Biological Opinions - Bureau of Reclamation

hydrograph will approximate the natural flow variability because the calculations are based on
actual real-time measurements of several hydrologic indicators including Williamson River flow.
Figure 11.15. Williamson River observed, proposed action and Observed IGD discharge for water year 2011.
Proposed action IGD daily discharge was calculated for water years 1981-2011 based on a 7-day moving
average.
11.4.1.1.4 Channel Maintenance Flows
The role of channel maintenance flows in managed river systems to maintain the integrity and
ecology of ecosystems and aquatic organisms and to facilitate sediment transport has been
widely recognized (Petts 1996; USFWS and HVT 1999; Bunn and Arthington 2002; Poff et al.
2009; NMFS 2010a). NMFS believes that over-bank flows are critical in creating and
maintaining in-channel and riparian habitat. In contrast, protracted drought conditions without
supplemental channel maintenance flows will result in extended periods of low velocity flows
and additional fine sediment deposition downstream from IGD (Holmquist-Johnson and
Milhous 2010). Protracted droughts can cause spawning gravels to become filled with fine
sediment and provide habitat conditions conducive to the establishment of aquatic vegetation,
two conditions that are favorable to the spread of C. shasta in the Klamath River Basin
(Stocking and Bartholomew 2007).
NMFS evaluated the effects of the proposed action on flood frequency relative to the POR to
evaluate whether the proposed action will result in a trend of the Klamath River hydrograph
towards, or away from, the natural flow regime in terms of channel maintenance flows (Table
11.7). Flood frequency analyses applying the Log-Pearson Type III distribution were performed
on the observed daily discharge and the modeled proposed action daily discharge for the POR at
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