Gravel Bed Composition in Oregon Coastal Streams

Abstract

The amount of fine sediments (generally < 1 mm in diameter) in gravel bedded streams is often used as an indicator of habitat quality and also as a measure of the impact from accelerated sedimentation resulting from land disturbance. Five streams in the Oregon Coast Range were studied to evaluate temporal and spatial variability of streambed composition, as well as the factors affecting the amount of fine sediment within the bed. The amount of fine sediments (< 1 mm) contained in frozen streambed cores and expressed as a percentage (by weight) of the total sample proved highly variable in time and space. During a 19-mo sampling period, temporal variability was caused by an occasional flushing of fines from the gravel beds during high flows. Percent fines also varied greatly between streams, between locations in the same stream, and between locations in the same riffle. Streams on 21 Coast Range watersheds were sampled during summer low flow. The amount of fines averaged 19.4% for all watersheds and ranged from 10.6 to 29.4% for streams on undisturbed watersheds. Regression analysis indicated that the watershed slope, area, relief, and land use influenced the amount of fine sediment in the bed. Bed composition varied greatly between locations in the same stream with about 75% of the within-stream comparisons indicating a significant (α = 0.05) difference. Within a single stream, gravel bed composition correlated significantly with channel sinuosity and bank-full stage. Regression analysis and field observations suggested that road construction and logging operations can increase the amount of fines; however, such increases may be temporary if high flows flush the gravelsKey words: bed sediments, forest harvesting, Oregon Coast Range, sedimentation, spawning gravels, stream channels, water quality