Aquatic assemblages and their relation to temperature variables of least-disturbed streams in the Salmon River basin, central Idaho, 2001

In the late 1990s, Idaho’s established stream
temperature criteria for the protection of coldwater
biota and salmonid spawning were considered
inadequate because the criteria did not agree with
observed biological conditions in many instances
and did not allow for variability in environmental
condition or species diversity across a broad area
such as the entire State of Idaho.
In 2001, benthic invertebrate and fish assemblages
in 34 least-disturbed streams in the Salmon
River Basin, central Idaho, were evaluated in relation
to stream temperature and other environmental
variables. The Salmon River Basin retains
watersheds that are minimally affected by human
activities. These “natural” stream conditions provide
a basis for deriving attainable stream temperatures
that can be used to set new, and revise existing,
water-quality criteria for stream habitats
affected by human activities.
During July through September 2001, data
were collected to document the thermal regime of
least-disturbed streams, characterize the distribution
of aquatic biota at streams representing a gradient
of temperature, and describe the relations
between environmental variables and benthic
invertebrate and fish assemblages. Nine stream
temperature metrics were compared with the U.S.
Environmental Protection Agency’s criterion of
10
°
C (degrees Celsius) for bull trout spawning and
juvenile rearing. The maximum weekly-maximum
temperature at all 33 sites where temperature data
were available exceeded this criterion. The maximum
daily-maximum temperature (MDMT) at 30
of the sites exceeded the Idaho Department of
Environmental Quality (IDEQ) criterion of 13.0
°
C
for the protection of salmonid spawning; and the
maximum daily-average temperature at all 33 sites
exceeded the 9.0
°
C criterion for the protection of
salmonid spawning. The thermal regime at most
sites did not exceed the IDEQ criteria for the protection
of coldwater biota. Nine environmental
variables—water-surface gradient, discharge,
wetted channel width, width:depth ratio, aspect,
total seasonal thermal input, open canopy, riparian
canopy density, and elevation were selected for
correlation with the nine stream temperature metrics.
Elevation showed the strongest inverse correlation
with the stream temperature metrics.
Two hundred and one benthic invertebrate
taxa from the 34 sampling sites were identified.
The most abundant taxa were Baetis tricaudatus,
Oligochaeta, Tvetenia bavarica gr., Acari,
Rhithrogena, Cinygmula, Heterlimnius,
Micropsectra, Eukiefferiella devonica gr.,
Drunella doddsi, and Cricotopus. Of the 201
benthic invertebrate taxa collected during this
study, 57 taxa (present at a minimum of 5 sampling
sites) were significantly correlated with one
or more of the stream temperature metrics. Among
the invertebrate taxa collected, 32 were identified
as coldwater taxa. Of the coldwater taxa collected,
Zapada oregonensis gr. showed the strongest
inverse correlation with the stream temperature
metrics and was collected at sites where maximum
weekly-maximum temperature (based on date of
sample and 6 days prior) ranged from 11.3
° to
18.5
°C.
Ten species of fish in the families Salmonidae,
Cottidae, and Cyprinidae were collected.
Two species (bull trout and chinook salmon) listed
as threatened under the U.S. Fish and Wildlife Service
Endangered Species Act were collected.
Among all species, bull trout showed the strongest
inverse correlation between relative fish abundance
and stream temperature. Bull trout and juvenile
bull trout densities were inversely correlated with stream temperature. The probability of occurrence
of juvenile bull trout was significantly correlated
with MDMT on the basis of results from a
logistic regression model developed during this
study. However, this model differed from a similar
model developed by the U.S. Forest Service on the
basis of regional data collected in the Pacific
Northwest. The regression model based on data
collected during this study showed higher probabilities
of occurrence of bull trout at colder stream
temperatures (10
° to 15
°
C) and lower probabilities
of occurrence at warmer stream temperatures (16
°
to 21
°
C) than did the model based on regional
data. The model comparisons suggest that regional
or local differences need to be considered when
deriving stream temperature criteria.