Biological Opinions - Bureau of Reclamation

Table 11.7. Flood frequency analysis on Klamath River for IGD gaging station observed daily discharge and
proposed action daily discharge for the period of record from 1981-2011.
Flood IGD Gaging Station Discharge (CFS)
Frequency Observed Daily Proposed Action Daily
1.5-yr Flood 3,712 3,958
2-yr Flood 5,168 5,454
5-yr Flood 9,710 10,160
10-yr Flood 13,390 14,040
25-yr Flood 18,740 19,800
50-yr Flood 23,210 24,700
100-yr Flood 28,060 30,120
11.4.1.1.5 Summary
The proposed action results in a hydrograph that approximates the shape of the natural flow
regime. However, partly as a result of operating the Project, the Klamath River annual flow
volume, magnitude, duration, flow variability and channel maintenance flows are reduced
relative to the natural hydrograph defined by the 1905-1913 discharge dataset at Keno, Oregon
(Figure 11.4). Under the proposed action, Klamath River will have lower base flows in the fall
and winter, lower and earlier peak discharge, reduced spring and summer discharge, and an
earlier return to base flow relative to the natural hydrograph (Figure 11.4). Spring and summer
flows in the mainstem Klamath River (i.e., EWA volume) have a strong positive relationship
with hydrologic conditions in the upper Klamath Basin defined by the three hydrologic
indicators: UKL net inflow, Williamson River inflow, and NRCS UKL inflow forecasts. The
relationship between EWA volume and the three hydrologic indicators ensures that spring and
summer flows in the mainstem Klamath River reflect hydrologic conditions in the upper
Klamath Basin.
Under the proposed action, Klamath River flows will have lower base flows and enhanced flow
variability in the fall and winter period compared to the observed hydrograph for the POR
(Figures 11.5 and 11.10). The spring peak discharge under the proposed action will generally
occur two weeks later than the observed POR. Additionally, compared to the POR, the proposed
action will have increased spring and summer discharge volume and a later return to summer
base flow (Figures 11.4, 11.5 and 11.10). Therefore, the proposed action hydrograph at IGD
trends towards the natural flow regime compared to the observed hydrograph at IGD for the
POR; however, both hydrographs have been reduced relative to the natural hydrograph (Figures
and 11.4 and 11.5).
The proposed action ensures daily variability will occur at IGD if variability exists naturally in
the basin. The proposed action hydrograph generally tracks daily changes in natural hydrologic
conditions and implementation of the proposed action is expected to result in enhanced flow
variability throughout the year relative to the POR (Figure 11.10 and Table 11.6). While the
proposed action enhances flow variability relative to past Project operations, the proposed action
will continue to contribute to diminished flow variability relative to a natural Klamath River flow
regime.
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