeast-oriented flood flow routes. The southeast-oriented tributary valleys were eroded by southeast-oriented flood flow routes eroding into newly eroded valley walls. The northwest-oriented tributary valleys were eroded by reversals of flood flow on the northwest ends of beheaded southeast-oriented flood flow routes. Because flood flow channels were anastomosing or interconnected the reversed flood flow often captured flood flow from yet to be beheaded flood flow routes. This captured flood water often enable reversed flood flow to erode significant northwest-oriented tributary valleys. The Elkhorn River valley apparently eroded its southeast-oriented valley shortly before arrival of the northeast-oriented South Branch Verdigre Creek valley in the figure 7 map area and was able to erode its valley deep enough that headward erosion of the South Branch valley could not capture its flood flow. As a result the Niobrara River-Elkhorn River drainage divide in the figure 7 map area is located along the northwest-southeast oriented ridge just southwest of the railroad line connecting Page and Orchard.

Niobrara River-Elkhorn River drainage divide in Eagle Creek area

Figure 8: Niobrara River-Elkhorn River drainage divide in Eagle Creek area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 8 illustrates a big picture view of the Niobrara River-Elkhorn River drainage divide area in the Eagle Creek area, which is located west of the Verdigre Creek area. The southeast-oriented Elkhorn River flows through Atkinson, Nebraska in the figure 8 southwest corner. The east-southeast oriented Niobrara River is located in the figure 8 northeast quadrant. Eagle Creek is the long northeast-oriented Niobrara River tributary originating northeast of Atkinson. Note where the east-oriented East Branch, northeast-oriented Middle Branch, and north-northwest oriented East Branch Eagle Creek meet in the figure 8 center. That area is shown in more detail in figure 9 below. The north and northeast-oriented Niobrara River tributary located east of Eagle Creek is Redbird Creek and its western north- and southeast-oriented tributary is Blackbird Creek. West of Eagle Creek the first north-oriented Niobrara River tributary with a forested valley is Turkey Creek and the second is Brush Creek. Note how these Niobrara River tributaries are draining a northeast-oriented slope. North of the Niobrara River is somewhat rougher topography. The Elkhorn River valley at Atkinson is almost 200 meters higher than the Niobrara River valley where Eagle Creek joins it. This northeast-oriented slope is probably a flood eroded erosion surface formed as southeast-oriented flood water gradually converged to flow in what is today the east-southeast Niobrara River valley. The northeast-oriented erosion surface suggests the region was deeply eroded by immense volumes of flood waters moving in an east-southeast direction along what is today the Niobrara River valley route. The southeast-oriented Elkhorn River valley apparently eroded deep enough that headward erosion of the northeast-oriented Niobrara River tributary valleys was not able to capture it, although it did not erode deep enough to capture much southeast-oriented flow in the region to the northeast of it. The northeast-oriented Niobrara River tributaries again have southeast-oriented tributaries (or there are southeast-oriented tributaries to the north and northeast-oriented tributaries) and there are also northwest-oriented tributaries to some of the northeast-oriented Niobrara River tributaries. The northeast-oriented Niobrara River tributary valleys eroded headward in sequence, with the easternmost tributary valley eroding headward first and the westernmost tributary valley eroding headward last.

Detailed map of North Branch-Middle Branch-East Branch Eagle Creek drainage divide area

Figure 9 illustrates the North Branch-Middle Branch-East Branch Eagle Creek drainage divide area seen in less detail in figure 8 above. Remnants of the northeast-oriented erosion surface are preserved between the various valleys. The Middle Branch Eagle Creek flows northeast from the figure 9 southwest corner. The North Branch Eagle Creek flows east from the figure 9 west edge to join the Middle Branch. The north-oriented East Branch flows north from the figure 9 south edge to join the Middle and North Branches and to form northeast-oriented Eagle Creek. Note southeast oriented tributary valleys to the east-oriented North Branch and also to the northeast-oriented Middle Branch. Also note the northwest-oriented tributaries to the north-oriented East Branch and the northeast-oriented Middle Branch. These southeast and northwest-oriented tributary valleys provide evidence the deep East Branch valley eroded headward across multiple southeast-oriented flood flow routes with flood waters on the northwest ends of the beheaded flood flow routes reversing flow direction to flow northwest into the newly eroded East Branch valley. Shortly thereafter, without sufficient time for flood waters to erode southeast-oriented tributary valleys into the newly eroded East Branch west valley wall, headward erosion of the deep Middle Branch valley captured the same southeast-oriented flood flow routes and diverted the water northeast. Flood flow reversals eroded short northwest-oriented tributary valleys into the northeast-oriented Middle Branch southeast valley wall. At about the same time the North Branch valley began to erode west to capture southeast-oriented flood flow coming into the figure 9 map area. Southeast-oriented flood flow did have sufficient time to erode southeast-oriented tributary valleys into the newly eroded North Branch north valley wall before headward erosion of valleys further to the northwest captured the flood waters.

West end of the Niobrara River-Elkhorn River drainage divide area

Figure 10: West end of the Niobrara River-Elkhorn River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 10 provides a big picture view of the west end of the Niobrara River-Elkhorn River drainage divide area. The Elkhorn River flows northeast in the figure 10 south center to near Stuart, Nebraska and then turns southwest to flow to Atkinson along the figure 10 south edge. The North Branch Elkhorn River flows generally east to join the northeast-oriented Elkhorn River just west of Stuart. The northeast-oriented Niobrara River is located in figure 10 northwest corner. The northwest oriented Niobrara River tributary originating northwest of Stuart and joining the Niobrara River at Mariaville is Ash Creek. West of Ash Creek is northwest and northeast-oriented Oak Creek and the next tributary to the west is northwest and northeast-oriented Willow Creek. Note how the tributaries further east where the Niobrara River is southeast-oriented tended to be northeast-oriented (see figure 8 and figure 2). In figure 10 where the Niobrara River is northeast-oriented the tributaries are predominantly northwest-oriented. Also note how the northwest-oriented Niobrara River tributaries are approximately aligned with the southeast-oriented Elkhorn River valley segment. Prior to Niobrara River valley headward erosion there was a major southeast-oriented flood flow route (or multiple southeast-oriented flood flow routes) moving across the figure 10 map area to what was then the actively eroding southeast-oriented Elkhorn River valley. The Niobrara River valley is northeast-oriented here because as it eroded headward to capture the southeast-oriented flood flow it eroded to the southwest across the flood flow routes moving water to the Elkhorn River valley. The northwest-oriented Niobrara River tributary valleys were eroded by reversals of flood flow on the northwest ends of the beheaded flood flow routes. The northeast-oriented Elkhorn River valley segment was formed at that time by a northeast-oriented Elkhorn River valley tributary that eroded southwest to capture southeast-oriented flood flow on flood flow routes further to the southwest. Headward erosion of the deep Niobrara River valley did not behead those flood flow routes until later, so the Elkhorn River valley was extended southwest from the Stuart area and then south of the figure 10 map it was extended northwest along a southeast-oriented flood flow route it captured (see figures 1 and 2).

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.