Vegetation Conversion and Channel Geometry in Monroe Canyon, Southern California

Abstract

Vegetation Conversion and Channel Geometry in Monroe Canyon, Southern California ANTONY R. ORME and ROBERT G. BAILEY9 The geometry of a stream channel is a product of several parameters , the interrelations of which are exceedingly complex. Throughout its length a channel must adjust continually to changes in these parameters. Some factors, namely watershed discharge and bedrock lithology, change independently of the channel itself. The nature and amount of debris supplied by tributaries are also independent , although a channel actively undercutting debris-laden hillslopes prone to mass failure can directly augment this supply. Semidependent factors, those related both to the environment and to the channel, include the pattern of the channel, mode of debris transport, size of debris as modified by sorting and abrasion, flow resistance or hydraulic roughness, and stream gradient. Factors more or less dependent on activity by or within the channel are stream velocity, channel width, and channel depth. Leopold, Wolman, and Miller recognize the longitudinal profile of a stream channel as a function principally of eight interrelated variables—discharge, supply of debris, size of debris, flow resistance, gradient, velocity, width, and depth.1 Similarly, the cross-profile of a stream channel at a given * Dr. Orme is Associate Professor of Geography at the University of California , Los Angeles, California 90024; Mr. Bailey is Hydrologist, U. S. Forest Service, South Lake Tahoe, California 95705. This paper was read at the 32nd annual meeting of die Association. The authors gratefully acknowledge the assistance of Dr. Raymond M. Rice of the Pacific Southwest Forest and Range Experiment Station, Glendora, and unpublished survey data for 1958 and 1963 provided by the Los Angeles County Flood Control District. 1 Luna B. Leopold, M. Gordon Wolman, and John P. Miller, Fluvial processes in geomorphology (San Francisco: W. H. Freeman and Company, 1964), pp. 248-252. 65 66ASSOCIATION OF PACIFIC COAST GEOGRAPHERS location is a function of discharge, of the nature and amount of debris moving through the section, and of the composition and adaptability of the materials comprising the bed and banks of the channel. A change in any one of these variables may produce a change in channel geometry. The variables listed above are related ultimately to the broad environmental parameters of physique, climate, and biota, parameters that control the energy and materials of individual drainage systems. Physique includes such factors as geologic structure, lithology, regolith, relief, slope, base level, drainage pattern, and infiltration capacity of the soil. Climatic factors, notably the nature and intensity of precipitation and évapotranspiration, ultimately control the amount of water available to individual watersheds, although the subsequent distribution of water within a drainage basin is also conditioned by physical and biotic factors. The biotic parameter is expressed most significantly by the nature and density of vegetation within the watershed, including its modification by land use and management practices. Studies of the adjustments of channel geometry to hydrologie fluctuations are common in the literature, but such fluctuations are usually linked to flood surges and seasonal flow characteristics related directly to climate. Studies in which variables affecting channel geometry have been grossly upset by changes in the vegetation cover are less common. Monroe Canyon, notched into the southern flanks of the San Gabriel Mountains in southern California, provides vivid evidence of the effect that has been produced on channel geometry by vegetation changes which have resulted in increased debris production and flood discharge . The Monroe Canyon Environment Monroe Canyon occupies a highly dissected 354-hectare watershed along the western margins of the San Dimas Experimental Forest, 7 kilometers northeast of Glendora. The watershed is underlain primarily by pre-Cretaceous schists, gneisses, and granites into which numerous quartz veins and basic dikes have been intruded. This bedrock complex is severely fractured, a characteristic which has greatly increased its permeability and groundwater storage capacity and has allowed the deep but selective penetration of weather- YEARBOOK VOLUME 33 1971 67 BIG DALTON DAM ONE KILOMETRE • ¦ ¦ ¦ · Mojave ' ' *&££&' *"7canyon"'" RESERVOIR LEVEL MAY 1969 500 45O0 Figure 5. Longitudinal profile of Monroe Canyon main channel, March 1942 and May 1969.» O O r ss co -4 in 76ASSOCIATION OF PACIFIC COAST GEOGRAPHERS Figure 6. Mouth of Monroe Canyon before conversion, April 1958. Stream-gauging shelters...