Environmental Engineering Reference
In-Depth Information
of a stream, either as a behavioral response to stable flows or in response to aquatic
invertebrate populations that thrive along the water's edge under stable flows. These
hypotheses should be thoroughly tested before they are applied to mitigation practices.
J
uvenILe
e
mIgratIon
(s
aLmonId
d
rIft
)
Flow fluctuations in an experimental stream channel caused juvenile Chinook to emi-
grate downstream (McPhee and Brusven, 1976). The pre-test rate of emigration under
stable flows was about 1% a day. Severe flow fluctuations (from 51 L/s to 17 to 3 to
51, with each flow being held for 24 hours) caused 60% of the Chinook to emigrate.
A high rate of emigration continued even after initial flows were reestablished. A
less severe daily fluctuation in flow (between 51 and 17 L/s for four 24-hour periods)
caused 14% of the Chinook to emigrate. Alternating flows between 51 L/s and 17 L/s
every 24 hours cause a greater rate of emigration than alternating the same flows
every 12 hours. Most of the emigration occurred at night, a behavior observed in
aquatic invertebrates. The behavioral response to flow fluctuations and how this may
affect the juvenile salmonid rearing capacity is not well understood. Under conser-
vative ramping requirements, flow fluctuations may cause downstream emigration,
driving many fish to habitats that may be less desirable or overcrowded and leav-
ing upstream rearing habitats underutilized. This could be a particular concern in a
stream with a falls or other barrier that prevents juveniles from returning upstream.
I
nCreased
p
redatIon
It has been suggested that juvenile fish forced from the river margins as a result of
declining flows suffer from predation by larger fish (Phillips, 1969). This effect does
not appear to have been documented anywhere, but it is a credible hypothesis under
some circumstances.
a
quatIC
I
nvertebrates
Like fish, aquatic invertebrates are not necessarily adapted to unnatural drops
in flow. Cushman (1985) extensively reviewed the effects of flow fluctuations on
aquatic life, especially aquatic invertebrates. Interested readers are encouraged to
DID YOU KNOW?
Stream stages are not always cooperative, so it's not uncommon for someone
to have to go measure a stream at 2:00 in the morning during a storm, some-
times in freezing conditions. Also, the stream can be uncooperative in that
it changes—a big storm may come along and scour out bottom material of
a creek or lodge a big log sideways in the creek, or sometimes do both at the
same time. These kind of changes result in changes in the relationship between
stage and discharge (USGS, 2014b).
