Coulee Alignment and the Wind in Southern Alberta, Canada
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Coulee Alignment and the Wind in Southern Alberta, Canada CHESTER B. BEATY Department of Geography, University of Lethbridge, Lethbridge, Alberta, Canada T1K 3M4 ABSTRACT GEOLOGIC AND PHYSIOGRAPHIC be little detectable relation between these SETTING larger structural features and details of sur- Study of the distinctive pattern of align- face form. ment and geographical distribution of more The study area includes all of the non- The Alberta Plain region of the Interior than 250 coulees in the plains of southern mountainous part of Alberta south of lat Plains province of western Canada is also Alberta shows that (1) the coulees in ques- 51° N., a region roughly 280 x 200 km, ex- part of the study area (Bostock, 1970). Ele- tion have a mean orientation of N. 70° E., tending from the Rocky Mountain Foothills vations decrease from >1,100 m in the west and (2) their spatial distribution is not east to the Saskatchewan border (Fig. 2). to <800 m in the east. The east to northeast ubiquitous but rather displays a concentra- The southern plains of the province are regional slope has controlled development tion in the area from Lethbridge west to the underlain predominantly by horizontal or of the preglacial and postglacial master Rocky Mountain front. near-horizontal sedimentary rocks of Late streams (Stalker, 1961; Farvolden, 1963). Several possible hypotheses of origin of Cretaceous age (Williams and Burk, 1964), Local relief in most of the region is <40 m, the aligned coulees are considered, includ- generally veneered by glacial, interglacial, although the larger postglacial rivers have ing subsurface structural control, the role of and postglacial deposits. Large subsurface incised segments of their courses to depths regional slope, the effect of lithologic dif- structures, which in places impart gentle >100 m, and Milk River Ridge, the Por- ferences, and wind action. easterly and westerly dips to the sandstone cupine Hills, and the Cypress Hills (see Fig. Action of postglacial wind, operating to and shale strata, include the northern ex- 2 for locations) rise hundreds of meters initiate surface furrows (by wind-driven tension of the Sweetgrass Arch and, in the above the prairie surface. The overall visual snow or rain) that were enlarged by run- west, the Alberta Syncline. There appears to impression, however, is of a broadly rolling ning water, accounts for the three outstand- ing characteristics of the aligned coulees: (1) their preferred orientation of N. 70° E., which approximates the mean direction of the strongest chinook winds in the southern Alberta plains; (2) their geographical dis- tribution, which coincides with that part of the region experiencing the most pro- nounced chinooks; and (3) their almost ex- clusive location on windward topographic surfaces. No other hypothesis of origin is known that can satisfactorily explain all of these observed facts. Key words: geomor- phology, Holocene, drainage patterns, coulee origin, erosion, wind action, Alberta. INTRODUCTION Casual examination of aerial photo- ^^fciS^^i graphs of the plains portion of southern Al- ~ "^HK^^^^B^l berta revealed the existence of a series of ^ ( B^. short, narrow, parallel or subparallel J^^^H^^ - coulees tributary to stream valleys (Fig. 1). ^ ^^ „, f lc Systematic examination of the photos indi- ^ jj^L^ * cated that the distribution of such coulees «^B 1 ^^ shows a decided concentration in the west- j,4 v ^^BL ^ ern half of the region. This study offers an explanation for the striking degree of paral- » lelism and pattern of geographical distribu- " tion of the aligned coulees based on the ac- Figure 1. Aligned coulees along Oldman River north of Lethbridge, Alberta; view toward north- tion of postglacial wind. east. Geological Society of America Bulletin, v. 86, p. 119-128, 9 figs., January 1975, Doc. no. 50116. 119 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/1/119/3428950/i0016-7606-86-1-119.pdf by guest on 28 September 2021 120 C. B. BEATY Longitudinal and Parabolic dunes s«. Aligned Coulees Kilometers 115°w Figure 2. Index map of southern Alberta showing major rivers, outstanding topographic features, and settlements mentioned in text. General locations of aligned coulees and four areas of stabilized dunes are indicated. till plain, interrupted by stream-cut valleys runoff route to the Hudson Bay depression periods of copious snowmelt. Average di- and an occasional gentle topographic emi- had opened 8,000 to 6,000 yr ago, and it is mensions of a coulee in the region would nence. here assumed that the major rivers of the fall within the following ranges: length, 100 region were in, or close to, their contem- to 500 m; width, 50 to 200 m; depth at LATE-GLACIAL AND porary locations by that time. The precise mouth, 50 to 80 m. POSTGLACIAL HISTORY courses attained by late-glacial and postgla- Students of glacial geology and geomor- cial streams must have been conditioned by PATTERN OF ALIGNMENT phology do not agree on the number and at least three factors: (1) "initial" ir- Though most of the valleys in the plains age of Laurentide glaciations that took regularities and regional slope of the emer- of southern Alberta have short, steep place in the plains of western Canada (for gent surface; (2) relative resistance of earth tributaries, the pattern of alignment of the an instructive introduction to the problem, materials encountered; and (3) positions of coulees differs throughout the area. Most see Reeves, 1973). This stud)' will not at- the receding or stagnating ice front(s). As coulees east of Lethbridge are oriented ap- tempt to bring order out of the conflicting the rivers became established in their pres- proximately normal to valley axes (Fig. 4). versions of the Pleistocene stratigraphy. ent courses and valley deepening pro- They appear to be consequent drainage The coulees are of late-glacial or postglacial ceeded, conditions favorable for coulee lines, cut by surface runoff taking the most age; thus, the late-glacial and postglacial formation came into being. direct route down the walls of the valleys. geomorphic history of the region is of In a zone extending roughly from primary interest. COULEES Lethbridge west to the mountains, how- Basing their estimates on physiographic General Morphology ever, a decidedly preferred orientation is evidence and a limited number of radiocar- evident in many clusters of coulees along bon dates, investigators have suggested that As defined for purposes of this study, a segments of the Oldman, St. Mary, Belly, the southern part of Alberta was ice free by "coulee" is a short, often straight, narrow, and Castle Rivers, as well as along several 15,000 to 12,000 yr ago (Gravenor and comparatively steep valley tributary to one smaller streams on the southern slopes of Bayrock, 1965; Westgate, 1968; Bryson of the perennial streams (Fig. 3). Such the Porcupine Hills (Fig. 5). As determined and others, 1969; Prest, 196S1). Maps pre- coulees generally are dry valleys; they carry from measurements on aerial photographs, pared by Bryson and others (1969) and surficial runoff only during and shortly the mean trend of more than 250 such Prest (1969) indicate that an ice-free surface after heavy rain and, more rarely, during coulees is N. 70° E., with orientation vary- Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/1/119/3428950/i0016-7606-86-1-119.pdf by guest on 28 September 2021 COULEE ALIGNMENT AND WIND, SOUTHERN ALBERTA, CANADA 121 unglaciated parts of North Dakota, South Dakota, Montana, Wyoming, and Ne- braska and discussed four possible hypoth- eses of origin (1929, p. 251): (1) influence of regional slope, (2) differential erosion of weak and resistant strata, (3) effect of struc- tural deformation, and (4) effect of wind. Russell concluded (1929, p. 255) that prevailing wind, acting to build longitudi- nal dunes with a northwest-southeast orien- tation, which would control the location and pattern of surface streams, was proba- bly responsible. As a result of later studies in South Dakota, Flint (1955, p. 156-160) and Crandell (1958, p. 45-50) reached comparable conclusions regarding the ef- fectiveness of wind, as have other inves- tigators of Great Plains surficial geology (for example, Price, 1943). On the other hand, a large body of litera- ture suggests that subsurface structural fea- tures, principally faults and joints, are re- sponsible for surficial lineaments in parts of the earth's surface, including many of those in the Great Plains (Blanchet, 1957; Sikabonyi, 1957; Mollard, 1957a, 1957b; Kupsch and Wild, 1958; Lattman and Matzke, 1961; Barton, 1962; Robinson and others, 1969; Nielsen, 1971; Ozoray, 1972). Speculation about causes for linea- ments in southern Alberta favors the Figure 3. Cluster of aligned coulees along Castle River west of Pincher Creek, Alberta; view to- hypothesis of subsurface control, the argu- ward northeast. Buildings of ranch on river bottom give scale. ment being that "reflection" of faults or joints through unconsolidated overburden ing from N. 60° E. to N. 90° E.; 85 percent rotational slumping has modified some creates linear zones of potential weakness of the coulees in question have an align- coulee walls, especially those facing north, that can be exploited by surficial weather- ment within 2°ofN. 70° E. The distribution northeast, and east (Beaty, 1972). Finally, ing and gradation. of the coulees with preferred alignment is the floor of the average coulee tends to be Nonetheless, the action of postglacial not random with respect to surface mor- flat and smooth, varying in width from 1 m wind provides an explanation for both the phology; most are on valley walls or the to as much as 10 to 12 m. Some coulees preferred orientation and geographical dis- flanks of topographic eminences with a have actively incising channels on their tribution of aligned coulees that is in many southwest (or windward) exposure.