Flood deposits of Arroyo Seco, Los Angeles County, California

Abstract

This study represents a continuation of earlier work in San Gabriel Canyon. Deposits left in Arroyo Seco by the flood of March 2, 1938, were mapped in the field, and samples were taken to determine how sedimentary characteristics varied with distance from source. In an area of Lowe granodiorite near the headwaters of the stream size and roundness of particular pebbles, free from contamination, were studied. As in San Gabriel Canyon, the flood deposits were arranged largely in log and boulder jam deposits, which formed a series of partially overlapping features along the valley. These jam deposits have reverse size gradients,—mean size of the boulders increased markedly downstream. In general, however, neither the size nor the sphericity of pebbles in the flood deposits, collected in corresponding portions of successive log jams, showed any change along the valley; roundness, on the other hand, increased noticeably downstream. These results agree with findings in San Gabriel Canyon and suggest that size and shape respond to the dynamical conditions at the immediate site of deposition, whereas roundness is a function of the abrasion suffered by the particles over the distance traveled. An attempt is made, in the light of available evidence, to reconstruct the physical conditions governing the movement and deposition of the flood material, as well as to evaluate the relative roles of wear and selective transportation during floods.