Chin Coulee-Milk River drainage divide area landform origins, south central Alberta, Canada

Authors

Abstract:

Topographic map interpretation methods are used to determine landform origins in the Chin Coulee-Milk River drainage divide area of south central Alberta, Canada. The Milk River and its North Fork originate on the east edge of Glacier National Park in Montana and flow in northeast directions into southern Alberta where they join and then flow in an east direction to southeast Alberta. Once in southeast Alberta the Milk River flows in a southeast direction back into Montana and water eventually reaches the Gulf of Mexico. North of the east-oriented Milk River in south central Alberta is east and east-southeast oriented Etzikom Coulee and north of Etzikom Coulee is southeast, east-southeast, and northeast-oriented Chin Coulee, with water eventually reaching the South Saskatchewan River and finally ending up in Hudson Bay. West and north of the Chin Coulee headwaters, Etzikom Coulee headwaters, and headwaters of southeast oriented North Fork Milk River tributaries are north and northeast-oriented Oldman River tributaries, with the Oldman River being a South Saskatchewan River tributary. Deep and broad through valleys bounded by well-defined escarpments crossing the present day north-south continental divide provide evidence of multiple flood flow routes carved by immense southeast-oriented melt water floods prior to headward erosion of the deep northeast-oriented South Saskatchewan River valley and its tributary valleys. Headward erosion of the deep South Saskatchewan River valley and its tributary valleys captured the southeast- and south-oriented melt water flood flow with flood waters on north and northwest ends of beheaded flood flow routes reversing flow direction to erode north- and northwest-oriented valleys. Elevations of through valley floors indicate melt water floods flowed across what are today the highest regional elevations, including the high Milk River Ridge area, which is today drained by the North Fork Milk River and the Milk River.

Preface:

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

Introduction:

  • The purpose of this essay is to use topographic map interpretation methods to explore the Chin Coulee-Milk River drainage divide area landform origins in south central Alberta, Canada. 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 essays in the Missouri River drainage basin landform origins research project 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 Chin Coulee-Milk River drainage divide area landform evidence in south central Alberta will be regarded as evidence supporting the “thick ice sheet that melted fast” paradigm (see paradigm related essay in menu at top of page). This essay is included in the Missouri River drainage basin landform origins research project essay collection.

Chin Coulee-Milk River drainage divide area location map

Figure 1: Chin Coulee-Milk River drainage divide area location map from Atlas of Canada Toporama index map (select and click on maps to enlarge).
 
  • Figure 1 is an index map from the Atlas of Canada Toporama web site to provide a location map for the Chin Coulee-Milk River drainage divide area in south central Alberta, Canada. Areas south of the numbered grid are located in Montana. The large grid numbers identify 1:250,000 scale hard copy topographic maps and the more detailed grid numbers identify 1:50,000 scale hard copy topographic maps. The more detailed grid numbers are shown on Toporama maps used in figures below and can be used for location identification purposes. North of the west to east oriented international border is the Province of Alberta with Saskatchewan to the east and British Columbia then to the west. Most drainage routes are not identified on figure 1 and I will here try to identify drainage routes of importance to this essay. The Milk River originates in Montana as two northeast-oriented streams, which enter Alberta and join in map 82H1. From map 82H1 the Milk River flows in an east direction across maps 72E4, 72E3, and into map 72E2 where it turns to flow in a southeast direction to reenter Montana and then to flow to the figure 1 southeast corner. East and south of figure 1 the Milk River joins the Missouri River with water eventually reaching the Gulf of Mexico. A southeast-oriented drainage route in map 82H8 draining toward the Milk River in map 72E4 is Verdigris Coulee, which is an identifiable valley with what is probably discontinuous southeast-oriented drainage. North of the Milk River in south central Alberta is east-oriented Etzikom Coulee which drains from the southeast corner of map 82H10 across maps 82H8, 72E5, and 72E6 to Lake Pakowki. Lake Pakowki is normally an internal drainage basin although should it overflow the water would flow in a north and northeast direction to join the northeast-oriented South Saskatchewan River at Medicine Hat (map 72L2). Water in the South Saskatchewan River eventually reaches Hudson Bay. North of Etzikom Coulee in maps 82H9, and 72E12 is east-southeast oriented Chin Coulee which turns in map 72E11 to drain in a north and northeast direction to join the South Saskatchewan River. Chin Coulee is shown on some maps as a discontinuous drainage route although a continuous valley can be seen on topographic maps. The northeast and north-northeast oriented river flowing from Cardston (map 82H3) to Lethbridge (map 82H10) is the St Mary River, which joins the Oldman River at Lethbridge. The Oldman River is a South Saskatchewan River tributary. The Chin Coulee-Milk River drainage divide area in south central Alberta is the north-south continental divide and is primarily located in the east half of large grid map 82H and the west half of large grid map 72E. The Milk River-Lodge Creek drainage divide area landform origins, southeast Alberta essay describes the region immediately to the east and the Sage Creek-O’Brien Coulee drainage divide area landform origins, Hill and Liberty Counties, Montana essay and the Milk River-Marias River drainage divide area landform origins, Sweet Grass Hills, Toole and Liberty Counties, Montana essay describe areas immediately to the south. Essays may be found by selecting the Milk River category from the sidebar category list.
  • Before looking at the Chin Coulee-Milk River drainage divide area in detail I will provide a brief description of the larger region erosion history. The Chin Coulee-Milk River drainage divide area was eroded by massive southeast and south oriented melt water floods late during the rapid melt down of a thick North American ice sheet. The ice sheet had been located in a deep “hole”, which had been created by deep glacial erosion and crustal warping caused by the ice sheet’s great weight. To understand the crustal warping it is important to understand the ice sheet was formed on a topographic surface preserved today, if it is preserved at all, on the highest level Rocky Mountain erosion surfaces. At that time the Rocky Mountains did not stand high as they do today and early in the ice sheet melt history immense south and southeast oriented melt water floods flowed across what are today the crests of high Rocky Mountain ranges. Ice sheet caused crustal warping, perhaps aided by crustal unloading as the large melt water floods eroded the surface bedrock, resulted in uplift of Rocky Mountain ranges beginning in the south and continuing in a north direction as the ice sheet melted. Rocky Mountain uplift diverted the immense south and southeast oriented melt water floods to both the east and west where flood waters eroded valleys now used by present day drainage systems, including the Missouri River valley and its many tributary valleys. In time the ice sheet surface was lowered to the point where the ice sheet no longer stood high above the surrounding region and giant supra-glacial melt water rivers carved huge ice-walled canyons into the decaying ice sheet surface. One such large ice-walled canyon extended in a southeast direction across Saskatchewan and northwest North Dakota and then in a south direction into South Dakota. This huge ice-walled canyon eventually became an ice-walled and bedrock-floored canyon and detached the ice sheet’s southwest margin. Because the floor of this large canyon was lower than the ice-marginal bedrock surface in present day North Dakota and Montana deep east and northeast oriented valleys eroded headward from the canyon to capture and divert the south and southeast-oriented ice-marginal melt water floods to the immense southeast- and south-oriented melt water river flowing on the canyon floor. Today the northeast and east-facing Missouri Escarpment in Saskatchewan, North Dakota, and South Dakota is what remains of the ice-walled and bedrock-floored canyon’s southwest and west wall, although north and northeast-oriented Missouri River headwaters and tributary valleys in Montana and northern Wyoming were eroded by melt water floods flowing to the deep canyon floor.
  • Chin Coulee-Milk River drainage divide area landforms were eroded during final stages of the decaying ice sheet’s rapid melt down. At that time the ice sheet’s south-west margin had been detached (or was in the process of being detached) and a large east and northeast-oriented valley eroded headward from the ice-walled and bedrock-floored canyon in the North Dakota northwest corner. This east and northeast-oriented valley eroded headward across the Medicine Lake region of northeast Montana to the Poplar, Montana and then along the alignment of the present day Missouri River valley. This “Missouri River” valley captured south and southeast oriented ice-marginal melt water floods and north and northeast-oriented ice-marginal melt water flood waters which had become trapped behind rising Canadian Rocky Mountain ranges and which had flowed into southwest Montana where rising Rocky Mountain ranges had blocked their south and southeast movement forcing flood waters to flow into north central Montana east of the Rocky Mountain front. A major tributary valley to this large east and northeast-oriented “Missouri River” valley also eroded headward on the present day Milk River alignment across northern Montana to capture south and south eastoriented melt water floods. Thee large “Missouri River” and Milk River” valleys were deep, probably hundreds of meters deep, as they eroded headward across the region. Headward erosion of the deep “Missouri River” valley was slightly in advance of “Milk River” valley headward erosion and massive south and southeast-oriented melt water floods lowered what is now the Milk River-Missouri River drainage divide prior to “Milk River” valley headward erosion. Following headward erosion of the deep east and southeast-oriented Milk River valley across north central Montana and southern Alberta immense southeast-oriented melt water floods carved broad and deep valleys which significantly lowered many regions to the north. Headward erosion of the deep northeast-oriented South Saskatchewan River valley and its north and northeast-oriented tributary valleys subsequently captured the southeast-oriented melt water floods and diverted the flood waters further to the north and east. About that time flood waters in the giant southeast- and south-oriented ice-walled and bedrock-floored canyon was captured by new ice-walled and bedrock-floored canyons leading to the North Atlantic and Hudson Bay regions. Diversion of south-oriented melt water floods from the Gulf of Mexico to the north changed the North American climate, which ended the ice sheet’s rapid melt down.

Pakowki Lake-Milk River drainage divide area

Figure 2: Pakowki Lake-Milk River drainage divide area from Toporama 1:300,000 scale topographic map.
  • Figure 2 uses a Toporama !:300,000 scale topographic map to illustrate the eastern end of the Chin Coulee-Milk River drainage divide area in south central Alberta. Montana is located south of the west to east oriented international border where no data is available. Figure 2 drainage routes are also not labeled so I will need to identify major streams. The Milk River flows in an east direction across map 72E3 into map 72E2 where it flows in a southeast direction to the figure 2 south edge and then into Montana. North-oriented tributaries joining the Milk River in map 72E3 originate in the Montana Sweet Grass Hills region and their headwaters are illustrated in maps included in the Milk River-Marias River drainage divide area landform origins, Sweet Grass Hills, Toole and Liberty Counties, Montana essay. Lake Pakowki is labeled and is located in a shallow internal drainage basin and has northwest and southeast-oriented streams flowing to it. Note how in the northeast corner of map 72E3 Lake Pakowki is linked by shallow through valleys with the Milk River valley and in the northwest corner of map 72E7 it is linked by a slightly lower and much broader through valley to north-oriented streams flowing to the figure 2 north edge. The east-oriented stream draining from map 72E12 into map 72E11 where it turns to drain in a north direction is Chin Coulee. Chin Coulee is shown in figure 2 as a continuous drainage route and north of figure 2 drains to northeast oriented Seven Persons Creek, which joins the South Saskatchewan River at Medicine Hat, Alberta (see figure 1). Today should Lake Pakowki overflow the water would eventually reach the South Saskatchewan River and finally end up in Hudson Bay. The through valleys seen in the map 72E3 northeast corner region however provide evidence that at one time water from the Milk River valley did flow in a north and northeast direction across the Lake Pakowki basin to the South Saskatchewan River. But, before that event massive southeast-oriented melt water floods flowed from the Lake Pakowki basin to the figure 2 southeast corner region and then in a southeast direction into Montana. Northwest-oriented Lake Pakowki tributaries are linked by northwest-southeast-oriented through valleys with southeast-oriented streams draining to the figure 2 southeast corner region. These through valleys cross the north-south continental divide and are on the floor of what is a much broader and deeper through valley defined on each side by high elevations in maps 72E10 and 72E3. The figure 2 map contour interval is 20 meters and elevations of 1120 meters are found in map 72E10 while elevations greater than 1140 meters can be found along the Montana border in map 72E3. Floors of the deeper northwest-southeast oriented through valleys in the map 72E2 northeast quadrant are in the 940 to 960 range with the Milk River valley being even deeper. This broad northwest-southeast oriented through valley is at least 160 meters deep (and by proceeding into the Sweet Grass Hills area in Montana and the Cypress Hills region east of figure 2 evidence for a much deeper through valley exists). The through valley was eroded by massive southeast-oriented flood flow moving to what was then the actively eroding and deep Milk River valley to the south. Headward erosion of the deep northeast-oriented South Saskatchewan River valley and its northeast- and north-oriented tributary valley (across the Lake Pakowki basin) captured the southeast-oriented flood flow and diverted the flood waters in a north and northeast direction. Flood waters on northwest ends of the beheaded flood flow channels reversed flow direction to erode the northwest-oriented Lake Pakowki tributary valleys. The figure 2 evidence suggests headward erosion of the north-oriented South Saskatchewan River tributary valley may have almost captured flood flow in the Milk River valley.

Chin Coulee-Milk River drainage divide area

Figure 3: Chin Coulee-Milk River drainage divide area from Toporama 1:300,000 scale topographic map.
  • Figure 3 uses a 1:300,000 scale Toporama topographic map to illustrate the Chin Coulee-Milk River drainage divide area in south central Alberta west of the figure 2 map area and includes overlap areas with figure 2. The Milk River flows in an east direction from the figure 3 west edge (south half) across maps 72E4, 72E3, and 72E2 to the figure 3 southeast corner. Pakowki Lake is located in map 72E7 and the east and east-southeast oriented drainage route across maps 72E5 and 72E6 to Lake Pakowki is Etzikom Coulee. North of Etzikom Coulee is east-southeast, east, northeast, and north-oriented Chin Coulee in maps 72E12 and 72E11. Remember from the figure 2 discussion north of figure 3 Chin Coulee drains to Seven Persons Creek, which flows to the South Saskatchewan River with water eventually reaching Hudson Bay. Also remember today Pakowki Lake is today an internal drainage basin, but should it ever overflow water would flow to Chin Coulee. Further, the Pakowki Lake basin is at the northwest end of a broad northwest-southeast oriented through valley linking the South Saskatchewan River basin with the Milk River valley. Note how elevations in the Pakowki Lake basin are less than 880 meters while elevations in the northwest corner of map 72E3 rise to more than 1000 meters. West of the high area in the map 72E3 northwest corner and map 72E4 northeast corner is another broad northwest-southeast oriented through valley linking the Etzikom Coulee valley with the  east-oriented Milk River valley. This map 72E4 through valley has a valley floor elevation of between 940 and 960 meters so it has not been eroded as deep as the through valleys in the Pakowki Lake basin area. However, there is a well-defined and identifiable through valley crossing the north-south drainage divide, which provides evidence of a major southeast-oriented flood flow channel which was captured by headward erosion of the deeper east-oriented Milk River valley. The northwest-southeast oriented intermittent lake near the figure 3 west edge in map 72E4 is located in northwest-southeast oriented Verdigris Coulee, which will be seen in more detail in figures 4, 5, 6, and 7. Headward erosion of the deep east-oriented Milk River valley across the figure 3 map area enabled southeast-oriented ice-marginal flood waters to significantly lower elevations along the Etzikom Coulee-Milk River drainage divide (and also along the Chin Coulee-Etzikom Coulee drainage divide and drainage divides further to the north). South of map 72E3 and the map 72E4 southeast quadrant are the Montana Sweet Grass Hills, which are a Rocky Mountain outlier and which limited erosion of the Milk River-Marias River drainage divide area to the south. The southeast-oriented through valley east of the Milk River southeast-oriented segment seen in figure 2 was eroded east of this erosion resistant rock mass and for that reason was able to erode a much deeper broad lowland.

Verdigris Coulee-Milk River drainage divide area

Figure 4: Verdigris Coulee-Milk River drainage divide area from Toporama 1:300,000 scale topographic map.
  • Figure 4 uses a Toporama 1:300,000 scale topographic map to illustrate the western end of the Chin Coulee-Milk River drainage divide area west of the figure 3 map area. The Milk River flows in an east direction from the figure 4 west edge (south half) across map 82H1 and into map 72E4 and then to the figure 4 south edge. The intermittent lake in the map 72E4 northwest quadrant is located in southeast-oriented Verdigris Coulee. Note how there is a discontinuous chain of northwest-southeast oriented elongate lakes and streams on that alignment with the southernmost stream being a Milk River tributary. These northwest-southeast oriented lakes and streams are all located in an identifiable valley, which is known as Verdigris Coulee. South and west of Verdigris Coulee the land rises into an upland region known as Milk River Ridge. Note how there is well-defined northeast-facing escarpment in the southwest corner of map 82H8 and adjacent maps. The Verdigris Coulee valley near the escarpment base has an elevation of less than 1000 meters while elevations along the escarpment crest exceed 1200 meters (even higher elevations are found further west). In other words this is a 200-meter plus high escarpment very similar to the Missouri Escarpment and other escarpments found further east in Saskatchewan, Manitoba, North Dakota, South Dakota, and southwest Minnesota. The escarpment is an erosional feature and was eroded by immense southeast-oriented ice-marginal melt water floods which once flowed into Montana west of the Sweet Grass Hills upland region probably to what at that time was the actively eroding Marias River valley. Subsequent headward erosion of the east-oriented Milk River valley captured the southeast-oriented floods and diverted flood waters to the actively eroding Milk River valley. Note also how in map 82H1 the Milk River has eroded its valley into the upland surface south and west of the northeast-facing Milk River Ridge escarpment. This evidence suggests at the time the Milk River valley eroded headward across the figure 4 map area massive southeast-oriented melt water floods were moving across a continuous upland surface at least as high as the Milk River Ridge upland surface. As will be seen in figures 8 and 9 today west of the figure 4 map area is the deep northeast-oriented St Mary River valley. Headward erosion of the Milk River valley across the Milk River upland surface would not have been possible if the St Mary River valley had existed at that time. In other words, the deep St Mary River valley eroded headward into the region after headward erosion of the Milk River valley.

More detailed map of Verdigris Coulee-Milk River drainage divide area

Figure 5: More detailed map of Verdigris Coulee-Milk River drainage region from Toporama 1:150,000 scale topographic map.
  • Figure 5 uses a 1:150,000 scale Toporama topographic map to illustrate in more detail the Verdigris Coulee-Milk River drainage area. The Milk River is labeled and is the continuous east-oriented stream flowing across map 72E4. Two discontinuous Verdigris Coulee drainage segments are labeled with the southeast-oriented segment draining to the Milk River. Note how elevations where Verdigris Coulee joins the Milk River are in the 940 to 960 meter range. Also note how south and west of Verdigris Coulee elevations rise along the face of the northeast-facing Milk River Ridge escarpment. North and east of the northeast-facing Milk River Ridge escarpment relief is generally low and  elevations are generally below 1000 meters except for a small region of 1000 meter plus elevations seen along the figure 5 east edge. Continuing east as seen in figures 4, 3, and 2 elevations north of the Milk River valley generally are less than 1000 meters all the way to the east edge of figure 2 where elevations are rising toward the Cypress Hills (which has an upland surface with elevations greater than 1400 meters). South of the Milk River valley in the map 72E4 southeast corner and map 72E3 southwest corner is a high area along the north edge of the Sweet Grass Hills region located primarily in northern Montana just to the south. As flood waters eroded southern Alberta and north central Montana the Sweet Grass Hills emerged to become a significant barrier dividing south-oriented flood flow to the actively eroding Marias River valley (west of the Sweet Grass Hills) and southeast-oriented flood flow to the actively eroding Milk River valley and its tributary valleys (east of the Sweet Grass Hills). The southeast-oriented Verdigris Coulee valley was apparently eroded along the base of the northeast-facing Milk River Ridge escarpment and was probably eroded during the final southeast-oriented flood flow events. While many details remain to be worked out what is important in figure 5 is the evidence for massive southeast-oriented flood flow into the east-oriented Milk River valley.

Very detailed map of Verdigris Coulee southeast end area

Figure 6: Very detailed map of Verdigris Coulee southeast end area from Toporama 1:40,000 scale topographic map.
  • Figure 6 uses a 1:40,000 scale topographic map to illustrate in even more detail where southeast oriented Verdigris Coulee enters the Milk River valley. The Milk River meanders from the figure 6 west edge in an east and southeast direction to the figure 6 south center area and then flows in an east-southeast direction to the figure 6 south edge (near figure 6 southeast corner). Verdigris Coulee drains in a south-southeast direction from the figure 6 northwest corner to join the Milk River in the figure 6 south center. Elevations in figure 6 are given in feet and the map contour interval is 25 feet. Note how the south-southeast oriented Verdigris Coulee valley has been sliced into a northeast-facing slope. Elevations east of the valley near the figure 6 center rise to more than 3200 feet yet gradually decrease toward the figure 6 northeast corner where elevations are lower than 3100 feet. Refer back to figure 5 to see how lower elevations in the figure 6 northeast corner area are linked by a less well-defined, but much broader northwest-southeast oriented through valley with the Milk River valley. The broad northwest-southeast oriented through valley east of the figure 6 map area provides evidence of a somewhat deeper and larger southeast-oriented melt water flood flow route than the Verdigris Coulee valley seen in figure 6. A possible explanation for these two parallel valleys is the larger and deeper through valley to the east was eroded first by massive southeast-oriented flood flow to what was then the actively eroding Milk River valley. At the time Milk River valley headward erosion was occurring in the figure 5 and 6 map areas the immense south-oriented melt water floods to the Gulf of Mexico were being captured on the ice sheet floor and diverted to the North Atlantic Ocean and Hudson Bay areas. These changes in melt water flood flow direction changed climates and may have caused freezing of melt water floods in the figures 5 and 6 map areas. Freezing of flood waters in the larger and deeper valley to the east of figure 6 could have forced remaining flood waters to flow in a southeast direction between the frozen flood water and the newly eroded northeast-facing escarpment (along the southwest wall of the larger and deeper northwest-southeast oriented valley which had been eroded by immense southeast-oriented flood flow prior to the freezing event). The remaining flow was significantly less than the massive floods which eroded the larger and deeper valley, however it was enough to erode the southeast-oriented Verdigris Coulee valley seen today.

Etzikom Coulee-Verdigris Coulee drainage divide area

Figure 7: Etzikom Coulee-Verdigris Coulee drainage divide area from Toporama 1:150,000 scale topographic map.
  • Figure 7 uses a 1:150,000 scale Toporama topographic map to illustrate the Etzikom Coulee-Verdigris Coulee drainage divide area located north and west of the figure 5 map area. Etzikom Coulee is labeled and drains in an east-southeast direction from map 82H9 into the map 82H8 northeast quadrant and then to the figure 7 east edge. Note how many figure 7 drainage routes flow to internal drainage basins and are not components of continuous drainage networks. The figure 7 map contour interval is 20 meters and most drainage routes are not in deep valleys. The northeast-facing Milk River Ridge escarpment can be seen in the figure 7 southwest quadrant. Verdigris Coulee extends in a northwest-southeast direction from Suds Lake in map 82H8 to the southeast corner and the figure 7 south edge, although is drained by discontinuous drainage routes. A northwest-oriented stream is located along the Verdigris Coulee alignment north and west of Suds Lake and a shallow through valley links the Suds and Tyrell Lake basins with the northwest-oriented stream (and also with the Weston Lake basin to the southeast. The railroad follows a second through valley west of the Verdigris Coulee alignment in a valley labeled as Middle Coulee (passing through the town of McNab). Note how a large north-northeast oriented valley and stream links the south end of the Middle Coulee valley with the Tyrell Lake basin. The Middle Coulee valley was probably initiated by a southeast-oriented flood flow channel adjacent to a southeast-oriented flood flow channel on the Verdigris Coulee alignment. These flood flow channels were probably channels in a large southeast-oriented anastomosing channel complex formed on the floor of the deep northwest-southeast oriented through valley east and north of the northeast-facing Milk River Ridge escarpment. Headward erosion of the deep northeast-oriented South Saskatchewan River valley and its tributary valleys (including the St Mary River valley north and west of figure 7) then captured the southeast-oriented flood flow channels in sequence from east to west. The Etzikom Coulee valley resulted from a capture in the Lake Pakowki region which was seen in figures 2 and 3. Southeast-oriented flood flow on the Verdigris Coulee alignment was captured prior to capture of southeast-oriented flood flow on the Middle Coulee alignment. Flood waters on the northwest end of the beheaded Verdigris Coulee flood flow channel reversed flow direction to erode northwest-oriented valleys and captured the southeast-oriented flood flow moving on the Middle Coulee. Probably aiding this capture were large influxes of melt water flowing in a northeast direction from the Milk River Ridge area to the south and west where flood waters were still flowing on a high upland surface toward what was then the actively eroding and deep and deep Milk River valley.

Oldman River-Etzikom Coulee drainage divide area

Figure 8: Oldman River-Etzikom Coulee drainage divide area from Toporama 1:150,000 topographic map.
  • Figure 8 uses a Toporama 1:150,000 scale topographic map to illustrate the Oldman River-Etzikom Coulee drainage divide area north and west of the figure 4 map area and includes overlap areas with figure 4. The city of Lethbridge is located in the figure 8 northwest quadrant. The Oldman River flows in a south-southeast direction from the figure 8 northwest corner where it is joined the north-oriented St Mary River and then flows in an east-northeast and north direction to the figure 8 north edge (north of Lethbridge). The St Mary River flows in an east-northeast direction from the figure 8 west edge (south of center) and is joined by north and north-northwest oriented Pothole Creek and then flows in a northwest and north direction to join the Oldman River. Pothole Creek flows in a north-northeast direction from the figure 8 southwest corner to join the St Mary River. Stirling Lake in the map 82H10 southeast corner drains in an east and east-southeast direction to the figure 8 southeast corner with the drainage route being the western end of Etzikom Coulee. Stafford Reservoir in the figure 8 northeast corner region drains in a southeast and east direction to Chin Coulee. Several of the drainage routes between the Oldman River valley and its major tributary valleys and the Etzikom Coulee and Chin Coulee headwaters areas appear to be man-made drainage ditches and/or to have been altered by human intervention. For this reason those drainage routes will not be used in the interpretation of the figure 8 map area. Without considering those drainage routes the figure 8 map shows an upland surface gently sloping in a northeast direction. Elevations in the figure 8 northeast corner and in the Oldman River valley near Lethbridge are below 840 meters (the map contour interval is 20 meters). South of the words “Nine Mile Coulee” in the figure 8 southwest quadrant elevations rise to more than 980 meters. The Oldman River and St Mary River valleys are generally defined by 3-4 contour lines suggesting they may be 60-80 meters deep (plus or minus at least 20 meters). Note how the south-southeast oriented Oldman River valley segment and the north-northwest oriented St Mary River valley segment are roughly on the alignment of the southeast-oriented Verdigris Coulee valley (see figure 4) and also point toward the Etzikom Coulee headwaters area. Headward erosion of the deep Oldman River valley (from the deep South Saskatchewan River valley) captured southeast-oriented flood flow moving on the Verdigris Coulee and Etzikom Coulee alignments (north of the figure 8 Oldman River valley headward erosion also beheaded flood flow routes to the Chin Coulee flood flow channel). Flood waters on the northwest end of the beheaded flood flow route reversed flow direction to erode the north-northwest-oriented St Mary River valley segment while continued southeast-oriented flood flow eroded the south-southeast oriented Oldman River valley segment.

Milk River Ridge area

Figure 9: Milk River Ridge area from Toporama 1:300,000 scale topographic map..
 
  • Figure 9 uses a 1:300,000 scale Toporama topographic map to illustrate the Milk River Ridge area located west of the figure 4 map area and includes overlap areas with figure 4. The North Fork Milk River flows from Montana in a northeast direction to the map 82H2 southwest corner and then in a northeast and east direction into map 82H1 where it is joined by the northeast-oriented Milk River and then flows to the town of Milk River and the figure 9 southeast corner. The northeast-facing Milk River Ridge escarpment can be seen in map 82H7, the map 82H8 southwest corner, and the map 82H1 north center area. Etzikom Coulee drains in an east and east-southeast direction from the Stirling area in the map 82H10 southeast corner. The lake in the figure 9 northeast corner drains to Chin Coulee. The reservoir along the figure 9 west edge in map 82H6 is flooding the St Mary River valley and the St Mary River flows from the dam in an east and north direction in map 82H6 before flowing in a northeast direction across the northwest corner of map 82H7 and into map 82H10 where it is joined by north-oriented Pothole Creek and then flows in a northwest direction to the figure 9 north edge. Pothole Creek is the major north-oriented drainage route in the west half of map 82H7 with headwaters in the map 82H2 northwest corner. A west-facing Milk River Ridge escarpment can be seen along the east edge of map 82H3 and marks the eastern margin of the deep and broad north-oriented St Mary River valley. Note how the North Fork Milk River in maps 82H2 and 82H1 has several southeast-oriented tributaries suggesting the North Fork Milk River valley eroded headward across multiple southeast-oriented flood flow routes. If so the high Milk River Ridge elevations mean at that time the deep north- and northeast-oriented St Mary River valley did not exist and elevations west and north of Milk River Ridge were at least as high as the Milk River Ridge elevations are today (at least in a relative sense). Headward erosion of the deep St Mary River valley beheaded south and southeast oriented flood flow to the actively eroding North Fork Milk River valley and flood waters on north and northwest ends of the beheaded flood flow routes reversed flow directions to erode north- and northwest-oriented St Mary River tributary valleys. Figure 10 below uses a more detailed topographic map to illustrate a major northwest-southeast oriented through valley eroded by the southeast-oriented flood flow and which was beheaded by headward erosion of the deep Pothole Creek valley.

Detailed map of Pothole Creek-Milk River drainage divide area

Figure 10: Detailed map of Pothole Creek-Milk River drainage divide area from Toporama 1:150,000 scale topographic map.
  • Figure 10 provides a more detailed look at maps 82H7 and 82H2 which were seen in less detail in figure 9 above. The North Fork Milk River is not labeled and meanders in a deep valley near the figure 10 south edge from the figure 10 southwest corner to the southeast corner. Pothole Creek is labeled in the map 82H2 northwest corner area and flows in a north and north-northeast direction to Jensen Reservoir and the figure 10 north edge. Of special interest is northwest-oriented Lonely Valley Creek which originates in the map 82H2 north center edge area and joins Pothole Creek at Jensen Reservoir. Also of special interest is southeast-oriented Lonely Valley Creek which begins near the northwest-oriented Lonely Valley Creek headwaters and which joins the North Fork Milk River in the map 82H2 east center region. Note how these two Lonely Valley Creeks flow in opposite directions in a deep and well-defined northwest-southeast oriented through valley carved into the Milk River Ridge upland area. The floor of the through valley has an elevation of between 1180 and 1200 meters while elevations rise to more than 1300 meters to the northeast and rise even higher in the west half of map 82H2 (where elevations rise to more than 1380 meters). This Lonely Valley Creek through valley is at least 100 meters deep and may have been deeper when initially eroded and served as a major southeast-oriented melt water flood flow channel. At that time the north-oriented Pothole Creek valley did not exist nor did the deep St Mary River valley west of the figure 10 map area exist. Headward erosion of the deep north-oriented Pothole Creek valley beheaded the southeast-oriented flood flow and flood waters on the northwest end of the Lonely Valley Creek through valley reversed flow direction to erode the northwest-oriented Lonely Valley Creek valley as a northwest-oriented Pothole Creek tributary valley. Note other northwest-oriented Pothole Creek tributaries and or tributary segments. Some of these are aligned with southeast-oriented North Fork Milk River tributaries such as southeast-oriented Mackie Creek flowing across the map 82H2 northeast corner and which is aligned with a northwest-oriented Pothole Creek tributary in map 82H7 (with a southeast- and south-oriented Lonely Valley Creek tributary between the two opposing streams). These two opposing streams are also linked by a northwest-southeast oriented through valley defined by 1300  meter high regions on either side (the through valley floor elevation is less than 1300 meters). While not as deep as the Lonely Valley Creek through valley this higher level through valley provides evidence of a southeast-oriented melt water flood flow channel that crossed a 1300-meter high upland surface and that existed probably prior to erosion of the much deeper Lonely Valley Creek through valley.

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 produced and/or compiled by Natural Resources Canada and were downloaded from the Natural Resources Canada Toporama web site. Hard copy maps can be obtained from dealers offering Natural Resource Canada topographic maps or may be observed in libraries located throughout Canada and elsewhere.

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