Study Plan - Biological Resource Responses to Fall Steady Experimental Flows Feruary 2010

Home Browse Search

accumulation of algae or invertebrate biomass. However, algae and invertebrate drift data are less variable and easier to collect than benthic biomass data, so we propose evaluating drift response to the FSEF and using these data to make inferences about potential biomass accumulation. Experimental Design Fall Steady Flows Selecting a discharge for the FSEF and determining whether discharge should vary across years of the experiment depends on .what science questions are of greatest interest to managers. Our assumption is that managers are most interested in determining whether steady flows in the fall have a positive impact on the recruitment of juvenile humpback chub into the adult population. Varying the discharge of stable flows across years would allow scientists to evaluate whether discharge volume affects the degree of nearshore warming or habitat selection by juvenile humpback chub. However, identical steady discharge across the remaining years of this experiment would simplify interpretation of humpback chub recruitment data. That is, having the same stable discharge across the remaining years of the experiment would make it easier for scientists to attribute any trends in humpback chub recruitment observed in subsequent years to the steady flow experiment. We believe managers are also interested in maximizing the amount of backwater habitat available during the steady flows, and recently published findings based on data collected in 2008 demonstrate that there is generally greater abundance of backwater habitat at lower flows (Grams and others, 2010). In October 2008, there was about 26 percent greater area of backwater habitat at 10,000 cfs than at 12,000 cfs and an additional 50 percent more habitat at 8,000 cfs than 10,000 cfs. Based on these results, there may be reason to implement the lowest possible discharge of 8,000 cfs, if there is a desire to maximize the quantity of backwater habitat. It is important to recognize that the amount of backwater habitat available in fall 2010 and thereafter will depend on the morphology of the sandbars that create the habitat. The results of Grams and others (2010) are based on morphology measured in October 2008 and, therefore, cannot be used to predict the amount of habitat that will be available. However, because the relation between habitat quantity and discharge was similar in February 2008, before the HFE, and October 2008, 6 months after the HFE, we believe that it is reasonable to assume that the relation will be similar in 2010 through 2012. In other words, we believe that there should still be more habitat at 8,000 cfs than higher discharges, even if the total amount of habitat is different in 2010 than it was in 2008. The only time the relation is expected to be different is for the first few months following a high flow, when backwater habitat size is not expected to vary as a function of discharge (Grams and others, 2010). If water releases must be moved into other months of the year to accommodate these steady flow levels, then it is recommended that releases in July and August not be increased, as daily peak discharges during these months are already relatively high. Further increases will increase sediment transport and sandbar erosion, an undesirable side effect because it would reduce backwater area. August to September Transition in Discharge Determining whether the transition from the high discharges of August to lower discharges of September has a negative impact on organisms downstream is of longstanding interest to managers. The BO calls for study of this topic, stating the following: