Packman, James J. 2004. Land-use effects on suspended sediment in Puget Lowland salmonid streams. M.S.
Sediment
is naturally delivered to and transported in streams. Anthropogenic activities
can alter watershed conditions
and affect processes that determine
the concentrations, sizes, and frequencies of suspended sediment found in
streams. These changes can potentially affect Pacific salmonids, which have
evolved to cope with the naturally occurring sediment regime. For this study,
suspended sediment samples were collected from streams in eight watersheds
in the Puget Lowland-two agricultural, three forested, and three urbanized-and
the data were related to watershed conditions that have the potential to
mobilize and transport sediment. The suspended sediment data were also used
in a model from Newcombe and Jensen (1996) to estimate potential effects
of suspended sediment on salmon.
Grab-samples were collected during 11 storm
events and one base flow event and analyzed for suspended sediment concentrations
(total suspended solids). Some samples were also analyzed for grain size
distributions. Suspended sediment concentrations were significantly 
higher
in the urban watersheds (median 43 mg/l) and not significantly different
between the agricultural (median 1.4 mg/l) and forested (median 4.5 mg/l)
watersheds. Grain sizes of suspended sediment were the largest in urban
streams followed by agricultural streams and then forested streams. In the
urban watersheds, impervious surfaces and low soil permeability likely were
the
primary factors affecting suspended sediment concentrations. In the forested
watersheds, moderate to high soil permeability and in-channel sediment
storage were likely the most controlling factors for suspended sediment.
In the agricultural
watersheds, topography and gradient were likely the primary factors affecting
suspended sediment. The multitude of factors affecting sediment created
an unavoidable scatter in the suspended sediment concentration data. More
careful
stratification of sites by anticipated sediment delivery and transport
processes should reduce this scatter. Future sampling programs like this
should also
benefit from a larger sample size, especially of particle size data, within
storm events and for the range of storm events across seasons.
Potential
effects of suspended sediment on three life stages of Pacific salmonids
(eggs/alevin, juveniles, and adults) were estimated based on observed
concentrations and
modeled durations of exposure of one hour and one day. For exposure of
one hour at observed concentrations, the 
model predicted potential effects
to
be minor (or less). However, exposure for longer periods could have more
significant effects, especially for eggs and alevin in urban streams.
The model results should be considered in context of other factors that can
affect fish response to suspended sediment, including stream habitat,
fish
condition,
and cumulative effects from multiple exposures. Although watershed conditions
related to land-use affected suspended sediment to varying degrees in
the urban and forested streams studied here, the observed concentrations
are
not likely of significant concern to salmon at short exposures.
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