TRACKING THE DOWNSTREAM FATE OF DIVERSE LITHOLOGIES IN MODERN FLUVIAL SYSTEMS, NORTHERN ROCKY MOUNTAINS, IDAHO

Bed sediments in stream channels reflect the integrated effects of supply, diminution, and hydraulic sorting of material through fluvial networks. As a result, the size distribution and lithologic composition of sediment should reflect the influence of these factors as sediment travels downstream. The Big Wood River (BWR) and North Fork Big Lost River (NBL) watersheds of south-central Idaho provide a unique setting to assess the interplay of these processes in basins of markedly different sediment yields. Rock types in both basins consist of Paleozoic clastic marine strata and Eocene volcanic and plutonic rocks of the Challis Volcanic Group. Lithologic variations and differential erodibility provide probable mechanisms for the observed differences in sediment yield and bed sediment composition.

Lithologic classification of surface pebble counts and subsurface bulk samples were used to document longitudinal trends in the quantity and caliber of individual lithologies comprising the modern stream sediment at 35 sites. The lithology of approximately 1500 individual grains ranging from 4 ϕ to -8 ϕ were characterized using hand samples (-2.5 to -8 ϕ) and a petrographic microscope (less than -2.5 ϕ). In both basins, sedimentary material in the fine fraction generally peaks in the very coarse sand range (0- -1 ϕ), and most faithfully reflects the watershed proportion of these lithologies. At finer sizes, particles are composed largely of the matrix of volcanic rocks and monominerallic grains derived from igneous phenocrysts or sedimentary sands. The coarse size fractions record divergent stories between the two basins. In the BWR, resistant plutonic lithologies become over-represented downstream in the coarse fractions, while sedimentary rocks are rapidly reduced in size and quantity. Alternatively, channel-proximal sources in the NBL provide a continued downstream supply of sedimentary lithologies across all size classes, generally offsetting any diminution that may occur during transport. As a result, the bed sediments of the NBL more generally reflect the upstream contributing area of each lithology. This analysis suggests that sediments in streams draining similar rock types are modified by the combined effects of differential diminution, hydraulic sorting, and local sediment supply.