Abstract:
Tracer-derived estimates of hydraulic resistance and transient hydraulic
storage were related to measures of pool volume and channel morphometric
variability in small streams of the Oregon coast, U.S.A. Fourteen
100 m study reaches in 3 streams were selected to compare channel
and hydraulic characteristics in streams representing a time series of recovery
since major torrent scour or deposition (2, 12 and 120 years).
Transient storage ("dead zone") volume fractions, ranging from 0.3 to 0.6
in the study reaches, were significantly (p <.01) correlated with aggregate
residual poo1 volume (r = +0.94) and the standard deviation of thalweg
depth (r = +0.95). Darcy-Weisbach friction factors (f) ranging from 2 to
90 were correlated (r values from +0.95 to +0.98) with the standard deviation
of thalweg depth (SDD) within restricted ranges of summer low
flow and elevated springtime discharge. Regressions of f versus SDD for
combined data collected over a range of discharges (0.019 to 0.11 m³/s)
showed increased scatter. A semi-logarithmic relationship (r² = 0.60, n =
40) between dimensionless velocity (8/f) ⁰·⁵ and a dimensionless measure indexing relative submergence of large scale bed features (mean thalweg
depth/SDD) was significant at p <.01.
Measures and indices of pool volume and transient storage were
positively correlated (r = +0.78 to +0.89) with volumetric loadings of
woody debris. High total pool and dead zone volumes in reaches were
largely due to plunge pools formed by scouring downstream of woody debris
accumulations. Among the study streams, the greatest reach pool volume
and channel complexity occurred in torrent deposit reaches of the intermediate
(12 yr.) recovery stage stream. Reaches scoured recently (2
yr.) by a debris torrent had the lowest pool volume and channel complexity.
The stream experiencing the longest period of "recovery" (120 yr)
had characteristics between those of the 2- and 12-year recovery streams.
Torrent scouring reduced pool volume, dead zone fraction and channel
morphometric variability. Torrent deposition and subsequent local reworking
of sediments by the stream increased values of these variables,
especially when torrent deposits contained woody debris and boulders.
The relative importance of pool-forming agents varied with recovery
time and amount of torrent deposits. Bedrock, cobbles, log clusters,
and single logs contributed about equally to the small residual pool volume
in reaches recently scoured by a torrent. Log clusters and boulders dominated
in two reaches of the intermediate recovery class stream where logs
and sediment were deposited by a torrent, and in two reaches where boulders
were left as lag deposits. Bedrock and log clusters contributed about
equally to pool formation in the relatively undisturbed stream.
