Model Development and Estimation of Short-term Impacts of Dam Removal on Dissolved Oxygen in the Klamath River

This report summarizes modeling approaches, assumptions and results of a modeling evaluation of short-term variations in dissolved oxygen (DO) due to sediment releases associated with the removal of one or more of four dams in the Klamath Hydroelectric Project (Figure 1; FERC Project No. 2082). Estimates of the volume of sediment retained by these dams include 10.0 million m3 (13.1 million yd3) (Greimann et al. 2011), 11.1 million m3 (14.5 million yd3) (Eilers and Gubala 2003), and 11.1 million m3 to 15.6 million m3 (14.5 to 20.4 million yd3) (GEC 2006), with a high proportion of fine sediments, organic matter, and nutrients (Shannon & Wilson, Inc. 2006). Sediment transport modeling of the impacts of dam removal on suspended sediment in the lower Klamath River indicates high short-term concentrations of suspended material (i.e., peak values of 9,000–13,600 mg/L) may occur immediately downstream of Iron Gate Dam for 2–3 months following reservoir drawdown under the Proposed Action (Greimann et al. 2011, Stillwater Sciences 2008). Using a combination of in situ sampling of sediments and water quality, combined with numerical modeling, this study was developed to estimate the potential influences that re-suspension of reservoir deposits may have on DO levels in the Klamath River downstream of the dams. A numerical model was developed to help in understanding these dynamics, using approaches similar to those described in USEPA (1985, 1987).