Paradox of downstream fining and weathering-rind formation in the lower Hoh River, Olympic Peninsula, Washington

Abstract

Although downstream fining of clasts is typical in modern and ancient river systems, the Hoh River, a cobble-and-boulder-bed river in Washington state, contains surprisingly little downstream fining of the coarsest tail of the grain-size distribution along its lower 63 km. Mean rate of fining of the coarsest size fraction is only 0.24 mm/km, barely significant given uncertainties in measurement. In addition, these same clasts have weathering-rind thicknesses that change very little along the length of the river (they decrease by 0.01 mm/km). Because weathering rinds are well developed in this setting and both tumbler studies and field observations show that abrasion of weathering rinds greatly accelerates clast diminution, there seems to be a paradox between predicted and observed downstream fining rates. Field study shows that detritus is constantly being resupplied to the river by erosion of late glacial materials along the river9s cutbanks and tributaries. The clasts supplied range in size but include abundant coarse grains with very thick weathering rinds. The continuous resupply of grains strongly attenuates the rate of downstream fining, despite the fact that these weathered grains abrade relatively rapidly. Thus, the dominance of reworked glacial debris overwhelms any processes by which grains are reduced in size in the modern river. These results suggest that relatively infrequent glaciation can have a long-lived effect on river sedimentation.