Pebble morphology in the Merced River (California)

Granitic pebble morphology was studied in the detritus of the Merced River, Sierra Nevada, California, where minimum distance of transport of granitic pebbles can be determined. Modern alluvium was sampled to determine: (1) percentage of granitic pebbles; (2) percentage of broken rounds; (3) roundness; (4) shape. Granitic pebbles downstream from a large cataract show bimodal distributions. A paucity of intermediate-sized clasts (8 to 32 mm), coupled with an abundance of broken rounds in the same size range, and a decrease in overall roundness occurs directly downstream from the high-energy environment of the cataract. Movement of the intermediate-sized pebbles through the cataract may be mainly by saltation, whereas larger pebbles move by rolling-sliding traction and smaller pebbles by suspension. The critical factor in pebble breakage is probably momentum, and in the cataract environment the saltating intermediate-sized clasts may attain the necessary velocity-mass product to break when they collide with boulders in the stream bed. Granitic pebble abundance, as determined by pebble counts, appears to reflect proximity to source rather than areal abundance of source rock in the drainage area. Comparison between granitic talus and stream pebbles shows equidimensional shape controlled by source-rock characteristics; no obvious evolution in shape results from transport, even where considerable breakage has occurred.