Environmental Engineering Reference
In-Depth Information
Herbivorous invertebrates —Impacts are similar to those on the detritus
community. Algae grows on exposed rock surfaces on which herbivorous
aquatic invertebrates graze. Fluctuations desiccate and disrupt the growth
of the exposed algae (Gislason, 1985) and reduce access by herbivores.
r edd d ewaterIng
Research has extensively documented the lethal impact of redd dewatering on salmo-
nid eggs and alevins (i.e., larval fish) (Frailey and Graham, 1982; Fraser 1972; Fustish
et al., 1988; Satterthwaite et al., 1985). Salmonid eggs can survive for weeks in dewa-
tered gravel (Becker and Neitzel, 1985; Neitzel et al., 1985; Reiser and White, 1983;
Stober et al., 1982), if they remain moist and are not subjected to freezing or high
temperatures. The necessary moisture may originate from subsurface river water or
from groundwater. If the subsurface water level drops too far, the inter-gravel spaces
will dry out, and the eggs will desiccate and die. Thus, redd dewatering is not always
lethal or even harmful to eggs; however, site conditions, weather, and duration of
exposure will all affect survival. Because alevins rely on gills to respire, dewatering
is lethal (Neitzel et al., 1985; Stober et al., 1982). Alevins can survive in subsurface,
inter-gravel flow from a river or groundwater source. If inter-gravel spaces are not
obstructed with pea gravel, sand, or fines, some alevins will survive by descending
through inter-gravel spaces with the declining water surface (Strober et al., 1982).
Both alevins and eggs may die from being submerged in stagnant water. Standing
inter-gravel water may lose its oxygen to biotic decay, and metabolic wastes may build
up to lethal levels. A redd can be dewatered between spawning and hatching with-
out harm to the eggs under some circumstances, and in one situation a hydropower
facility is operated to allow limited redd dewatering (Neitzel et al., 1985). However,
in most Pacific Northwest rivers, anadromous fish spawn over an extended period.
Different species spawn in different seasons and individual species may spawn over
a range of 2 to 6 months. As a result, when eggs are present, alevins and fry are also
present, both of which are highly vulnerable to flow fluctuations.
s pawnIng I nterferenCe
Bauersfeld (1978b) found that repeated dewatering caused Chinook salmon to
abandon attempts to spawn and move elsewhere, often to less desirable or crowded
locations. Hamilton and Buell (1976) performed a highly detailed study using obser-
vation towers situated over spawning beds to track activity on the spawning bed
and to observe individual tagged fish. They observed that spawning Chinook were
frequently interrupted by flow fluctuations. Females repeatedly initiated redd dig-
ging and then abandoned the redd sites when flows changed. The authors concluded
that flow fluctuations decrease viability due to untimely release of eggs, failure to
cover eggs once they were released, and a failure of males to properly fertilize eggs
laid in incomplete redds. Other researchers offered conflicting conclusions. Stober
et al. (1982) noted that Chinook salmon successfully spawned in an area that was
dewatered several hours a day, and Chapman et al. (1986) found that 8 hours a day of
dewatering still permitted successful spawning.
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