8255 - Laserfiche WebLink

Home Browse Search

Grams and Schmidt 9 5.2 Mapping of Surficial Geology Surficial geology was interpreted on aerial photographs (1:5000 scale) taken in 1993 during low discharge (33.4 m3s-1 and 45.3 m3s-1 above and below Echo Park, respectively). The preliminary maps were field checked then transferred to a 1:12,000 scale topographic base map using a reflecting projection table. A multi-layered deposit classification strategy was devised to incorporate a maximum amount of information in a manageable format. Six geomorphic characteristics were specified for each mappped deposit or feature: (1) bedrock exposure at river level, (2) reach in which each deposit is located (Ladore Canyon, Echo Park, Whirlpool Canyon, Island Park, or Split Mountain Canyon), (3) whether or not the deposit is located within a debris-fan eddy complex, (4) grain size, (5) depositional environment, and (6) elevation of the deposit in relation to the low-discharge water surface. Bedrock type at river level and reach were determined from geologic (Hansen and others, 1983) and topographic maps. Each discrete assemblage of debris fan, eddy bar, upper-pool deposit, and expansion bar was grouped and assigned a unique fan-eddy complex number. Completed maps were digitized into a geographic information system for areal analysis of deposits. 5.4 Inventory of Rapids and Riffles The correlation between debris fans and rapids was evaluated by an inventory of all rapids and riffles on the Green River within the study reach. Our criteria were similar to that of Graf (1979) who defined a rapid as a location where 'debris particles are numerous enough or large enough to break the water surface at mean annual discharge.' We defined a rapid as any discrete river segment where breaking water was visible across most of the channel on the 1993 aerial photographs. The cause of the channel constriction and the material of each river bank were recorded for each rapid. -- 6. RESULTS 6.1 Map Unit Descriptions 6.1.1 Deposit Materi~_