Biological Opinions - Bureau of Reclamation

temperature, and changes in water temperature rates estimated by the RBM-10 model for the
Klamath River at the confluence of the Shasta River and Scott River, respectively.
Results and Summary
The change in mean monthly water temperature (T-Rate) with mean monthly discharge (cfs) for
each month (March to June) and simulated year (2012 to 2019) is presented in Table 1. In
general, higher discharges resulted in less warming of the river (lower temperature rates)
downstream of Iron Gate Dam to the Shasta River (14 RMs below Iron Gate) and temperature
change rates decreased as flows progressed downstream to the confluence of the Scott River (47
RMs below Iron Gate). The addition of cold snow melt runoff contributions from the Scott
River, in combination with a decrease in the effects of the thermal mass in Iron Gate reservoir,
are likely responsible for the decrease in the warming rates observed at this location. For
example, simulation results for both May of 2013 and June of 2014 show that water temperature
in the mainstem Klamath River between Iron Gate Dam and the confluence of the Shasta River
were warming, and were then cooling (negative temperature rates) at the confluence of the Scott
River (Table 1).
The total change in water temperature between Iron Gate Dam and the confluence of the Shasta
River and Scott River is displayed in Table 2. The maximum change in the modeled monthly
water temperature at the confluence of the Shasta River never exceeded 1.30 °C and only
exceeded 1 °C for 9 of the 64 scenarios (14%) examined. The differences in water temperature
between paired discharge model scenarios were always 0.5 °C or less, regardless of the
magnitude in the difference between discharged simulated in each pair. The maximum change in
simulated water temperatures at the confluence of the Scott River only exceeded 2 °C on one
occasion and was generally less than 1.6 °C. Differences in water temperature between paired
discharge scenarios never exceeded 0.6 °C, which indicates that discharge (within reasonable
operating ranges) has little effect on water temperature and that effect diminishes further
downstream as the influence of Iron Gate reservoir is diminished and ambient conditions begin
to control water temperatures.
Although higher discharges generally resulted in less warming of water (lower rates of
temperature change) downstream, the differences between the rates in which water temperatures
changed by river mile relative to flow magnitude were found to be very small (< 0.036 °C) at the
confluence of the Shasta and was even less (< 0.012 °C) by the time water passed by the
confluence of the Scott River (Table 3). Examination of these paired data indicate the that
discharge has very little effect on the rate in which temperatures change (warm or cool)
downstream of Iron Gate Dam to the Shasta River and the Scott River confluence for those
months and discharges considered.
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