This essay uses topographic map evidence to interpret landform origins in the Boyer River-Nishnabotna River drainage divide area located in Crawford, Carroll, Shelby, and Audubon Counties, Iowa. Evidence presented and discussed also includes regions between Nishnabotna River tributaries located in the four counties. The Boyer River and Nishnabotna River are southwest and south-southwest oriented Missouri River tributaries located in western Iowa with the Boyer River being located north and west of the Nishnabotna River. Both rivers, especially the Nishnabotna River, have multiple tributaries many of which are also oriented in southwest and south-southwest directions. Topographic map evidence including orientations of secondary and tertiary tributaries and through valleys eroded across most drainage divides indicate the Nishnabotna River valley and its tributary valleys eroded headward from what was then an actively eroding Missouri River valley across an immense southeast-oriented anastomosing channel complex. Flood waters in that southeast oriented anastomosing channel complex were flowing to what were then actively eroding southeast-oriented Des Moines River tributary valleys. Headward erosion of the Nishnabotna River tributary valleys in a progressive sequence first beheaded southeast-oriented flood flow routes to the Des Moines River tributary valleys and then one after another beheaded flood flow routes to the previously eroded Nishnabotna River tributary valley to the east. Headward erosion of Nishnabotna River tributary valleys ended when headward erosion of the Boyer River valley and tributary valleys beheaded all flood flow routes to the newly formed Nishnabotna River drainage basin and also to some actively eroding Des Moines River tributary valleys.

Preface:

The following interpretation of detailed topographic map evidence is one of a series of essays describing similar evidence for all major drainage divides contained within the Missouri River drainage basin and for all major drainage divides with adjacent drainage basins. The research project is interpreting evidence in the context of a previously unexplored deep glacial erosion paradigm, which is fundamentally different from most commonly accepted North American glacial history interpretations. Project essays available at this site may be found by selecting desired Missouri River tributaries and/or states from this essay’s sidebar category list.

Introduction

• The purpose of this essay is to use topographic map interpretation methods to explore the Boyer River-Nishnabotna River drainage divide area landform origins in Crawford, Carroll, Shelby and Audubon Counties, Iowa, USA. 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 and/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 essay in a comment here.

• This essay is also exploring a new geomorphology 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 other Missouri River drainage basin landform origins research project essays is a thick North American ice sheet, comparable in thickness to the Antarctic ice sheet, occupied the North American region usually recognized to have been glaciated, and through its weight and erosive actions created a deep North American “hole”. The southwestern rim of that deep “hole” is today preserved in the high Rocky Mountains. The ice sheet 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 Boyer River-Nishnabotna drainage divide area landform evidence in Crawford, Carroll, Shelby, and Audubon Counties, Iowa will be regarded as evidence supporting the “thick ice sheet that melted fast” paradigm (see menu at top of page for paradigm related essay). This essay is included in the Missouri River drainage basin landform origins research project essay collection.

Boyer River-Nishnabotna River drainage divide area location map

Figure 1: Boyer River-Nishnabotna River drainage divide area 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 regional map to illustrate the Boyer River-Nishnabotna River drainage divide area in western Iowa. The south-southeast oriented Missouri River is located near the figure 1 west edge and Omaha, Nebraska is the large city located in the figure 1 southwest quadrant with Council Bluffs, Iowa located directly across the Missouri River to the east. Des Moines is the large city located near the figure 1 east center area and the Des Moines River flows in a south-southeast direction from the figure 1 north edge to Des Moines and then in a southeast direction to the figure 1 east edge. East of figure 1 the Des Moines River flows directly to the Mississippi River and the Des Moines River drainage basin is not included in the Missouri River drainage basin. The Raccoon River is the tributary joining the Des Moines River at Des Moines. Note how Raccoon River tributaries are generally oriented in south-southeast and southeast directions and how several northeast-oriented Des Moines River tributaries south of the Raccoon River drainage basin have southeast-oriented headwaters. The south-southeast oriented Thompson River and some other south-oriented streams in the figure 1 south center edge area do drain to the Missouri River south of the figure 1 map area. West of the Des Moines River drainage basin is the Missouri River drainage basin where almost all Missouri River tributaries from the east are oriented in a south-southwest direction. Note how in Iowa west of the Des Moines River drainage basin the south-southwest oriented Missouri River tributaries parallel each other, have south-southwest oriented tributaries, and have narrow drainage basins. Also note how on figure 1 how the Missouri River-Des Moines River drainage divide can be identified by orientations of tributaries to the two rivers.

• The Boyer River flows in a south direction from the figure 1 north edge (near Early) to Wall Lake and then in a south-southwest direction to Denison, Woodbine, and Missouri Valley before joining the Missouri River a short distance north of Omaha. The Nishnabotna River is not labeled on figure 1 and has multiple south-southwest oriented tributaries flowing to the figure 1 south edge between the Missouri River and Red Oak, Iowa. These tributaries converge south of the figure 1 map area and the Nishnabotna River then joins the Missouri River just south of the Iowa south boundary in the Missouri northwest corner. Major tributaries are the East Nishabotna River, which originates south of Templeton and which flows in south direction to Atlantic, Iowa and then in a south-southwest direction to near Red Oak and then to the figure 1 south edge. The West Nishnabotna River flows in a south-southwest direction through Manning, Avoca, and Carson before flowing to the figure 1 south edge. The West Fork West Nishnabotna River flows through Manila and Defiance before joining the West Nishnabotna River. Further south the south-southwest oriented stream with many south-southwest oriented tributaries which joins the West Nishnabotna River near Malvern is Silver Creek. As we will see in the more detailed maps below there are many more south, south-southeast, and south-southwest oriented Nishnabotna River tributaries. The Boyer River Nishnabotna River drainage divide area in Crawford, Carroll, Shelby, and Audubon Counties is located north of Interstate highway 80 and the discussion in this essay also includes drainage divides between various Nishnabotna River tributaries found in that region.

• The difference in tributary orientations for the Des Moines River drainage basin and the Missouri River drainage basin was caused by headward erosion of the south-southwest oriented Missouri River tributary valleys in sequence from east to west across an immense southeast-oriented anastomosing channel complex which was supplying flood waters to what were then actively eroding southeast-oriented Des Moines River tributary valleys. Flood waters were derived from a rapidly melting North American ice sheet located north of the figure 1 map area and were being captured by headward erosion of the southeast-oriented Des Moines River valley and its tributary valleys. These captures were diverting south-oriented flood flow in western Iowa in a southeast direction toward the actively eroding Des Moines River valley and its tributary valleys. At that time the south-southeast oriented Missouri River valley was just beginning to erode headward into the figure 1 map area. As the deep Missouri River valley eroded headward along what was a major south-southeast oriented flood flow route, south-southwest oriented tributary valleys eroded headward across the southeast and south-southeast oriented flood flow routes in advance of the Missouri River valley erosion. Headward erosion of the East Nishnabotna River valley captured the south-southeast and southeast oriented flood flow first and diverted the water in a south-southwest direction to the newly eroded Missouri River valley. Other Nishnabotna River tributary valleys also eroded headward across the south-southeast and southeast oriented flood flow in sequence from east to west, with each successive tributary valley beheading flood flow routes to the newly eroded tributary valley to the east. Eventually Missouri River valley headward erosion proceeded to the point where the south-southwest oriented Boyer River valley eroded headward and in time beheaded all south-southeast and southeast oriented flood flow routes to the Nishnabotna River drainage basin.

Boyer River-Nishnabotna River drainage divide area in Crawford, Carroll, Shelby, and Audubon Counties, Iowa detailed location map

Figure 2: Boyer River-Nishnabotna River drainage divide area in Crawford, Carroll, Shelby, and Audubon Counties, Iowa detailed location map. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 2 provides a somewhat more detailed location map for the Boyer River-Nishnabotna River drainage divide area in Crawford, Carroll, Shelby, and Audubon Counties, Iowa. The Missouri River flows from the figure 2 northwest corner to the figure 2 south edge. Omaha, Nebraska is the city straddling the south edge just west of the Missouri River. Iowa county names are given and county boundaries area shown. The Boyer River flows in a southwest direction across Crawford County to the Harrison County northeast corner and then across Harrison County to join the Missouri River in the Pottawattamie County northwest corner. Mosquito Creek begins just north of the Crawford County south center border and flows in a south-southwest direction across Shelby County, the Harrison County southeast corner, and into Pottawattamie County. The Boyer River-Mosquito Creek drainage divide area is discussed in the Boyer River-Mosquito Creek drainage divide area landform origins in Pottawattamie, Harrison, Shelby, and Crawford Counties, Iowa essay. The Little Sioux River-Boyer River drainage divide area located north and west of the Boyer River is discussed in the Little Sioux River-Boyer River drainage divide area landform origins, western Iowa essay. Essays can be found under Iowa on the sidebar category list.

• The West Nishnabotna River originates in the Carroll County southwest corner and flows in a south-southwest direction to Harlan in Shelby County and then to Hancock in Pottawattamie County and finally to the figure 2 south edge. The West Fork West Nishnabotna River originates in southeast Crawford County and flows in a south-southwest direction to Manila and Defiance before turning to flow in a south-southeast and south direction to join the West Nishnabotna River near Harlan. The East Branch West Nishnabotna River originates near Gray in the Audubon County northwest quadrant and flows in a southwest and south-southwest direction across Shelby County to join the West Nishnabotna River in northern Pottawattamie County. The East Nishnabotna River originates south of Templeton in southern Carroll County and flows in a southeast direction into northern Audubon County. Once in Audubon Couny the East Nishnabotna River turns to flow in a south and south-southwest direction across the Cass County northwest quadrant and to the figure 2 south edge. This knob illustrates and discusses only a few samples of topographic map evidence found in the region between the Boyer River and the East Nishnabotna River drainage divide area located in Crawford, Carroll, Shelby, and Audubon Counties. Readers are encouraged to study topographic maps for the entire region.

East Boyer River-West Fork West Nishnabotna River drainage divide area

Figure 3: East Boyer River-West Fork West Nishnabotna River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 3 provides a reduced size topographic map of the East Boyer River-West Fork West Nishnabotna River drainage divide area located south and east of Denison, Iowa. The south-southwest oriented Boyer River flows across the figure 3 northwest corner and is joined by the southwest-oriented East Boyer River at Denison (town located near figure 3 northwest corner). Note how most East Boyer River tributaries from the south are oriented in northwest directions. Manila is the town located near the figure 3 south center edge and the West Fork West Nishnabotna River is the stream flowing to the figure 3 south edge at Manila. Note how shallow through valleys link the northwest-oriented East Boyer River tributary valleys with valleys of West Fork Nishnabotna River tributaries. The northwest-southeast oriented through valleys and the northwest-oriented East Boyer River tributary orientations provide evidence of multiple southeast-oriented flood flow channels that existed prior to East Boyer River valley headward erosion. Just prior to East Boyer River valley headward erosion flood waters were flowing to what was then the newly eroded and actively West Fork West Nishnabotna River valley and its tributary valleys. The multiple flood flow channels suggest the flood flow channels were components of a giant southeast-oriented anastomosing channel complex. Headward erosion of the deep southwest oriented East Boyer River valley captured the southeast oriented flood flow one channel at a time from the southwest to the northeast. Flood waters on northwest ends of beheaded flood flow channels reversed flow direction to erode the northwest-oriented East Boyer River tributary valleys.

• The town located in the figure 3 southeast corner is Manning and is located on the south-southwest oriented West Nishnabotna River. West of Manning on the railroad line is the small town of Aspinwall. The south-southwest oriented stream at Aspinwall is Elk Creek, which south of the figure 3 map area flows to the West Nishnabotna River. Note how multiple and shallow northwest-southeast oriented through valleys cross the West Fork West Nishnabotna River-Elk Creek drainage divide, the Elk Creek-West Nishnabotna River drainage divide, and (further to the northeast) the West Fork West Nishnabotna River-West Nishnabotna River drainage divide. The through valleys provide further evidence of multiple southeast-oriented flood flow channels that once crossed the entire figure 3 map area. Headward erosion of the West Nishnabotna River valley first captured the southeast-oriented flood flow and flood waters on northwest ends of beheaded flood flow routes reversed flow direction to erode northwest-oriented tributary valleys (see figure 3 southeast corner). Elk Creek valley headward erosion next captured the southeast-oriented flood flow and beheaded the southernmost of the southeast-oriented flood flow routes to the newly eroded West Nishnabotna River valley. Next West Fork West Nishnabotna River valley headward erosion beheaded all flood flow routes to the actively eroding Elk Creek valley, causing headward erosion of that valley to cease. The West Fork West Nishnabotna River valley and subsequently its northern tributary valley then beheaded additional southeast-oriented flood flow routes to the newly eroded West Nishnabotna River valley. Completing the figure 3 drainage system development headward erosion of the East Boyer River valley next beheaded all southeast-oriented flood flow routes to West Fork West Nishnabotna River and tributary valleys.

Detailed map of West Fork West Nishnabotna River-Elk Creek drainage divide area

Figure 4: Detailed map of West Fork West Nishnabotna River-Elk Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 4 provides a detailed topographic map of the West Fork West Nishnabotna River-Elk Creek drainage divide area north and east of Manila. Manila is the town located in the figure 4 southwest corner. The West Fork West Nishnabotna River flows in a south and south-southwest direction along the figure 4 west edge. Elk Creek flows in a south-southwest direction across the figure 4 southeast corner area. Note how in sections 18 and 19 northwest-southeast oriented through valleys link northwest-oriented West Fork West Nishnabotna River tributary valleys with southeast-oriented Elk Creek tributary valleys. The map contour interval is 20 feet and while some contour lines are faint and hard to read floors of the deeper through valleys have elevations in the 1420 to 1440 foot range. Elevations on either side of the through valleys rise to at least 1500 feet suggesting the through valleys are at least 60 feet deep and may have been deeper when eroded. Additional northwest-southeast oriented through valleys can also be seen. For example in the section 9 southwest corner (figure 4 northeast quadrant) the railroad is located in a through valley with a somewhat higher floor elevation. Another significant northwest-southeast oriented through valley is located in section 17. These through valleys were all eroded by southeast-oriented flood flow channels moving flood waters to what was once the actively eroding Elk Creek valley. At least for a period of time flood waters were flowing simultaneously in all of the southeast-oriented channels, which were components of a large southeast-oriented anastomosing channel complex. Headward erosion of the deep West Fork West Nishnabotna River valley beheaded the southeast-oriented flood flow channels in sequence from the south and southeast to the north and northwest. Flood waters on northwest ends of the beheaded flood flow channels reversed flow direction to erode the northwest- and west oriented West Fork West Nishnabotna River tributary valleys.

East Boyer River-West Nishnabotna River drainage divide area

Figure 5: East Boyer River-West Nishnabotna River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 5 provides a reduced size topographic map of the East Boyer River-West Nishnabotna River drainage divide area and is located north and east of the figure 3 map area and includes overlap areas with figure 3. Note the north-south oriented Crawford-Carroll County boundary line located in the figure 5 center. Westside is the small town located just west of the county boundary in the figure 5 north center area. Vail is the slightly larger town located just north of the figure 5 west center area. Vail is located in the southwest-oriented East Boyer River valley and Westside is located in the valley of the northwest-oriented East Boyer River. The south and southwest oriented stream in the figure 5 southwest quadrant is the West Fork West Nishnabotna River and was seen in figure 3. The south-southeast and south oriented stream flowing through the word WASHINGTON in the figure 5 southeast quadrant is the West Nishnabotna River headwaters. Note how the south-southeast oriented West Nishnabotna River valley segment is linked by a northwest-southeast oriented through valley with the northwest-oriented East Boyer River tributary valley. The map contour interval is ten meters and the through valley is defined by two contour lines on the northeast and at least three contour lines to the southwest (figure 6 below better illustrates the through valley). Arcadia is the small town located east and slightly north of Westside. South of Arcadia are north and east-northeast oriented headwaters of south-southeast oriented Brushy Creek, which flows to the figure 5 southeast corner. The Middle Raccoon River flows in a southeast direction across the figure 5 northeast corner and Brushy Creek is a South Raccoon River tributary. South and east of the figure 5 map area Brushy Creek joins the South Raccoon River, which then joins the Middle Raccoon River and other tributaries to form the Raccoon River, which flows to the southeast-oriented Des Moines River at Des Moines (see figures 1 and 2). Note how south of Arcadia the Brushy Creek headwaters are linked by a northwest-southeast oriented through valley with headwaters of a west-oriented East Boyer River tributary. Through valleys such as this through valley provide evidence headward erosion of the Boyer River-East Boyer River valley from the Missouri River valley captured southeast-oriented flood flow moving to actively eroding Des Moines River tributary valleys as well as to the actively eroding West Nishnabotna River valley and West Fork West Nishnabotna River valley.

Detailed map of East Boyer River-West Nishnabotna River drainage divide area

Figure 6: Detailed map of East Boyer River-West Nishnabotna River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 6 provides a detailed topographic map of the East Boyer River-West Nishnabotna River drainage divide area seen in less detail in figure 5 above. The East Boyer River flows in a north-northwest direction to the figure 6 northwest corner area. As seen in figures 3 and 5 north and west of figure 6 map area the East Boyer River turns to flow in a southwest direction to join the south-southwest oriented Boyer River. The West Nishnabotna River flows in a south-southeast direction to the figure 6 south center edge. As seen in figures 1, 2, and 3 south of the figure 6 map the West Nishnabotna River turns to flow in a south-southwest direction. Note how the north-northwest oriented East Boyer River is linked in the section 32 southeast corner by a well-defined through valley with the south-southeast oriented West Nishnabotna River valley. The map contour interval is ten feet and the through valley floor elevation is between 1420 and 1430 feet. The hill in the section 33 northwest quadrant rises to at least 1500 feet and the hill in the section 5 northeast quadrant rises to at least 1510 feet. The through valley is a water eroded feature and was eroded by southeast-oriented flood flow moving to what was at one time the actively eroding West Nishnabotna River valley. Headward erosion of the southwest-oriented East Boyer River valley beheaded the southeast-oriented flood flow channel. Flood waters on the northwest end of the beheaded flood flow route reversed flow direction to erode the north-northwest and northwest oriented East Boyer River valley segments and also to create the East Boyer River-West Nishnabotna River drainage divide. Note how northwest, north, and east-northeast oriented Brushy Creek headwaters originate in section 34 and are linked in sections 28 and 29 by very shallow northwest-southeast oriented through valleys with a northwest-oriented East Boyer River tributary valley. Just north of the figure 6 map area there is a northwest-southeast oriented through valley (comparable in depth and width to the section 32 East Boyer River-West Nishnabotna River through valley) linking northwest-oriented East Boyer River tributary valleys with southeast-oriented Brushy Creek tributary valleys. The through valleys provide evidence that for a time at least flood water was flowing simultaneously in a southeast direction to both the actively eroding West Nishnabotna River valley and the actively eroding Brushy Creek valley and then was beheaded and reversed by Boyer River-East Boyer River valley headward erosion.

West Nishnabotna River-East Branch West Nishnabotna River drainage divide area

Figure 7: West Nishnabotna River-East Branch West Nishnabotna River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 7 uses a reduced size topographic map to illustrate the West Nishnabotna River-East Branch Nishnabotna River drainage divide area located south and east of the figure 3 map area and includes overlap areas with figure 3. Note the east-west and north-south oriented and labeled county boundary lines. Manning is the town located near the north edge just west of the center and Aspinwall is the smaller town located near the north edge near the figure 7 northwest corner. The south-southwest oriented river flowing from Manning to the figure 7 west edge (south half) is the West Nishnabotna River. Elk Creek is the south-southwest oriented stream flowing across the figure 7 northwest corner. The southeast-oriented river located in the figure 7 northeast quadrant is the East Nishnabotna River, which east and south of the figure 7 map area turns to flow in a south and south-southwest direction (see figures 1 and 2). The south-southeast and south-southwest oriented stream flowing to the figure 7 south center edge is East Branch West Nishnabotna River, which south of figure 7 joins the West Nishnabotna River. The south-southwest oriented stream in the figure 7 southwest quadrant, located between the West Nishnabotna River and the East Branch West Nishnabotna River, is the Long Branch East Branch West Nishnabotna River. Note how East Branch West Nishnabotna River headwaters are oriented in a southeast direction and are linked by multiple northwest-southeast oriented through valleys with northwest-oriented West Nishnabotna River tributaries. The map contour interval is ten meters. Many of the through valleys are only defined by a single contour line on each side, although the large through valley southeast of Manning is defined by three contour lines on each side. Again the through valleys are water eroded features and provide evidence of multiple southeast-oriented flood flow channels that crossed the present day West Nishnabotna River-East Branch West Nishnabotna River drainage divide prior to headward erosion of the West Nishnabotna River valley. Northwest-oriented West Nishnabotna River tributary valleys were eroded by reversals of flood flow on northwest ends of beheaded flood flow routes.

East Branch West Nishnabotna River-East Nishnabotna River drainage divide area

Figure 8: East Branch West Nishnabotna River-East Nishnabotna River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 8 provides a topographic map of the East Branch West Nishnabotna River-East Nishnabotna River drainage divide area located south and east of the figure 7 map area and includes overlap areas with figure 7. Note the north-south oriented Shelby-Audubon County boundary located near the figure 8 west edge. Audubon is the larger town located just east of the figure 8 south center edge. Gray is the much smaller town located just west of the figure 8 north center area. The stream flowing in a south-southeast direction from the north edge to Gray and then turning to flow in a south-southwest and southwest direction to the figure 8 west edge (just north of southwest corner) is the East Branch West Nishnabotna River. Note southeast-oriented East Branch West Nishnabotna River tributaries and headwaters in the figure 8 northwest quadrant and northwest and west oriented tributaries from the east. Lone Willow Creek is the northwest-oriented tributary located west of Audubon. The East Nishnabotna River is located in the figure 8 east half and flows in a southeast, south, southwest, and south direction from the figure 8 north edge to the south edge. Note several northwest-oriented tributaries from the east and southeast-oriented tributaries in the figure 8 southeast quadrant. Bluegrass Creek is the south-oriented stream flowing to the figure 8 south edge at Audubon and is an East Nishnabotna River tributary. Ross is a small town located in the Bluegrass Creek valley north of Audubon. Note how the south oriented Bluegrass Creek valley is linked by a northwest-southeast oriented through valley with headwaters of a southeast-oriented East Nishnabotna River tributary. Also note how northwest of Ross the Bluegrass Creek valley is linked by a northwest-southeast oriented through valley with headwaters of a northwest-oriented East Branch Nishnabotna River tributary, which joins the East Branch at the elbow of capture where the East Branch turns from flowing in a south-southeast direction to flowing in a south-southwest direction. The through valleys define what was once a significant southeast oriented flood flow channel to what was then the newly eroded East Nishnabotna River valley. Headward erosion of the Bluegrass Creek valley first captured the southeast-oriented flood flow and subsequently East Branch West Nishnabotna River valley headward erosion captured the flood flow. Flood waters on the northwest end of the beheaded flood flow channel reversed flow to erode the northwest-oriented East Branch Tributary valley.

East Nishnabotna River-Davids Creek drainage divide area

Figure 9: East Nishnabotna River-Davids Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 9 provides a topographic map of the East Nishnabotna River-Davids Creek drainage divide area east of the figure 8 map area and includes overlap areas with figure 8. Audubon is the town located in the figure 9 southwest corner. Ross is the small town located in the Bluegrass Creek valley north of Audubon. The East Nishnabotna River in located in the figure 9 west half and flows in a southeast, south, southwest, and south direction from the figure 9 north to south edges. Larland is a place just east of the figure 9 south center area. The south-oriented stream near Larland is Davids Creek, which flows in a south-southwest, south-southeast, and south direction to the figure 9 south edge (south of Larland). South of the figure 9 map area Davids Creek turns to flow in a southwest direction to join the East Nishnabotna River. Note how a south-southeast oriented Davids Creek tributary valley segment is linked by a northwest-southeast oriented through valley with a northwest-oriented East Nishnabotna River tributary valley. Figure 10 below provides a more detailed topographic map of that through valley area. Brushy Creek flows in a southeast direction across the figure 9 northeast quadrant and the southeast-oriented South Raccoon River originates in the region between north and northwest oriented tributaries to a northeast-oriented Brushy Creek tributary (located just north of figure 9) and the Davids Creek headwaters area. East of the figure 9 map area Brushy Creek joins the South Raccoon River, which joins with other tributaries to form the Raccoon River, which in turn flows to the southeast-oriented Des Moines River. In other words, the East Nishnabotna River-South Raccoon River drainage divide seen in the figure 9 map area is the divide between the Missouri River drainage basin to the west and the Mississippi River drainage basin to the east. Shallow through valleys can be seen crossing the drainage divide. These through valleys are defined by a single ten meter contour line on each side, but they provide evidence of multiple southeast-oriented channels that were beheaded by headward erosion of first the Davids Creek valley and subsequently by headward erosion of the East Nishnabotna River valley.

Detailed map of East Nishnabotna River-Davids Creek drainage divide area

Figure 10: Detailed map of East Nishnabotna River-Davids Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 10 provides a detailed topographic map of the East Nishnabotna River-Davids Creek drainage divide area seen in less detail in figure 9 above. Davids Creek originates in the figure 10 northeast corner area and flows in a south and south-southeast direction near the figure 10 east edge. Note the north-northeast and southeast oriented Davids Creek tributary in sections 16, 15, and 14 near the figure 10 south edge. A northwest oriented stream flows to the figure 10 northwest corner and is a tributary to the south-oriented East Nishnabotna River. Note how that tributary has a northwest and north oriented tributary originating in section 10. Also note how headwaters of that northwest- and north-oriented tributary are linked by through valleys in sections 9 and 10 with the southeast-oriented Davids Creek tributary valley located in section 15. The map contour interval is 20 feet and elevations of the floors of the deepest through valleys are between 1400 and 1420 feet. Hills on either side of the through valleys rise to elevations greater than 1460 feet. Other shallower through valleys are also present. The through valleys, especially the deeper through valleys, provide evidence of southeast-oriented flood flow channels to what at one time was the actively eroding Davids Creek valley and tributary valley. The north-oriented tributary valley segment in west half of section 15 was probably eroded by a reversal of flood flow on the north end of a beheaded south-oriented flood flow route. In the section 17 southeast quadrant (along the figure 10 south edge) a through valley links the Davids Creek tributary valley with a north-oriented tributary to the northwest-oriented East Nishnabotna River tributary. Prior to being beheaded and reversed by East Nishnabotna River valley headward erosion one flood flow route was south into sections 16 and 17 and then east, north, and southeast to Davids Creek along the Davids Creek tributary valley seen in figures 16, 15, and 14. East Nishnabotna River valley headward erosion then beheaded and reversed the multiple south- and southeast-oriented flood flow routes to create the north- and northwest-oriented East Nishnabotna River tributary valley complex seen today.

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.