9510 - Laserfiche WebLink

Home Browse Search

f:.! :") i.:.;..) are considered to be se~idependent, self-regulating variables; and slope is considered to be dependent on all these other factors. As an extrinsic factor, climate can, of course, account for changes 1n discharge, sediment load and base level, and thereby cause changes in each of the semidependent and dependent variables in such a way that either aggradation (deposition) or degradation (erosion) occur. Of all the hydrologic variables involved, variations in the ratio between di scharge and sediment load seem to have the greatest influence on channel morphology and fluvial response (Leopold 1969; SchuMM 1971). As sediFlent load increases relative to di scharge the stream becomes overloaded and tends to aggrade, and conversely, decreases in sediment load at a given discharge will result in streaM underloading and dissection. Slope changes which result from changing regional or ultimate base levels can also cause deposition or erosion. Aggradation in the fluvial system may be accomplished by vertical accretion or overbank deposition of fine-grained sediments duri nga flood which exceeds bankful discharge, or by lateral accretion via deposition of coarser-grained sands and gravels in point bars which build during channel-confined floods. \~hen stream level cuts below the level of former floodplains, a new floodplain forms at the new lower level, and the abandoned floodplain forms a terrace. All uvi al terraces may be products of deposition, as in the case of cut and fill terraces (Fig. 2.1a),or products of erosion, as in the case of fill-strath terraces (Fig. 2.1b). In addition, alluvial deposi ts may be superposed on 01 der terraces, or, more commonly, may include a mixture of superposed and inset terraces (Fig. 2.1c). Therefore, correct interpretati on and correl ati on of al J uvi al deposi ts requires accurate dating of alluvial depositional units and a thorough understanding of alluvial depositional and erosional processes. Numerous workers have postul ated a posi ti ve rel ati onship between climate change and alluvial terraces. However, models of fluvial response to climate change differ. The "Huntington principle" (Fairbridge 1968) relies heavily on the influence of vegetation and postulates that reduced vegetation during arid climates causes increased Mass wasting, excessive sediment loads, and results in aggradation. Conversely, stabilized vegetation during moister periods is thought to f m- ~. ~ :j ,:! ~-:.'! " "-. ~. :-i \- -~ U 23