A geomorphic history based on topographic map evidence

The Big Sioux River-Floyd River drainage divide area is located in northwest Iowa. Present day Big Sioux-Floyd River drainage divide area landforms include the Big Sioux River and Big Sioux River tributary valleys, Floyd River and Floyd River tributary valleys, and drainage divides between the various drainage routes. Shallow through valleys cross the drainage divides. Topographic map evidence suggests these landforms originated during an immense southeast-oriented flood, when headward erosion of the Floyd River valley and tributaries first captured the flood flow and diverted the water to the what was probably a newly eroded Missouri River valley and subsequently headward erosion of the Big Sioux River valley captured the southeast-oriented flood flow and diverted the water to the newly eroded Missouri River valley.

Preface:

• The purpose of this essay is to use topographic map interpretation methods to explore Big Sioux River-Floyd River drainage divide area landform origins in northwest 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 essays 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 Big Sioux River-Floyd River drainage divide area landform evidence in northwest Iowa will be regarded as evidence supporting the “thick ice sheet that melted fast” paradigm. This essay is included in the Missouri River drainage basin landform origins research project essay collection.

Big Sioux River-Floyd River drainage divide area location map

Figure 1: Big Sioux River-Floyd 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 location map for the Big Sioux River-Floyd River drainage divide area. The state of Iowa is located in the figure 1 southeast quadrant and Minnesota is located north of Iowa. South Dakota is the state occupying the northern three-fourths of the figure 1 west half and Nebraska is the state south of South Dakota. The Missouri River is the southeast-oriented river flowing along the South Dakota-Nebraska border and the Nebraska-Iowa border. The Big Sioux River flows south in eastern South Dakota from the figure 1 north edge to Brookings and Sioux Falls. At Sioux Falls the Big Sioux River flows east to Brandon, South Dakota and then flows south along the South Dakota-Iowa border to near Sioux City, Iowa, where the Big Sioux River joins the Missouri River. An important Big Sioux River tributary is the unlabeled Rock River, which flows south from Luverne, in the Minnesota southwest corner, into the Iowa northwest corner and flows through Rock Rapids, Doon, and Rock Valley, Iowa before joining the Big Sioux River near Hudson, South Dakota. The Floyd River is not labeled in figure 1, but is the south-southwest oriented stream in northwest Iowa flowing from near Sheldon to Alton, LeMars, and Merrill, before flowing to Sioux City, where it joins the Missouri River. The West Branch Floyd River begins near Boyden and joins the Floyd River near Merrill. The Big Sioux River-Floyd River drainage divide area illustrated and discussed in this essay is almost completely located in northwest Iowa. Essays describing nearby drainage divide areas include the  Rock River-Des Moines River drainage divide area essay in southwest Minnesota and the James River-Big Sioux River drainage divide area south of Mitchell and Sioux Falls, South Dakota essay, and can be found under Big Sioux River on the sidebar category list. The interpretation provided here is the Floyd River valley eroded headward to capture what was southeast-oriented flood water, which was subsequently captured by headward erosion of the Big Sioux River valley. More detailed maps shown below provide evidence to support the interpretation the Floyd and Big Sioux River valleys, and their tributary valleys, eroded headward to capture southeast-oriented flood flow. Flood water was probably derived from a rapidly melting thick North American ice sheet, and it is probable the Floyd River and Big Sioux River valleys originally eroded headward into what was a decaying ice sheet margin

Big Sioux River-Floyd River drainage divide area detailed location map

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

Figure 2 provides a somewhat more detailed map of the Big Sioux River-Floyd River drainage divide area.Lyon, Osceola, Sioux, O’Brien, Plymouth, and Cherokee are Iowa county names. Lincoln, Clay, and Union are South Dakota county names, and Dixon is a Nebraska county name. The Big Sioux River forms the eastern boundary for Lincoln and Union Counties and joins the southeast-oriented Missouri River near Sioux City, Iowa (located near the figure 2 south center edge). Northwest from Sioux City the Missouri River is the South Dakota-Nebraska border. The Floyd River originates in northwest O’Brien County and flows west to the Sioux County-O’Brien County border and then turns to flow south-southwest through Sioux and Plymouth Counties to join the Missouri River near Sioux City. Just north of the Floyd River headwaters is Otter Creek. Otter Creek flows in a south-southwest direction in Osceola County and at the Osceola County southwest corner turns west, southwest, and northwest to flow into Lyon County and to join the Little Rock River. The northwest-oriented Little Rock River then turns southwest to flow to the southwest-oriented Rock River, which joins the south-oriented Big Sioux River near Hudson, South Dakota. The West Branch Floyd River originates near Boyden (located in the Sioux County northeast corner) and flows in a south-southwest direction through Sioux County and into Plymouth County, where it joins the Floyd River near Merrill (located in the Plymouth County center). Note how west and northwest oriented Otter Creek is located immediately north of the West Branch Floyd River headwaters. Detailed maps illustrated below provide evidence the Floyd River, West Branch Floyd River, and other Floyd River tributary valleys eroded headward to capture southeast-oriented flood waters. The maps also provide evidence the Big Sioux, Rock River, and Rock River tributary valleys (including the Otter Creek valley) eroded headward to capture the southeast-oriented flood flow and in the process to behead flood flow routes to what was then the newly eroded Floyd River drainage basin. Maps at the scale shown in this essay do not provide evidence Sioux and Plymouth Counties were glaciated, although essays for other Big Sioux River drainage divide areas include maps providing such evidence. For that reason southeast-oriented flood water is interpreted as having been derived from a rapidly melting ice sheet north of the figure 2 map area.

Otter Creek-Floyd River drainage divide area near Ashton, Iowa

Figure 3: Otter Creek-Floyd River drainage divide area near Ashton, Iowa. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 3 illustrates the Otter Creek-Floyd River drainage divide area near Ashton, Iowa. The corner between Lyon and Osceola, Sioux and O’Brien Counties Iowa is located in the figure 3 southwest quadrant. Ashton is located near the figure 3 center and is in the Osceola County southwest corner. Southeast-oriented drainage in the figure 3 northeast corner flows to the Little Sioux River, which eventually flows to the Missouri River.Floyd River originates in the figure 3 southeast quadrant and flows south, northwest, west, and finally southwest to the figure 3 south center edge. The remarkable direction changes in the Floyd River headwaters area from south, to northwest, to southwest oriented is typical of the figure 1 drainage features that need to be explained. Also, note how the North Fork Floyd River begins as a northwest-oriented stream and then turns to flow in a southwest direction. Otter Creek flows south and southwest from the figure 3 north center edge to Ashton and then continues in a southwest and west direction to the figure 3 west edge. The southwest-oriented Otter Creek tributary west of Ashton is Rat Creek and west of Rat Creek is West Rat Creek. Note the northwest-oriented Otter Creek tributary near Cloverdale in the figure 3 northeast quadrant. That barbed tributary along with other northwest-oriented tributaries are also representative of the figure 3 drainage features that must be explained. How does a southwest-oriented stream develop a series of northwest-oriented tributaries? Probably the best explanation for these figure 3 drainage oddities (which really are quite common in the Missouri River drainage basin) is to interpret the figure 3 landscape to have been eroded during a large-scale southeast-oriented flood, where flood waters flowed across the entire figure 3 map area. The figure 3 map area was then eroded by headward erosion of southwest-oriented valleys. The figure 3 west half is lower than the east half, which suggests broad shallow valleys eroded north-northeast across the region as headward erosion of the present day deeper valleys was taking place. The Floyd River valley eroded northeast and then east from what was a broader valley to capture the southeast-oriented flood water. The northwest-oriented Floyd River headwaters valley segment was formed by a reversal of southeast-oriented flood flow on the northwest end of a beheaded flood flow channel. The south-oriented Floyd River headwaters valley segment is evidence the reversed flood flow captured south-oriented flood flow and diverted the water northwest to the newly eroded Floyd River valley. The same explanation applies to the North Fork Floyd River northwest-oriented valley segment. The Otter Creek valley eroded headward next and beheaded southeast-oriented flood flow to the newly eroded Floyd River valley. The northwest-oriented Otter Creek tributaries formed as flood water waters on the northwest ends of beheaded southeast-oriented flood flow channels were reversed to flow in a northwest direction to newly eroded and deeper Otter Creek valley.

Otter Creek-Floyd River drainage divide area near Sheldon, Iowa

Figure 4: Otter Creek-Floyd River drainage divide area near Sheldon, Iowa. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 4 illustrates the Otter Creek-Floyd River drainage divide area near Sheldon, Iowa and is located south and west from figure 3 and includes overlap areas with figure 3. The Floyd River is the southwest-oriented stream located in the figure 4 east half and flows along the Sheldon northwest city boundary. The Little Floyd River is the southwest-oriented Floyd River tributary seen in the figure 4 southeast corner. Otter Creek flows from Osceola County in the figure 4 northeast quadrant into Lyon County and then turns to flow northwest and southwest before turning to flow in a northwest direction to the figure 4 northwest corner. Note northwest-oriented Otter Creek tributaries to southwest-oriented Otter Creek valley segments. Those tributary valleys were formed by reversals of southeast-oriented flood flow on the northwest ends of beheaded flood flow routes. Southeast-oriented flood flow had been moving in what were probably anastomosing channels to the newly eroded Floyd River valley. Southeast-oriented Floyd River tributaries and tributaries to southwest-oriented Floyd River tributaries provide evidence the Floyd River valley (and southwest-oriented Floyd River tributary valleys) had previously eroded headward to capture the same southeast-oriented flood flow channels. The long northwest-oriented Otter Creek valley segment was initiated by a reversal of flood flow on a beheaded southeast-oriented flood flow channel, which was beheaded by headward erosion of the Rock River-Little Rock River valley. Rat Creek is the southwest-oriented Otter Creek tributary in the figure 4 north center area. Northwest of the figure 4 map area Otter Creek joins the Little Rock River, which first flows in a northwest direction and then turns to flow in a southwest direction to join the south-oriented Rock River (see figure 5 below). The southwest-oriented stream flowing from Boyden to the figure 4 southwest corner is the headwaters of the West Branch Floyd River. Note how the West Branch Floyd River valley alignment is the same as the Rat Creek alignment and the alignment of southwest-oriented Otter Creek valley segment immediately downstream from where Rat Creek joins Otter Creek. The Rat Creek-West Branch Floyd River similarity of alignments suggests a southwest-oriented valley did erode headward on that alignment prior to headward erosion of the Rock River-Little Rock River valley to the west. Headward erosion of the Rock River-Little Rock River valley then beheaded a major southeast-oriented flood flow to that newly eroded southwest-oriented valley. Reversed flow on the northwest end of that beheaded flood flow route captured southwest-oriented flow in the newly eroded southwest-oriented Rat Creek-West Branch Floyd River valley and diverted the flow to the even newer Rock River valley and beheaded flow to the present day West Branch Floyd River valley.

Little Rock River near Doon, Iowa

Figure 5: Little Rock River near Doon, Iowa. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 5 illustrates the Little Rock River near Doon, Iowa and is located west and slightly north of the figure 4 map area and includes overlap areas with figure 4. The Rock River is the stream flowing in a southwest direction from the figure 5 north center area to near Doon and then to the figure 5 south edge. Note southeast-oriented Rock River tributaries and short northwest-oriented tributaries from the southeast. Burr Oak Creek in the figure 5 south center flows in a northwest and west direction to join the south-southwest oriented Rock River in the figure 5 southwest quadrant. The Rock River tributaries provide evidence the southwest-oriented Rock River valley eroded headward to capture multiple southeast-oriented flood flow routes, such as might be found in a southeast-oriented anastomosing channel complex. The northwest-oriented tributaries were initiated by reversals of flood flow on the northwest ends of beheaded flood flow channels. George, Iowa is the town located along the figure 5 east edge in the figure 5 northeast quadrant. The Little Rock River flows in a southwest direction into WHEELER township where it joins northwest-oriented Otter Creek and flows in a northwest direction before turning to flow in a southwest direction to join the Rock River near Doon. The northwest-oriented Little Rock River-Otter Creek valley segment was initiated by a reversal of flood flow on a flood flow channel beheaded by headward erosion of the southwest-oriented Little Rock River valley segment (from the Rock River valley). As noted in the figure 4 discussion reversed flood flow in the northwest-oriented Otter Creek valley segment beheaded what was probably a newly eroded southwest-oriented valley on the Rat Creek-West Branch Floyd River alignment and diverted flood water from that valley to the even newer Little Rock River valley. A similar capture probably took place in figure 5 where the reversed flow on the northwest-oriented Little Rock River valley segment captured flow from a southwest-oriented valley that is now the southwest-oriented Little Rock River valley segment near George, Iowa. Note how that southwest-oriented Little Rock River valley segment is on the same alignment as an unnamed southwest-oriented Burr Oak Creek tributary located in the figure 5 south center area. Elevation of the Little Rock River-Burr Oak Creek drainage divide provides evidence of how much regional flood water erosion took place following the capture.

Rock River-West Branch Floyd River drainage divide area south of Rock Valley

Figure 6: Rock River-West Branch Floyd River drainage divide area south of Rock Valley, Iowa. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 6 illustrates the Rock River-West Branch Floyd River drainage divide area south of the figure 5 map area and there is a small gap between figures 5 and 6. The southwest-oriented Rock River is located in the figure 6 northwest quadrant and west center area and flows from Rock Valley, Iowa to the figure 6 west edge. Note northwest-oriented Rock River tributaries from the southeast. Dry Creek is the southwest-oriented Rock River tributary flowing through PLATO township in the figure 6 center area to join the Rock River just west of the figure 6 west edge. Note the presence of southeast oriented Dry Creek tributaries (in addition to several southwest-oriented tributaries) and northwest-oriented Dry Creek tributaries. Sixmile Creek is the southwest-oriented Rock River tributary located southeast of Dry Creek. Note again the southeast oriented Sixmile Creek tributaries and northwest-oriented Sixmile Creek tributaries. The southwest-oriented West Branch Floyd River is located east of Sioux Center in the figure 6 southeast quadrant and unlike the southwest-oriented valleys further east flows to the Floyd River rather than the Big Sioux River. Again, note the presence of southeast and northwest oriented tributaries. The figure 6 drainage history can be explained in the context of a large southeast-oriented flood moving across the entire figure 6 map area, probably in a large-scale southeast-oriented anastomosing channel complex. Southwest-oriented valleys then eroded headward into the figure 6 map area to capture the southeast-oriented flood waters, with the West Branch Floyd River valley eroding headward before the Sixmile Creek valley, which eroded headward before the Dry Creek valley, which eroded headward before the Rock River valley. Northwest-oriented tributary valleys were eroded by reversals of flood flow on the northwest ends of beheaded flood flow channels. Because major valleys eroded headward they captured and beheaded flood flow channels one channel at a time. Also, because flood flow channels were anastomosing (or interconnected) reversed flood flow in one channel could often capture yet to be reversed flood flow in an adjacent channel. Such captures of yet to be reversed flood flow helped erode the northwest-oriented tributary valleys.

Big Sioux River-West Branch Floyd River drainage divide area south of Hawarden, Iowa

Figure 7: Big Sioux River-West Branch Floyd River drainage divide area south of Hawarden, Iowa. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 7 illustrates the Big Sioux River-West Branch Floyd River drainage divide area south of the figure 6 map area and there is a small gap between figure 6 and 7. Hawarden, Iowa is the city located in the figure 7 northwest corner. The Big Sioux River flows south and south-southwest from Hawarden to the figure 7 southeast quadrant and west edge. Sixmile Creek is the south-southwest oriented Big Sioux River tributary east of Hawarden and joining the Big Sioux River near Chatsworth, just north of the Sioux County-Plymouth County line. Indian Creek is the west, southwest and west oriented Big Sioux River tributary flowing from east of Ireton (in the figure 7 north center) to join the Big Sioux River just south of the Sioux County-Plymouth County line. The West Branch Floyd River flows south just west of the figure 7 east edge. Mink Creek is a south-southeast oriented West Branch Floyd River tributary flowing from near Craig (near the northwest corner of the figure 7 southeast quadrant) to the figure 7 south edge. While southwest and south-southwest oriented drainage routes dominate the figure 7 map area a close look does reveal the presence of southeast and northwest oriented tributaries to the larger southwest and south-southwest oriented valleys. The Figure 7 drainage history is similar to the drainage histories determined for regions further north. A large south-oriented flood moved across the figure 7 map area, probably in a large-scale southeast-oriented anastomosing channel complex. Headward erosion of the West Branch Floyd River captured the flood water and diverted the water south to the Floyd River valley. Southeast-oriented tributary valleys eroded headward along southeast-oriented flood flow routes. Headward erosion of the Big Sioux River-Indian Creek valley next captured the southeast-oriented flood flow and beheaded flood flow channels to the newly eroded West Branch Floyd River valley. Next headward erosion of the Big Sioux River-Sixmile Creek valley captured the southeast-oriented flood flow and beheaded flood flow routes to the Indian Creek valley. Finally continued headward erosion of the Big Sioux River valley captured the southeast-oriented flood flow and beheaded flow routes to the Sixmile Creek valley.

Big Sioux River-West Branch Floyd River drainage divide area northwest of Merrill, Iowa

Figure 8: Big Sioux River-West Branch Floyd River drainage divide area northwest of Merrill, Iowa. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 8 illustrates the Big Sioux River-West Branch Floyd River drainage divide area northwest of Merrill, Iowa and south of the figure 7 map area and includes overlap areas with figure 7. The south-southwest oriented Big Sioux River is located in the figure 8 west half. Indian Creek is the southwest and west oriented Big Sioux River tributary near the figure 8 north edge. Merrill, Iowa is located in the figure 8 southeast corner. The south-southwest oriented stream at Merrill is the Floyd River and the south-southeast oriented stream joining the Floyd River near Merrill is the West Branch Floyd River. Broken Kettle Creek is the south-southwest oriented stream flowing from Ruble (in the figure 8 center) to the figure 8 south edge. At the scale shown in figure 8 evidence for southeast-oriented and northwest-oriented tributaries to the major south-southwest and south-southeast oriented streams and rivers is not as apparent as in regions further to the north. However, a close look at figure 8 reveals headward erosion of the various major stream and river valleys has captured southeast-oriented flood flow. Further there is evidence of short northwest-oriented tributary valleys, suggesting headward erosion of the south-southwest oriented valleys beheaded southeast-oriented flood flow channels. As previously described flood waters on the northwest ends of the beheaded flood flow routes reversed flow direction to erode the northwest-oriented tributary valleys. The figure 8 drainage history determined from this evidence is consistent with drainage histories for figures seen further to the north. Southeast-oriented flood flow moved across the entire figure 8 map area, probably in a large-scale southeast-oriented anastomosing channel complex. Headward erosion of the Floyd River-West Branch Floyd River valley captured this southeast-oriented flood flow and diverted the water in a south-southwest direction to the Missouri River valley further to the south (see figure 10). Headward erosion of the Broken Kettle Creek valley next captured the flood water and beheaded flood flow channels to the newly eroded Floyd River valley. Headward erosion of the Big Sioux River-Indian Creek valley next captured the flood flow and beheaded flood flow routes to the newly eroded Broken Kettle Creek and West Branch Floyd River valleys

Big Sioux River-Floyd River drainage divide area southwest of Merrill, Iowa

Figure 9: Big Sioux River-Floyd River drainage divide area southwest of Merrill, Iowa. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 9 illustrates the Big Sioux River-Floyd River drainage divide southwest of Merrill and south of the figure 8 map and includes overlap areas with figure 8. Merrill is located in the figure 9 northeast corner. The Floyd River flows in south-southwest direction from Merrill to the figure 9 south edge. The Big Sioux River flows in a south-southeast direction in the figure 9 southwest corner. Broken Kettle Creek is the south-southwest oriented Big Sioux River tributary flowing through Millnerville in the figure 9 northwest quadrant. Perry Creek and West Branch Perry Creek are the south-oriented streams flowing to the figure 9 south center edge area and south of figure 9 Perry Creek flows directly to the Missouri River valley (see figure 10 below). Note how the south-southwest oriented Floyd River valley has both southeast-oriented tributaries from the west and northwest-oriented tributaries from the east. These tributaries provide evidence the Floyd River valley eroded headward to capture multiple southeast-oriented flood flow routes such as might be found in a southeast-oriented anastomosing channel complex. Figure 9 detail does not show evidence of northwest-oriented Perry Creek tributary valleys, although such tributary valleys are seen on detailed topographic maps. Figure 9a below provides a more detailed map of the Perry Creek valley and shows southeast-oriented and northwest-oriented Perry Creek tributary valleys. Also note on figure 9a headwaters of southeast-oriented Floyd River tributaries and through valleys linking southeast-oriented Floyd River tributary valleys with the northwest-oriented Perry Creek tributary valleys. Southeast and northwest oriented Perry Creek tributary valleys and the through valleys linking northwest-oriented Perry Creek tributary valleys with southeast-oriented Floyd River tributaries provide evidence the Perry Creek valley eroded headward to capture multiple southeast-oriented flood flow channels and beheaded flood flow moving to what was then the newly eroded Floyd River valley. Headward erosion of the Broken Kettle Creek valley next captured the southeast-oriented flood flow and beheaded flood flow to what was then the newly eroded Perry Creek valley. Finally headward erosion of the Big Sioux River valley beheaded southeast-oriented flood flow to the Broken Kettle Creek and Perry Creek valleys.

Figure 9a: Detailed map of Perry Creek valley showing southeast and northwest oriented tributary valleys. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Big Sioux River-Floyd River drainage divide area near Sioux City, Iowa

Figure 10: Big Sioux River-Floyd River drainage divide area near Sioux City, Iowa. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 10 illustrates the Big Sioux River-Floyd River drainage divide area near Sioux City, Iowa and is located south of the figure 9 map area and includes overlap areas with figure 9. The Missouri River is the large river in the figure 10 southwest quadrant and south center area. Floyd River flows south-southwest from the figure 10 northeast corner to join the Missouri River at Sioux City, Iowa. The Big Sioux River flows south-southeast along the east edge of the lowland area located in the figure 10 west half and joins the Missouri River just west of Sioux City. Perry Creek flows south-southwest in the highland region between the Big Sioux River and the Floyd River valleys to Sioux City, Iowa and flows through the city area to join the Missouri River. Note how the Floyd River valley has northwest oriented and south-east-oriented tributary valleys providing evidence the Floyd River valley eroded headward from what was then the newly eroded Missouri River to capture southeast-oriented flood flow. Again, more detailed topographic maps  show Perry Creek tributaries better than figure 10 and figure 10a below provides a more detailed map of the Perry Creek-Floyd River drainage divide area just north of Sioux City. The south-southwest oriented Floyd River is located in the figure 10a southeast corner. Note the multiple southeast-oriented tributaries to the Floyd River. Perry Creek flows from the figure 10a north center edge in a south-southwest and south direction to the figure 10a south edge. Note multiple northwest oriented Perry Creek tributaries from the east. Also note how those northwest oriented Perry Creek tributaries are linked by shallow through valleys (across the ridge that now serves as the Perry Creek-Floyd River drainage divide) to headwaters of southeast-oriented Floyd River tributaries. Further, note the southeast-oriented Perry Creek tributaries. The northwest- and southeast-oriented tributaries and the through valleys linking those tributary valleys provide evidence headward erosion of the Perry Creek valley beheaded multiple southeast-oriented flood flow routes to what was then the newly eroded Floyd River valley. Referring back to figure 10 above, headward erosion of the Big Sioux River next beheaded southeast-oriented flood flow routes to the Perry Creek valley. Finally southeast-oriented flood flow from west and northwest of the Big Sioux River valley seen in figure 10 eroded the lowland west of the Big Sioux River valley.

Figure 10a: Detailed map of Perry Creek-Floyd River drainage divide north of Sioux City, Iowa. 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.