A geomorphic history based on topographic map evidence

Topographic map interpretation methods are used to determine Loup River-Platte River drainage divide area landform origins in the Nebraska Sand Hills region. Headwaters of major Loup River tributaries originate in the Nebraska Sand Hills region and flow in an east direction before turning to flow in a southeast direction east of the Sand Hills area. Based on study of drainage divides in surrounding regions the Nebraska Sand Hills area and adjacent regions are interpreted to have been crossed by massive southeast-oriented floods that converged in central and western Nebraska with large east-oriented floods from further west. Convergence of flood waters probably resulted in the ponding of flood waters and deposition of deltaic sediments. Headward erosion of what were then deep Platte River, South Loup River, Middle Loup River, and North Loup River valleys, in that sequence, drained the ponded flood waters. Subsequent eolian activity developed sand hills on the flood deposited deltaic sediments and probably significantly altered Loup River tributary routes, although the original valley sequence probably is preserved.

Preface:

• The purpose of this essay is to use topographic map interpretation methods to explore Loup River-Platte River drainage divide area landform origins in the Nebraska Sand Hills region, 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 the 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 Loup River-Platte River drainage divide area landform origins in the Nebraska Sand Hills region 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.

Loup River-Platte River drainage divide area in Nebraska Sand Hills region location map

Figure 1: Loup River-Platte River drainage divide area in Nebraska Sand Hills region location map (select and click on maps to enlarge). National Geographic Society map digitally presented using National Geographic Society TOPO software.

Figure 1 illustrates a large region in central Nebraska, with the Colorado northeast corner located in the figure 1 southwest corner and a sliver of southeast South Dakota located in the figure 1 northeast corner  just north of the Missouri River. The east-northeast oriented South Platte River flows from the Colorado northeast corner to North Platte, Nebraska where it joins the southeast-oriented North Platte River to form the Platte River. From North Platte the Platte River flows in a southeast direction to Kearney and then turns to flow in a northeast direction to the figure 1 east edge. East of the figure 1 map area the Platte River turns to flow south and east to join the Missouri River, which flows in a south-southeast direction along the Nebraska eastern border. Near the figure 1 north edge the east-oriented Niobrara River flows from the figure 1 west edge to join the Missouri River near Niobrara, Nebraska (located near the figure 1 northeast corner). Between the Niobrara River and the Platte River in the figure 1 west half is a large region labeled as the Sand Hills. Streams or rivers originating in this sand hills region from north to south include Snake River and Gordon Creek (which are both Niobrara River tributaries), the North Loup River and its tributary the Calamus River, the Middle Loup River and its tributary the Dismal River, and the South Loup River. Within the Sand Hills region itself drainage history reconstructions are difficult if not impossible because sand dunes obscure most drainage routes and also because wind deposited sand may have significantly altered drainage routes. However, drainage histories can be reconstructed for regions surrounding the Nebraska Sand Hills region, which suggest an immense southeast-oriented flood flowed into the region and converged with a massive east-oriented flood. Flood waters were probably derived from a rapidly melting thick North American ice sheet and flood flow routes from western Nebraska headward (or up flood) into north central Montana and are documented essays listed under Niobrara River, White River, Cheyenne River, Belle Fourche River, Little Missouri River, Powder River, Yellowstone River, Musselshell River, MT Missouri River, and Milk River (among others) on the sidebar category list.  Southeast-oriented flood waters flowed across the Sand Hills region into regions described in essays listed under Elkhorn River and Loup River on the sidebar category list. Sand hills in the Nebraska Sand Hills region are probably formed on flood deposited deltaic sediments, deposited where converging flood waters were temporarily ponded.

Loup River-Platte River drainage divide area in Nebraska Sand Hills region detailed location map

Figure 2: Loup River-Platte River drainage divide area in Nebraska Sand Hills region detailed location map. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 2 provides a slightly more detailed map of the Loup River-Platte River drainage divide area in the Nebraska Sand Hills region. Sheridan, Cherry, Keya Paha, Brown, Rock, Garden, Deuel, Keith, Lincoln, Grant, Arthur, Hooker, Mc Pherson, Thomas, Logan, Blaine, Loup, and Custer are Nebraska county names and the county boundaries area shown. The North Platte River flows from the figure 2 west edge across Garden County into Keith County and into Lincoln County, where it joins the east-northeast oriented South Platte River at North Platte. From North Platte the Platte River flows in southeast direction to the figure 2 south edge. The Niobrara River flows in a northeast direction in the figure 2 northwest quadrant in Sheridan County and western Cherry County and then in a southeast and northeast  direction near the figure 2 north center edge before turning to flow along the Keya Paha County southern border. Major Niobrara River tributaries in central Cherry County include east- and northeast-oriented Snake River and Gordon Creek, which are located south of the Cherry County Nebraska National Forest area and then turn to flow in northeast direction east of the National Forest area east boundary. The North Loup River originates south of Gordon Creek in central Cherry County and flows in an east, east-southeast, and southeast direction to the Blaine County northwest corner and then in southeast direction into Loup County and then to join the southeast-oriented Calamus River, which is a North Loup River tributary. The Calmus River originates in west-central Brown County and flows in a southeast direction to join the North Loup River. Several east-oriented branches of the Middle Loup River originate in southern Cherry County and along the Cherry County border with Grant and Hooker Counties and after combining flow in an east-southeast and southeast direction across Thomas County, the Blaine County southwest corner and the Custer County northeast corner to the figure 2 southeast corner. The Dismal River North and South Forks originate in southern Hooker County and the Dismal River flows across southern Thomas County before joining the Middle Loup River in Blaine County. The South Loup River originates as an east-oriented stream, but turns to flow in southeast direction to the figure 2 south edge (east half). All major Loup River tributaries are east-oriented in the Sand Hills region and turn to become southeast-oriented upon leaving the Sand Hills region. The northwest-southeast oriented drainage alignment seen in the region east of the Sand Hills region (in figure 2) is absent in the Sand Hills region, yet from other essays is known to exist in regions surrounding the Sand Hills region. The northwest-southeast oriented drainage alignment has been interpreted in other essays to have been formed by immense southeast-oriented floods that flowed across all of Nebraska, including the Sand Hills region. If that interpretation is correct major drainage routes in the Sand Hills region have since been probably altered by eolian activity, while secondary surface drainage routes have probably been completely obscured. However, major drainage routes probably still permit a rough reconstruction of the figure 2 drainage history, which began with headward erosion of the Platte River valley to capture southeast-oriented flood flow. Headward erosion of the South Loup River valley was next, until flood water to that actively eroding valley was beheaded by headward erosion of the Middle Loup River-Dismal River valley. And the sequence continues as illustrated and described in the more detailed maps below.

North Loup River-Middle Loup River drainage divide area

Figure 3: North Loup River-Middle Loup River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 3 provides an enlarged version of the figure 2 map to illustrate the North Loup River-Middle Loup River drainage divide area in southern Cherry County and northern Hooker and Thomas Counties. The entire drainage divide area is today a sand hill region similar to that seen in the topographic maps illustrated in figures 4 and 5 below. Surface drainage is generally limited to major drainage routes shown on figure 3, which means more detailed topographic maps have very limited evidence from which drainage histories can be reconstructed. Drainage routes across western and central Cherry County are generally east or east-southeast oriented and from north to south are Snake River, Boardman Creek, and Gordon Creek, all of which turn to flow in a northeast direction to the Niobrara River, which is located north of the figure 3 map area. South of Gordon Creek is the east-oriented Big Dry Valley and then the east-southeast oriented North Loup River. Big Creek is an east-oriented North Loup River tributary south of the North Loup River in central Cherry County. South of Big Creek is the Calf Creek valley, where surface drainage is absent for a considerable distance, although surface drainage is present at the western end and also in the Cherry County southeast corner where Calf Creek flows to join the North Loup River. The North Branch Middle Loup River is located south of the west end of the western Calf Creek surface drainage route and is joined by the Middle and South Branches before the Middle Loup River flows in an east-southeast direction across the Hooker County northeast corner and into Thomas County. Evidence for reconstructing drainage histories in this North Loup River-Middle Loup River drainage divide area is limited. However, with knowledge from other essays that the region was crossed by an immense southeast-oriented flood and that present day valleys eroded headward to capture the southeast-oriented flood waters the sequence of valley formation can probably be determined. Headward erosion of the Middle Loup River valley would have occurred first. Next headward erosion of the North-Loup River-Calf Creek valley would have beheaded flood flow to the newly eroded Middle Loup River valley. Headward erosion of the North Loup River-Big Creek valley and then the North Loup River valley would have next beheaded southeast-oriented flood flow to the newly eroded Calf Creek valley. Finally headward erosion of the Niobrara River-Gordon Creek valley would have beheaded southeast-oriented flood flow to the newly eroded North Loup River valley. It is probable subsequent eolian activity has significantly changed drainage routes in the Sand Hills region, although it probably has not altered the valley formation sequence, which can be easily confirmed using much better evidence in the region just east of the Sand Hills region.

North Loup River-Big Creek drainage divide area

Figure 4: North Loup River-Big Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 4 provides a topographic map view of the North Loup River-Big Creek drainage divide area seen in less detail in figure 3 above. The North Loup River flows from the figure 4 west edge just south of the northwest corner in an east-southeast direction to the figure 4 east center edge. Big Creek originates in the figure 4 west center area and flows in an east-southeast direction to the figure 4 east edge (near the figure 4 southeast corner). In the figure 4 southeast quadrant Horse Creek drains Allen Valley to Big Creek. Otherwise the figure 4 map area is devoid of surface drainage routes. The figure 4 map area hills all appear to be sand hills and their west-northwest to east-southeast alignment is probably related to wind direction and not to a previous drainage system. The North Loup River, Big Creek, and Horse Creek drainage routes all appear to have been influenced by the sand hill formation, meaning little or no figure 4 evidence can be used to reconstruction drainage histories. The presence of a large sand hill area is evidence that large quantities of sand were deposited in the Nebraska Sand Hills region. Further north in North Dakota and Manitoba sand hill areas are common on sandy deltas, which were deposited by melt water floods flowing into what are commonly considered to have once been glacial lakes. [The thick ice sheet that melted fast paradigm interprets what are commonly considered glacial lakes to have been formed by ponding of flood waters where huge glacial melt water rivers converged rather than by glacial damming, however the thick ice sheet that melted fast paradigm does not question the existence of bodies of ponded water in many of those glacial lake locations and nor does the paradigm question the deltaic deposits formed where large glacial melt water floods flowed into those ponded bodies of water]. Using the northern sand hill areas as analogies and flood route evidence supported by the hundreds of published essays (on this website) in this Missouri River drainage basin research project essay series, the Nebraska Sand Hills region is located in a position near where two major glacial melt water flood flow routes converged with considerable ponding of flood waters. Further, the Nebraska Sand Hills region is in a location where flood waters entering the ponded flood water area might have deposited significant sediment to produce large sandy deltas similar to sandy deltas developed where flood waters entered lakes or ponded water areas further to the north.

North Loup River-Calf Creek Valley drainage divide area

Figure 5: North Loup River-Calf Creek Valley drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 5 uses reduced size topographic maps to illustrate the North Loup River-Calf Creek valley drainage divide area. The North Loup River flows in an east direction in the figure 5 north center edge area and then turns to flow in a southeast direction to the figure 5 east center edge. The Middle Loup River is the east and southeast oriented river in the figure 5 southwest quadrant and flowing to the figure 5 south edge. Seneca is the town located in the Middle Loup River valley. Calf Creek (as a surface stream) is located in the figure 5 east center edge and flows east and northeast to join the North Loup River. West of the Calf Creek surface drainage area is the extensive Calf Creek Valley, which extends in a west to east direction across the figure 5 center area. The Calf Creek Valley appears to be one of the longer continuous figure 5 valleys without a surface drainage route, although other extensive figure 5 map area valleys can be identified. West of the figure 5 map area there is a surface drainage in what appears to be the Calf Creek Valley west end, suggesting the Calf Creek Valley may have originated as a surface drainage route. Study of the Calf Creek Valley on more detailed topographic maps suggests it would be very difficult for surface drainage to flow through the valley today. Figure 5a below provides a detailed topographic map of an eastern Calf Creek Valley segment seen in less detail in figure 5 above. Note how the Calf Creek Valley floor is uneven and contains no evidence of a surface stream channel. If the Calf Creek Valley originated as a surface drainage route eolian activity has since completely buried that drainage route with sand and from topographic map evidence alone it is impossible to determine whether the Calf Creek Valley as it exists today has much, if any, relationship to the original surface drainage route. The same problem exists with the present day surface drainage routes. Sand hill development has almost certainly altered drainage routes, and from topographic map evidence alone it is difficult or impossible to determine whether the present day routes are related to the pre-sand hill drainage routes.

Figure 5a: Detailed map of a Calf Creek valley segment. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Middle Loup River-Dismal River drainage divide area

Figure 6: Middle Loup River-Dismal River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 6 enlarges the Middle Loup River-Dismal River drainage divide area in Hooker, Thomas, and southern Cherry Counties seen previously in figure 2 above. The Middle Loup River originates in the figure 6 northwest quadrant in three branches, the northern two of which are located in southern Cherry County and the southern branch originates in southern Cherry County and then flows along the Cherry County-Hooker County border before joining the other two branches in southern Cherry County. Note how the east-oriented Calf Creek Valley is located north of the North Branch Middle Loup River. The Dismal River has a North and South Fork in western Hooker County and after these two forks join the Dismal River flows east into southern Thomas County and then in an east-southeast direction to the figure 6 east map edge and joins the Middle Loup River east of the figure 6 map area. Again, other than the streams shown, the Middle Loup River-Dismal River drainage divide area has no significant figure 6 surface drainage routes. Sand hills occupy the entire figure 6 map area and have obscured surface drainage routes important to using topographic map evidence for the purpose of reconstructing drainage histories. The size of the Nebraska Sand Hills region is significant. This region was not formed on a small delta developed by a small or modest sized flood moving into a small or modest size body of water. The body of water into which the flood waters moved had to be large and may have covered much of central and eastern Nebraska, if not a larger area. The flood event moving flood waters (and sediment) into that standing body of water must also have been immense and evidence of an immense flood moving from the northwest has been documented in other essays (see figure 1 discussion). Probably the body of water was drained by headward erosion of the deep Missouri River valley from what may have been an evolving south-oriented Mississippi River valley. Headward erosion of the Platte River valley and its tributary Loup River valley system may also have played an important role in draining ponded flood waters from eastern and central Nebraska. Once flood water was drained from the region and flood flow across the region ceased wind action began to create the sand hills seen today. Sand hill development probably obscured most of the flood eroded valleys and probably altered drainage routes of all surviving surface drainage routes.

North Branch Middle Loup River-South Branch Middle Loup River drainage divide area

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

Figure 7 provides a topographic map of the North Branch Middle Loup River-South Branch Middle Loup River drainage divide area seen in less detail in the figure 6 map area above. The North Branch Middle Loup River flows in a southeast and south-southeast direction in the figure 7 northwest quadrant and then in an east-southeast direction across the figure 7 north half. The Middle Loup River flows in an east direction across the figure 7 central area. The South Branch Middle Loup River flows in an east direction just north of the figure 7 south edge. Note how all streams are flowing in east-oriented valleys located between sand hills. The question can be asked, were the east-oriented valleys formed by the east-oriented streams or by east-oriented sand hills? Probably the valley origin is some combination of the two. Without east-oriented surface drainage routes the valleys would probably look like the Calf Creek Valley and might not even be continuous valleys. On the other hand, it is doubtful the valleys represent routes flood waters used when they flowed across the Nebraska Sand Hills region. Topographic map evidence alone in this figure 7 map area and also in all surrounding regions is not adequate to reconstruct pre-sand hills drainage routes. Such reconstructions must be done by using evidence from adjacent regions and then projecting evidence into the Nebraska Sand Hills region. Drainage histories can be reconstructed from all regions surrounding the Nebraska Sand Hills region, and those reconstructed drainage histories document immense southeast-oriented floods from the northwest. Essays illustrating and describing evidence along all major drainage divides extending from central and western Nebraska into north central Montana have been published and can be found under appropriate Missouri River tributary names on the sidebar category list. Essays listed under Loup River on the sidebar category list for regions east of the Sand Hills region document large southeast-oriented floods.

Dismal River-Platte River drainage divide area

Figure 8: Dismal River-Platte River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 8 provides an enlarged version of the figure 2 map for the Dismal River-Platte River drainage divide area. The Dismal River flows from Hooker County across southern Thomas County to Blaine County and joins the Middle Loup River east of the figure 8 map area. The southeast-oriented Middle Loup River can just barely be seen in the figure 8 northeast corner. The South Loup River originates in Logan County and after flowing east it turns to flow in a southeast direction to the figure 8 east edge (south half). The Platte River begins at the confluence of the North and South Platte Rivers near North Platte in the figure 8 south center edge area and flows in a southeast direction to the figure 8 south edge. Note the south-southeast oriented Platte River tributaries in the figure 8 southeast corner. Those tributaries are located on the southeast margin of the Sand Hills region and are not buried in sand. Also note the east-southeast and south-southeast oriented Birdwood Creek drainage routes in Mc Pherson County and northwest Lincoln County. The Birdwood Creek drainage system is the only significant surface drainage network flowing south to the Platte River valley from the Sand Hills region. Figure 9 below provides a topographic map of the northern Birdwood Creek drainage basin to illustrate how Birdwood Creek originates in the Sand Hills region. The south-southeast-oriented Birdwood Creek valley may be a relic a south-southeast oriented flood flow route that once crossed the figure 8 map region, although it may have been altered by eolian activity. Figure 8a below illustrates the Platte River-Republican River drainage divide area south of the figure 8 map area. The east-northeast oriented Republican River is located just south of the figure 8a map area and the southeast-oriented streams in Perkins, Chase, Hayes, and Frontier Counties are Republican River tributaries. Southeast-oriented drainage routes in the Platte River-Republican River drainage divide area were initiated by southeast-oriented flood waters to what was at that time a newly eroded Republican River valley (just prior to headward erosion of the Platte River valley). Headward erosion of the Platte River valley captured the southeast-oriented flood flow and beheaded all flood flow routes to the newly eroded Republican River valley. Note also the southeast-oriented South Platte River tributaries located in the figure 8a northwest corner. Those southeast-oriented tributaries provide evidence headward erosion of the North Platte River valley beheaded southeast-oriented flood flow routes to what was then the newly eroded South Platte River valley. Similar evidence can be found surrounding the Nebraska Sand Hills region, supporting the interpretation that sand in the Sand Hills region was deposited by an immense southeast-oriented flood. Figure 10 below illustrates why flood waters may have been ponded north of the Platte River valley, which might explain why sandy deltas were deposited in that location.

Figure 8a: Platte River-Republican River drainage divide area south of the figure 8 map area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Birdwood Creek drainage basin

Figure 9: Birdwood Creek drainage basin. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 9 provides a topographic map of the northern Birdwood Creek drainage basin area. North Birdwood Creek originates in the figure 9 north center area and flows in a southeast direction to join east-southeast oriented West Birdwood Creek. Birdwood Creek then flows in a south-southeast and south direction to the figure 9 south edge and then to the North Platte River (see figure 10 below). Note how the entire Birdwood Creek drainage basin is located within the Sand Hills region. However, a look at more detailed maps suggests sand deposits in the figure 9 map area are thinner than in areas further to the north. Figure 9a below provides a detailed map of the North Fork Birdwood Creek-West Fork Birdwood Creek confluence area. Note how additional intermittent surface drainage routes can be identified. While the figure 9a map evidence suggests the presence of sand hills, the evidence also suggests surface flow is able to keep at least some surface drainage routes open. At least some of these surface drainage routes may have originated when southeast-oriented flood waters flowed across the figure 9 map area to what was then the newly eroded Platte River valley. Headward erosion of the Middle Loup River-Dismal River valley to the north probably beheaded flood flow to what was then the actively eroding Birdwood Creek valley, although it is possible the flood flow to the Birdwood Creek valley was not beheaded until headward erosion of the Niobrara River valley captured all southeast-oriented flood flow across western Nebraska.

Figure 9a: Detailed of West Fork and North Fork Birdwood Creek confluence area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

North Platte River-South Platte River confluence area

Figure 10: North Platte River-South Platte River confluence area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 10 illustrates the North Platte River-South Platte River drainage divide area and the southern section of the Birdwood Creek drainage basin located south of the figure 9 map area (and includes overlap areas with figure 9). North Platte is the city located in the figure 10 southeast corner. The two parallel east-flowing rivers, which join just east of the figure 10 map area, are the North and South Platte Rivers. The North Platte River flows north of the city of North Platte while the South Platte River flows south of the city of North Platte. Birdwood Creek is the south-oriented North Platte River tributary located west of the figure 10 center. South of the South Platte River Sutherland Reservoir and the canals are components of the regional irrigation system. North of the North Platte River the figure 10 landscape is dominated by sand hills and surface drainage routes are rare. The North and South Platte Rivers here are flowing along the southern margin of what appears to be the region where sand deposits are thickest, although evidence for sand hills is definitely seen south of the North and South Platte Rivers. What is intriguing about the figure 10 map area is how the North and South Platte Rivers flow parallel to each other across the entire figure 10 map area. Grid squares on the map are one mile across and provide a measure of the distance these two river flow adjacent to each other without actually joining to becoming one river. In the western third of figure 10 the North and South Platte Rivers are located in separate valleys, but in the eastern two-thirds of figure 10 the two rivers are flowing parallel to each other in the same large valley. Other essays describing the Platte River valley downstream from the figure 10 map area noted a similar phenomena where major tributaries from the north enter the Platte River valley. For example, the Loup River-Platte River drainage divide area between Kearney and Columbus essay illustrates how the Loup River and Platte River flow parallel to each other on opposite sides of a large valley for a considerable distance before joining to become one river. The interpretation provided here for the phenomena is the Platte River valley was eroded headward to capture not only southeast-oriented flood flow, but also to capture flood waters from further to west. In other words, the North and South Platte River valleys are recording routes used by two different and converging large floods (or at least flood waters that had used different routes to reach the figure 10 map area). Flood water flowing northeast along the South Platte River valley route converged with flood waters flowing southeast along the North Platte River valley route. The volume of flood water arriving in the figure 10 map area from each of these two different flood routes was so great that flood waters from each route flowed parallel to each other on opposite sides of the large valley for considerable distances before being forced to merge by the introduction of additional southeast-oriented flood flow from the northwest. To properly appreciate this phenomena study the Platte River valley downstream from this figure 10 map area.

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.