|
Abstract:
|
Human activities in Northwestern watersheds, including logging, grazing, agriculture,
mining, road building, urbanization, and commercial construction contribute to periodic
pulses or chronic levels of suspended sediment in streams. Suspended sediment is
associated with negative effects on the spawning, growth, and reproduction of salmonids.
Effects on salmonids will differ based on their developmental stage. Suspended
sediments may affect salmonids by altering their physiology, behavior, and habitat, all of
which may lead to physiological stress and reduced survival rates. A sizable body of data
(laboratory and field-based) has been gathered in North America focusing on the
relationship between turbidity, total suspended sediments, and salmonid health. The
controlled environment of laboratory studies tends to give clearer results than field
studies.
Understanding the relationship between turbidity measurements, suspended sediments,
and their effects on salmonids at various life stages will assist agencies implementing
transportation projects to devise techniques to reduce temporary and chronic erosion and
sedimentation associated with these activities. There are three primary ways in which
sediment in the water column is measured: turbidity, total suspended solids, and water
clarity. While these measures are frequently correlated with one another, the strength of
correlation may vary widely between samples from different monitoring sites and
between different watersheds. Turbidity is currently in widespread use by resource
managers, partially due to the ease of taking turbidity measurements. In addition, current
state regulations addressing suspended sediment are usually NTU-based. The
disadvantage of turbidity is that it is only an indicator of suspended sediment effects,
rather than a direct measure, and may not accurately reflect the effect on salmonids.
Protection of Washington State’s salmonids requires that transportation officials consider
the effect of suspended sediments released into streams during transportation projects.
Many state and provincial criteria are based on a threshold of exceedance for background
levels of turbidity. However, determining natural background levels of turbidity is a
difficult endeavor. Turbidity measures may be affected by 1) differing physical
processes between watersheds including geologic, hydrologic and hydraulic conditions;
2) legacy issues (activities historically conducted in the watershed); and 3) problems with
instrumentation and repeatability of turbidity measurements. Altered systems may not
provide accurate baseline conditions.
The inconsistent correlation between turbidity measurements and mass of suspended
solids, as well as the difficulty in achieving repeatability using turbidimeters contributes
to concerns that turbidity may not be a consistent and reliable tool determining the effects
of suspended solids on salmonids. Other factors, such as life stage, time of year, size and
angularity of sediment, availability of off-channel and tributary habitat, and composition
of sediment may be more telling in determining the effect of sediment on salmonids in
Northwestern rivers.
Although salmonids are found in naturally turbid river systems in the Northwest, this
does not necessarily mean that salmonids in general can tolerate increases over time of
suspended sediments. An understanding of sediment size, shape, and composition,
salmonid species and life history stages, cumulative and synergistic stressor effects, and
overall habitat complexity and availability in a watershed is required.
For short-term construction projects, operators will need to measure background
turbidities on a case by case basis to determine if they are exceeding regulations.
However, transportation projects may also produce long-term, chronic effects. Shortterm
pulses will presumably have a different effect on salmonids than chronic exposure.
To adequately protect salmonids during their freshwater residence, TSS data on
physiological, behavioral, and habitat effects should be viewed in a layer context
incorporating both the spatial geometry of suitable habitat and the temporal changes
associated with life history, year class, and climate variability. Spatial and temporal
considerations provide the foundation to decipher legacy effects as well as cumulative
and synergistic effects on salmonid protection and recovery. |