Considerations Related to Equal Monthly Volumes

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Af % Considerations Related to Equal Monthly Volumes • The AMWG developed twelve resource goals in order to maintain the "balance" between impacts to electrical power and improvement in downstream environmental resources. Experimentation should be focused on environmental improvement while minimizing the impact to the electrical power resource (EIS ROD). • The proposal for equalized monthly volumes goes against the this approach by focusing exclusively on sediment conservation without consideration of other downstream resources and without attempting to accomplish environmental improvement at minimum power loss. • The necessity for a transition to equalized monthly volume operation is not supported by the scientific evidence: • The sediment modeling results presented by GCMRC to the TWG at its January annual reporting meeting compared sediment transport under MLFF (with fall steady flows), MLFF without daily fluctuations, equalized monthly volumes (EMV), and steady year - round flows. Given a starting input of 1.5 million metric tons of Paria sand, the results indicate that all, or nearly all, of the sand would be vacated by the end of one year under all four scenarios.' • When comparing sediment transport between MLFF and EMV, less sediment appears to be vacated under MLFF than EMV for the initial four months. For the remaining 8 months of the modeling period, the two trajectories are nearly identical. The EMV line actually crosses the zero- line (pre -input sediment condition) before the MLFF line. Only in the last two months of the year does the MLFF line dip below the EMV line. o The model is sensitive to the timing and amount of sediment input from the Paria River and could affect the results and the comparison among scenarios. o Sediment input from the Little Colorado River (LCR), or other side drainages, could affect comparisons among scenarios that only consider input from the Paria River. o Because most of the sediment is found below 8,000 cfs (up to 90%), it is unclear how total sediment relates to habitat availability under MLFF or EMV. o Sediment grain size greatly affects transport rates, and is not considered under the analysis by GCMRC. o It is likely that HFEs can increase sediment retention over the short-term and that flows post -HFE will transport sediment out of the system. It is unclear what the relationship between sand inputs, frequency of HFEs, and flow operations should be based on current modeling results. Because sediment inputs are uncertain, this creates even greater uncertainty in speculating whether one flow regime is likely to meet resources goals. ' The graphical representation of the modeled comparison among four operational scenarios: Example — Evaluation of dam operations, is attached.