A study of the temporal and spatial variability of stream gravel bed
composition and the factors affecting the amount of fine sediment within
the bed was conducted in the streams of the Oregon Coast Range.
Streambed samples were obtained by frozen core techniques and the
amount of sediment smaller than 1.0 mm in diameter was expressed as
a percentage by weight of the total sample.
The amount of fine sediment in stream gravel beds was found to
be highly variable in time and space. Temporal variability in percent
fines was caused by flushing of fines from the gravel beds during high
streamfiow events. This flushing of fine sediment seemed to occur
randomly during winter freshets. Seven of 13 total streambed sample
locations on five small streams showed trends of decreasing amounts of
fine sediment during the winter high streamflow season.
The percent fines within the stream bed was also found to display
large variation (a) between streams, (b) between locations in the same
stream, and (c) between locations in the same riff 1. Bed samples were
collected on 21 watersheds in the Coast Range during the summer of
1978. The amount of fine sediment averaged 19.4% and ranged from
10.6% to 29.4%. Comparisons between locations on the same stream
showed bed composition to be highly variable. Approximately 75% of
the bed composition comparisons were significantly different at the 95%
confidence level. One gravel bed was sampled on a 1.2 by 1.2 m grid
design and significant (95% confidence level) changes in percent fines
were found to exist both perpendicular and parallel to the stréamflow in
this small area of the stream.
Regression analysis on the samples collected on the 21 streams
indicated that the amount of fine sediment in the bed is influenced by
the slope, area, relief, and land use characteristics of the watershed.
Within a single stream, however, regression analysis indicated that
gravel bed composition was dependent on sinuosity and ankful1 stage.
These two variables suggest that the intrusion of fines into the stream
bed is influenced locally by hydraulic conditions within the channel.
Regression analysis and field observations suggest that the
amount of fine sediment in stream gravel beds might be increased by
road construction and logging operations. However, increases in
levels of bed fines after disturbance should be temporary due to the
flushing of fines with high flows.Keywords: Oregon Coast Range, Bed sediments, Stream channels, Sedimentation, Water quality, Forest harvestin
