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A comparative study of multiply-controlled valley asymmetry in S.E. Wyoming and S.W. Manitoba Kennedy, Barbara A.

Abstract

The nature of the inter-relationship of valley-side slopes and the streams at their base is regarded as being of fundamental significance in the study of landscape morphology. One aspect of this relationship, the problem of the development of valley-side asymmetry in east-trending valleys under the joint influence of microclimatic and stream controls, is investigated in two areas of uniform, flat-lying beds, using the approach and techniques of experimental design. Valleys in both study areas have been formed during the last 12,000 years, so that the importance of fossil slope forms is minimal. The variation in lithology between the two areas is held to be of less consequence than the difference in climatic regimes, as far as the type of asymmetry developed. The moister of the two areas, S.W. Manitoba, shows the typical, valley-wide asymmetry regarded as characteristic of non-periglaclal regions, with north-facing slopes significantly steepened by 3.1°, on average: the effects of basal steepening by meandering streams are additive with the microclimatic differences. In the drier area, S.E. Wyoming, the effects of the two controls are non-additive, and asymmetry is a purely localised development created by the over-steepening of shaded, north-facing undercut slopes. The moisture availability in this environment is probably increased by the formation of snowdrifts in the winter months. Asymmetry resulting from differences in aspect is statistically insignificant both in east-trending valleys without defined stream channels and in south-trending valleys with meandering streams. The absolute maximum angle is found to be an excellent indicator both of local changes in slope form and of the nature of the profiles as a whole. The degree of organisation of all profiles studied, in relation to the maximum angle, is high, with the exception of the south-facing slopes in those east-trending valleys in Wyoming which lack defined channels. In general, the maximum angle appears more directly related to the other geometric features of the profile than to the characteristics of soil, vegetation or nearby stream channel. A consideration of all available data on multiply-controlled asymmetry in non-periglacial areas leads to the conclusion that differences in steepness of slopes with northern and southern aspects are more likely to develop in regions of comparatively low humidity and that the valley-wide asymmetry found in Manitoba is the most common form. The strictly localised asymmetry developed in the Wyoming valleys appears to be a specialized phenomenon confined to a narrow area of semi-arid climate. It is suggested that the local conditions of moisture availability are the prime controls of multiply-controlled valley asymmetry, in that they will determine the relative importance of slope and channel processes and hence the nature of the valley-side development.

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