ABSTRACT

Fragmentation loads, the loads under which individual particles fragment between plattens, were measured for quartz from weathered samples of a granite and from points along a stream system draining the granite surface. Stream transportation caused major destruction of 2-5 mm quartz in a few kilometers. 1-2 mm particles were less affected and breakage of 0.5-1.0 mm quartz was more than compensated by production of new grains created by breakage of coarser ones.

Most breakage probably occurs in the bedload zone during floods, particularly in the presence of moving pebbles. Greater mobility enables small grains to outlast larger, stronger ones in this environment. Not only are small grains more easily kept above the breakage zone in suspension but they are probably much more readily moved away from impending impact points between pebbles by momentary radiating currents generated around such points just before the pebbles impact.

Downstream fining of river sediments is due to both hydraulic separation and breakage. The latter is very important in headwater regions but its effectiveness probably wanes downstream in large rivers.