Effects of flow variation on channel and floodplain biota and habitats of the Sacramento River, California, USA

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Abstract
Despite progress in the development of conceptual models of river processes, the validation and application of these models to conservation may be limited by a deficit of tools for intermediate‐scale (1–100km) reaches. A model was developed to examine the effect of variation in flow on the responses of two trophic levels in a large temperate river (Sacramento River) and its seasonal floodplain (Yolo Bypass). Field data and hydrologic simulations were evaluated for three hydrologically diverse years. The simulations showed much more hydrologic variability in the floodplain than the river, with greater total surface and shallow area, longer hydraulic residence times, and lower water velocities for the floodplain. Chlorophyll a levels were significantly higher in the floodplain than in the river, and were negatively associated with flow. These results were consistent with longer hydraulic residence times, increased surface area of shallow water, and warmer water temperatures. Copepods and cladoceran densities were similar in the river and its floodplain, and were mostly negatively associated with flow. There were significantly higher densities of Diptera and terrestrial invertebrates in the floodplain than the river. Diptera densities in the floodplain were positively associated with flow. These results provide evidence of the incremental value of floodplain habitat to the conservation of large rivers. In particular, it appears that providing river–floodplain connectivity can enhance production of lower trophic levels at relatively rapid time scales, probably benefitting secondary consumers, including Chinook salmon (Onchyrhynchus tshawytscha). Copyright © 2004 John Wiley & Sons, Ltd.