Topographic Map Analysis of the North Platte River-South Platte River Drainage Divide Area, Western Larimer County, Colorado, USA

The United States Supreme Court settled legal disputes concerning four different Larimer County (Colorado) locations where water is moved by gravity across the high elevation North Platte-South Platte River drainage divide, which begins as a triple drainage divide with the Colorado River at Thunder Mountain (on the east-west continental divide and near Colorado River headwaters) and proceeds in roughly a north and northeast direction across deep mountain passes and other low points (divide crossings) first as the Michigan River (in the North Platte watershed)-Cache la Poudre River (in the South Platte watershed) drainage divide and then as the Laramie River (in the North Platte watershed)-Cache la Poudre River drainage divide. The mountain passes and nearby valley and drainage route orientations and other unusual erosional features can be explained if enormous and prolonged volumes of south-oriented water moved along today’s north-oriented North Platte and Laramie River alignments into what must have been a rising mountain region to reach south-oriented Colorado River headwaters. Mountain uplift in time forced a flow reversal in the Laramie River valley while flow continued in a south direction along the North Platte River alignment only to be forced to flow around the Medicine Bow Mountains south end and then to flow northward in the Laramie River valley and later to be captured by headward erosion of the east-oriented Cache la Poudre River-Joe Wright Creek valley (aided by a steeper gradient and less resistant bedrock). Continued uplift next reversed flow on the North Platte River alignment to create drainage routes seen today. While explaining Larimer County North Platte-South Platte drainage divide area topographic map drainage system and erosional landform evidence this interpretation requires a completely different Cenozoic history than the geologic history geologists usually describe.


Statement of the Problem
The United States Supreme Court settled two legal disputes over the movement of water by gravity across the Larimer County (Colorado) North Platte-South Platte River drainage divide, yet the published geomorphology literature does not explain how that hard fought-over drainage divide came to be. How does a drainage divide between two divergent tributaries to what is now a major river originate? The North and South Platte Rivers today originate on either side of the south-and southwest-oriented Colorado River headwaters (flowing to the Pacific Ocean) and flow along completely different and divergent routes before eventually joining in western Nebraska to form the east-oriented Platte River (which joins the Missouri River with its water eventually reaching the Gulf of Mexico). Today the North Platte River originates in north central Colorado (see figure 1) as a north-northwest oriented river and after crossing the state line flows northward into central Wyoming where it makes a turn to flow in an east and then southeast direction to western Nebraska and to join the South Platte River. Most South Platte River headwaters and tributaries flow from the Colorado Front Range in southeast and east directions before joining the north-northeast oriented South Platte River, which flows through mountain canyons and then onto the plains before turning in an east and finally in a northeast direction to enter western Nebraska where it joins the southeast-oriented North Platte River.
The western Larimer County North Platte-South Platte River drainage divide origin has never been satisfactorily explained. In fact, geomorphologists have failed to explain how most United States drainage divides originated including the North American east-west continental divide. The United States Geological Survey (USGS) has published detailed topographic maps on which all evidence needed to interpret drainage divide origins is shown, yet when topographic map interpretation techniques are used drainage divide origins obtained are not consistent with accepted regional geologic histories. Those regional geologic histories were developed by following accepted Cenozoic history paradigm rules and assumptions, which raises the question, is the problem with the topographic maps, the topographic map interpretation techniques, or the Cenozoic history paradigm? The topographic maps represent a huge collection of mostly yet to be interpreted drainage system evidence and were prepared according to extremely high standards so the problem is probably not with the maps. Topographic map interpretation techniques are based on basic geologic principles such as the principle of cross cutting relationships and are unlikely to be causing the problem. If the maps and the map interpretation techniques are not causing the problem then the problem must be arising from a Cenozoic history paradigm flaw. This paper will determine whether topographic map interpretation techniques, without the constraint of accepted Cenozoic history paradigm rules and assumptions, can determine an internally consistent North Platte-South Platte River headwaters drainage divide origin and whether that origin is consistent with what similar topographic map interpretation techniques have determined for other regional drainage divides.

Geographic Setting
Larimer County is located in northcentral Colorado just south of the Wyoming state line with Jackson County located immediately to the west. The Larimer-Jackson County line follows the Medicine Bow Mountains crest (see figures 1 and 2) from the state line southward to Cameron Pass and then southward along the Never Summer Range crest. To the west in Jackson County is North Park, a mountain enclosed structural basin where the north-oriented North Platte River begins. The west -and northwest-oriented Michigan River originates just south of Cameron Pass, which in figure 2 is between the Michigan River (M on figure 2) and Joe Wright Creek (J on figure 2) headwaters. The Michigan River flows to the North Platte River, which then flows in a north direction to central Wyoming where the North Platte River turns in an east and then southeast direction to join the South Platte River with their water eventually reaching the Gulf of Mexico. Immediately south of Cameron Pass and the Michigan River headwaters are south-oriented Colorado River headwaters (with water flowing to the Pacific Ocean) while north of Chambers Lake (1 on figure 2) are north-oriented Laramie River headwaters. The Laramie River flows into the Laramie Basin and where the river turns in an east and northeast direction to cross the Laramie Range (see figure 1) before joining the North Platte River. Joe Wright Creek flows in a north-northeast direction from Cameron Pass to Chambers Lake and then joins the north-and east-oriented Cache la Poudre River, which is one of many east-oriented streams flowing from the high Colorado mountains to the north-northeast, east, and northeast oriented South Platte River. The Laramie Mountains are located north of the east-oriented Cache la Poudre River with the Mummy Range in the Colorado Front Range located immediately to the south. Today, a three-way drainage divide is located at Thunder Mountain, where the North Platte-South Platte drainage divide's southwest end meets the Colorado River drainage basin. For a short distance the North Platte-South Platte drainage divide is between the Cache la Poudre River and the Michigan River drainage basins, but for most of its distance in Larimer County the drainage divide is between the Cache la Poudre River and the Laramie River drainage basins. Also, for a distance the North Platte River-South Platte River drainage divide follows high mountain ridges including the crest of the southern Medicine Bow Mountains, before turning in an eastward direction to cross the north-oriented Laramie River valley head at Chambers Lake and then proceeding in a north-northeast direction across less well-defined uplands in the southern Laramie Mountains. Poudre River (P). The red number 1 locates Chambers Lake and the red number 2 locates Sand Creek Pass

Previous Work
Detailed topographic maps available at the USGS National Map website represent this paper's most significant previous work. The maps show drainage routes, drainage divides, through valleys (or divide crossings including mountain passes), drainage orientations, barbed tributaries, and other features required to determine a region's drainage history. Most information, while available on topographic maps since the mid 20 th century, has not been used to reconstruct western Larimer County geomorphic history. For example, Harris (1963) in a paper titled "Geomorphology of Larimer County, Colorado" does not mention topographic map interpretation nor does he discuss the North Platte-South Platte drainage divide or the mountain passes and unusual features such as abrupt drainage direction changes and numerous barbed tributaries. Topographic map interpretation is rarely mentioned in more recent regional geologic history papers including an Anderson et al paper (2006) with a title starting with the words "Facing Reality" in which aerial photos and digital elevation models are discussed, but in which topographic map interpretation is not mentioned.
The first workers to recognize unusual Larimer County (Colorado) North and South Platte River drainage divide features were not geomorphologists, but irrigators seeking additional water sources. Laflin (2005, p.26-27)  While courts discussed water movements across the North Platte-South Platte drainage divide geomorphologists focused their attention on high-level erosion surfaces. William Morris Davis (1911) first described what he named the "Highland peneplain of the Front Range." Researchers soon saw more erosion surfaces with Lee (1922) and others describing two such surfaces while Van Tuyl and Lovering (1935) argued for five erosion surfaces. Not only did workers see different numbers of erosion surfaces, but workers could not agree on the erosion surface ages. For example, Mears (1993, p. 609) observes, "A late Eocene age for the prominent sub-summit surface is the doctrine for most geologists in Colorado. However, dissenters in Wyoming consider the surface late Miocene." As a Colorado geologist Bolyard (1997, p. 125 Blackstone (1996) in a report on the structural geology of the Laramie Mountains. More recently Wohl (2008) published a report describing bedrock jointing in Cache la Poudre drainage basin areas downstream from this paper's study region.
Using topographic map evidence and topographic map interpretation techniques Clausen (2020a) suggested high mountain passes now crossing the east-west continental divide between the North and South Platte River headwaters and the Colorado River headwaters had been eroded by diverging and converging flood flow channels moving water southward from what is now the north-oriented North Platte River drainage basin to what are now southeast-and east-oriented South Platte River headwaters while the south-oriented Colorado River valley eroded headward into a rising mountain region. This interpretation requires a different Cenozoic history than is accepted, however it explains topographic map evidence not only in the Colorado River headwaters area, but also along the North Platte River drainage route. For example, headward erosion of the southeast-oriented North Platte River valley along the Laramie Range northeast flank is described by Clausen (2019) as beheading and reversing multiple south-oriented flood flow channels. Along the southcentral Wyoming Great Divide Basin eastern margin Clausen (2020b) describes evidence for south-oriented drainage on what is now the north-oriented North Platte River alignment. Turning to the Laramie River drainage basin Clausen (2018) describes an anastomosing canyon complex crossing the Laramie Mountains and leading to the Goshen Hole basin. Further, Clausen (2020c) describes how a large east-oriented flood-formed anastomosing channel complex heading along the Laramie Mountains crest ridge was systematically captured by headward erosion of south-oriented valleys from an actively eroding northeast-oriented South Platte River valley and by headward erosion of a north-oriented valley from an actively eroding southeast-oriented North Platte River valley. These Clausen papers all used detailed topographic maps and topographic map interpretation techniques to describe drainage divide origins, but describe a different Cenozoic geologic history than the accepted paradigm describes.

Research Method
More than 40 years ago the author of this paper, Clausen, began using detailed topographic maps to determine where continental ice sheet meltwater had once flowed. At first those efforts were unsuccessful because accepted regional glacial histories do not satisfactorily explain the detailed topographic map evidence. Finally, in 1999 a decision was made to do what became a three-year a systematic study of drainage divide evidence found on the thousands of detailed topographic maps covering the entire Missouri River drainage basin. The goal of that intensive study was to determine how drainage divides within and surrounding the Missouri River drainage basin originated. Results suggested the need for a new regional Cenozoic history paradigm defined by a rule requiring most Missouri River drainage basin valleys large enough to be shown on the detailed maps to have eroded headward across large south-and southeast-oriented floods. That rule forces recognition of a large North American continental ice sheet (in the recognized North American continental ice sheet location), which deeply eroded underlying bedrock (deep continental ice sheet erosion is not usually recognized in many glaciated areas) and which weighed enough to raise adjacent mountain ranges and continental areas as massive south-oriented meltwater floods flowed across the rising regions (accepted geologic histories do not usually link regional and mountain uplift to continental ice sheets). The initial 1999-2001 study was done using hard copy topographic maps and the study was repeated beginning in 2011 and ending in 2013 using National Geographic TOPO maps and software.
This investigation used detailed topographic maps available at the USGS National Map website and the cited geologic maps to follow the North Platte-South Platte River drainage divide across Larimer County. Low points located along the drainage divide were interpreted to be evidence water had once flowed across the drainage divide. Also noted were unusual nearby erosional landform and drainage features, such as abrupt drainage direction changes, barbed tributaries, water-eroded diverging and converging channels, and bedrock unit ages that might help date when deep erosion began.
Water was assumed to have eroded all existing stream and river valleys (including through valleys and mountain passes), although subsequent alpine glaciation was recognized to have probably deepened and otherwise altered the higher elevation valleys. Observed evidence was treated as a solvable puzzle in which it was necessary to move water across what was probably a rising mountain region in a way that would result in the present-day set of valleys, abandoned valleys (including through valleys and mountain passes), and drainage divides. Water movements and mountain and regional uplifts as suggested in Clausen (2020a) were tested and found consistent with observed map evidence. Those water movement and regional and mountain uplift interpretations were further tested for consistency with water movements and uplifts suggested in other cited and uncited Clausen papers. Faulting along what is mapped as a concealed thrust fault extending through Cameron Pass explains rock type differences on either side of the pass. However, Cameron Pass is also a 500-to 600-meter-deep erosional landform and had to be eroded by large volumes of water that flowed across what is now the North Platte-South Platte drainage divide.

Thunder Mountain to Diamond Peaks
To better understand the drainage divide history, we need to look at a short stretch of the North Platte-Colorado River drainage divide. Immediately to the southwest of Thunder Mountain (T) is Lulu Mountain (elevation 3727 meters) and then Thunder Pass (elevation 3454 meters) with the drainage divide then rising in a south direction to cross more high peaks. The Michigan River originates near Thunder Pass and flows in a north direction before turning in a west direction so as to create an enclosed Michigan River headwaters basin, which today drains in a west direction, but which has three significant abandoned outlets or inlets (Cameron Pass, the unnamed pass at number 1, and Thunder Pass). Today an irrigation ditch begins just south of the letter M and diverts water from north-oriented Michigan River tributaries and headwaters (in the North Platte River drainage basin) in a large semi-circle along the enclosed Michigan River headwaters basin wall to cross Cameron Pass and to reach Joe Wright Creek (in the South Platte River drainage basin). While the region was subsequently glaciated water almost certainly eroded the valleys and abandoned valleys (including the mountain passes). If water flowed through the two high elevation mountain passes from the Platte drainage basin to the Colorado River, the Michigan River valley and the Cameron Pass floors at that time must have been at least as high as the two mountain pass floors, which are now approximately 300 meters higher than the Cameron Pass floor (if the Cameron Pass floor and/or Michigan River valley floor had been lower, no water could have flowed from the Michigan River headwaters basin to the Colorado River.) The next question is how did enough water get into the Michigan River headwaters basin so as carve two 300-meter deep diverging channels into resistant bedrock (i.e. upper Oligocene igneous rock)? One possibility is large volumes of water flowed in a south direction from the now north-oriented Laramie River drainage basin and then across Cameron Pass, another possibility is large volumes of water flowed in a south direction along the now north-oriented North Platte River drainage route and then in a southeast and east direction along the Michigan River alignment, and a third possibility is large and prolonged volumes of water initially flowed in a south direction on the now north-oriented North Platte River and Laramie River alignments, but flow on the Laramie River alignment was reversed while south-oriented flow continued on the North Platte River alignment. However, each possibility requires major mountain uplift since upper Oligocene time to create drainage routes seen today.
Today the North Platte River flows in a north direction into central Wyoming (see figure 1) where it turns to flow in an east and then southeast direction into western Nebraska. The Laramie River joins the southeast-oriented North Platte River in western Nebraska and it is difficult to imagine large volumes of south-oriented water in the Laramie River while water in the North Platte River headwaters area was flowing in a north direction, so the first possibility needs to be rejected. It is possible large volumes of water flowed in a south direction on what are now the north-oriented Laramie and North Platte River alignments and as mountain uplift occurred Laramie River flow was reversed (to flow in a north direction to reach the downstream southeast-oriented North Platte River) while south-oriented flow continued on the now north-oriented North Platte River alignment. Cameron Pass is approximately 300 meters deeper than Thunder Pass and the unnamed pass between Thunder Mountain and Iron Mountain, which suggests south-oriented water on the North Platte River alignment would have made a U-turn in what is now the Michigan River headwaters basin to then flow through Cameron Pass so as to reach the then north-oriented Joe Wright Creek and Laramie River valleys. Continued regional uplift would have eventually reversed flow on what is now the north-oriented North Platte River alignment, which would have ended flow across Cameron Pass and created the west and northwest-oriented Michigan River drainage route. After following the Medicine Bow Range crest northward from Diamond Peaks and Montgomery Pass to Clark Peak the North Platte-South Platte drainage divide leaves the crest ridge and crosses an unnamed pass (elevation about 3363 meters) near Blue Lake and then rises to the Cameron Peak top (elevation 3696 meters). The unnamed pass links northeast-oriented West Branch Laramie River with southeast-and east-oriented Fall Creek flowing to Chambers Lake (and Joe Wright Creek) and may have been eroded by south-oriented water diverging from the Laramie River alignment to flow on a route defined by the north-northeast oriented West Branch Laramie River and southeast-and east-oriented Fall Creek alignments to reach the Cache la Poudre valley. An irrigation ditch today leads from the West Branch Laramie River along the Cameron Peak eastern slope to Joe Wright Creek at Chambers Lake. From Cameron Peak the North Platte-South Platte drainage divide descends to the Chambers Lake north shore (elevation 2790 meters). Harris (1963) describes Chambers Lake as impounded behind glacial moraines. Today Chambers Lake drains to north-northeast oriented Joe Wright Creek, which flows to a north-oriented Cache la Poudre River segment (in the South Platte watershed) and it is probable that when the Blue Lake pass (between Clark and Cameron Peaks) was eroded water in the now north-oriented Cache la Poudre headwaters valley was flowing in a south direction to reach an actively eroding south-oriented Colorado River headwaters valley. Workman et al (2018, p. 5) state the Laramie River (in the North Platte watershed) begins to the north of the lake's northernmost point and "flows in a graben formed in a structural syncline." Bolyard (1997, p. 131) notes an "obvious north-south lineament approximately 40 miles (54 km) long is formed by the upper reaches of the Colorado and Laramie Rivers".

Chambers Lake to Green Ridge Area
The distance from Chambers Lake at the Laramie River headwaters valley head to the north-oriented Cache la Poudre River valley is about three kilometers and abandoned valleys now cross the North Platte-South Platte drainage divide. The North Platte-South Platte drainage divide continues northward on the narrow upland between the north-oriented Laramie River to the west and north-oriented Joe Wright Creek and Cache la Poudre River to the east until the Tunnel Creek through valley is reached. Tunnel Creek drains that east-oriented valley to the Cache la Poudre River valley, which turns in an east and then northeast direction (see figure 4). The Laramie River at the west end of that through valley is roughly 160 meters higher than the Cache la Poudre River and the 3-km long Laramie-Poudre Tunnel extends under the through valley (which at the drainage divide is almost 300 meters higher than the Laramie River) and diverts Laramie River water to the Cache la Poudre River. Northward from the Laramie-Poudre Tunnel the North Platte-South Platte drainage divide follows Green Ridge, the crest of which is approximately equidistant from the north-oriented Laramie River and the northeast-oriented Cache la Poudre valley until the drainage divide crosses a shallow upland valley between Nunn Creek (west) and south-and southeast-oriented Roaring Creek (east).
Roaring Creek is one of several south-oriented tributaries flowing to the Cache la Poudre River near where the Cache la Poudre River turns abruptly from flowing in a northeast direction to flow in a southeast and then east direction. The Cache la Poudre River direction change suggests a capture took place with headward erosion of a deep southeast-and east-oriented Cache la Poudre River valley capturing north-and northeast-oriented Cache la Poudre River drainage. However south-oriented tributaries flowing to the Cache la Poudre River suggest headward erosion of the Cache la Poudre River valley also captured multiple south-oriented drainage routes. How can this conflicting evidence be reconciled? Clausen (2020a) suggests south-oriented flood flow moving in the now north-oriented Cache la Poudre River headwaters valley flowed to and across La Poudre Pass to erode the Colorado River headwaters valley, but mountain uplift forced a flow reversal to create the north-oriented Cache la Poudre River headwaters seen today. South-oriented water diverging from the Laramie River (before flow was reversed) probably carved the Tunnel Creek through valley as the water moved in a south direction on what is now the north-oriented Cache la Poudre River headwaters alignment.
Other through valleys radiate from the Chambers Lake area to the north-oriented Cache la Poudre River headwaters valley and suggest multiple channels moving south-oriented water once diverged from the now north-oriented Laramie River valley head. South-oriented flood flow in the Laramie River valley would also have once moved to the Colorado River headwaters along the Cameron Pass route, but mountain uplift blocked that route at an earlier time.  (Clausen) of Steamboat Rock, which stands above the Upper Grand Coulee floor (in Washington State) and which according to Bretz (1928, p. 461) is an erosional remnant left after massive floods eroded deep channels on either side. Map evidence suggests massive flooding also carved Middle Mountain, perhaps first by south-oriented floodwaters moving to the Colorado River and later by north-oriented floodwaters.

Sand Creek Pass Area
One of the most intriguing North Platte-South Platte drainage divide segments is illustrated in figure 5 and is located along the Laramie Basin southern margin (just south of the Wyoming state line). The Laramie River flows in a northwest direction to the west of figure 5 at the foot of Bull Mountain (partially seen in the figure northwest quadrant) and Sand Creek flows in a north direction from highlands in the figure south center area and descends in the figure northeast quadrant (and eventually joins the north-oriented Laramie River). Note how Sand Creek for a short distance is almost flowing along the North Platte-South Platte drainage divide and how Sheep Creek originates along that drainage divide segment and then flows in northeast and east directions (to the east of figure 5 Sheep Creek water eventually reaches the Cache la Poudre River Creek and north-oriented flows were of sufficient magnitude and duration to erode the Sand Creek Pass through valley. These features are difficult to explain unless massive and prolonged floods flowed around the Medicine Bow Mountains southern margin and then in a north direction into the Laramie Basin as Clausen (2018Clausen ( , 2020aClausen ( , and 2020c suggested.

Discussion and Conclusions
Topographic map interpretation techniques suggest the entire Colorado and Wyoming region and mountains were being uplifted as large and prolonged south-oriented floods moved along the now north-oriented Laramie and North Platte River alignments to reach an actively eroding south-oriented Colorado River headwaters valley. Colorado and Wyoming regional and mountain uplift eventually blocked south-oriented flood flow routes to the Colorado River valley and forced a flow reversal in the Laramie River valley while south-oriented floodwaters continued to flow on the North Platte River alignment with the water then flowing in a southeast and east direction along the Michigan River headwaters alignment before turning to flow northward through (and further deepen) Cameron Pass and then further north along the Laramie River alignment to reach the Laramie Basin. Once in the Laramie Basin (in Wyoming north of this paper's study region) the floodwaters crossed the rising Laramie Mountains to reach the southeast-oriented North Platte River valley (see Clausen, 2018). At the same time Cache la Poudre River valley headward erosion (from the actively eroding South Platte River valley) began to capture floodwaters making the large U-turn (in the Chambers Lake and Sand Pass regions). Headward erosion of the southeast-oriented North Platte River valley into central Wyoming combined with regional and mountain uplift eventually beheaded and reversed south-oriented flow on the north-oriented North Platte River alignment to create drainage routes and drainage divides seen today (see Clausen 2019).
While the above-described water movements and tectonic and erosion events explain the detailed topographic map drainage system and erosional landforms discussed here and are consistent with water movements and topographic map drainage system and erosional landform evidence described in the cited Clausen papers (2018Clausen papers ( , 2019Clausen papers ( , 2020aClausen papers ( , 2020b, and 2020c) a major problem exists. Immense and prolonged volumes of water are required and while large North American continental ice sheets might generate huge and long-lived south-oriented meltwater floods the accepted Cenozoic history paradigm considers the high Colorado and Wyoming plateaus and mountains and many of their drainage features to have existed before any large North American continental ice sheets developed. In other words, the map evidence described here is from the accepted Cenozoic history paradigm perspective what Thomas Kuhn (1970) refers to as anomalous or unexplainable evidence. Paradigms according to Kuhn are judged based on their ability to explain evidence and large quantities of anomalous evidence can lead to new paradigms. This paper along with the cited (and other uncited) Clausen papers are using vast quantities of yet to be explained North and South Platte River drainage basin and other topographic map drainage system and erosional landform evidence to describe a fundamentally different and new Cenozoic history paradigm in which a large North American continental ice sheet caused crustal warping as immense south-oriented meltwater floods flowed across and eroded what must have been rising Wyoming and Colorado regions and mountains.
Paradigms are difficult to change, but the vast quantities of unexplained topographic map drainage system and erosional landform evidence suggest a future Cenozoic geologic history paradigm change is inevitable.