HYPORHEIC FLOW, PORE WATER GEOCHEMISTRY AND BAR FORM INFLUENCE ON SALMONID SPAWNING IN THE AMERICAN RIVER, CALIFORNIA

Human impacts have reduced native salmon and steelhead production in the American River, California. Limited spawning gravel (habitat) is an obvious factor, and this study examines the hydrogeologic and geochemical conditions that relate to spawning site selection by anadromous fish.

Field measurements were used to determine surface water velocity, influent and effluent conditions, temperature, dissolved oxygen content, and pore water field parameters. Nested mini piezometers allowed periodic sampling of intergravel conditions. Surface water and pore water samples were analyzed in the lab for major and trace element concentrations, and results of all measurements were compared to spawning density.

Results indicate that spawning Salmonids have a strong preference for pool tailouts and riffles. These geomorphic features direct surface flow and influence hyporheic flow patterns. Physical and geochemical conditions that are common in heavily used spawning areas include strong influent OR effluent conditions, current velocity that ranges from 1-2 m/s, water depth less than 0.5 m, and pore water dissolved oxygen content higher than 3 mg/l.

Major element geochemistry of gravel pore waters does not indicate a clear relationship to spawning density, although there are measurable seasonal changes in some components. Chloride, sulfate and sodium concentrations are highest after prolonged Spring meltwater events, and decrease during the Fall dry season. This suggests contribution from upstream sources. Potassium concentration is highly variable, and increases in pore waters during low flow Fall conditions. This suggests rock-water interaction and dissolution of soluble substrate material. Calcium and Magnesium are relatively invariant. Pore waters equilibrate with new flow conditions over a period of weeks or months, and are relatively homogeneous to depths of 90 cm and between gravel bars that are spaced several km apart.

These results suggest that physical factors have the greatest influence on spawning site selection, with influent and effluent conditions, surface water velocity, and surface water depth most closely related to spawning site selection and habitat quality. Results can be used to plan future gravel restoration projects and manage a complex aquatic system.