Abstract
Dam construction, gravel mining, and water diversions have degraded river channels in Central Valley streams, and salmon have been affected by these changes. To mitigate this damage and improve egg deposition sites, habitat restoration often involves adding gravel and structural features such as woody debris and boulders. While structural features have shown the ability to capture spawning gravels, biologists do not have a detailed understanding of how structural features affect salmonid reproduction. This research tested several ideas, namely (1) that large woody debris and boulders can influence hyporheic flow, and (2) that changes in hyporheic flow and water quality around structure result in increased survival of Chinook salmon (Oncorhynchus tshawytscha) embryos.
Project data were collected in an eight-kilometer reach of the lower Mokelumne River, California, within three independent river sections. Located in upstream and downstream spawning reaches, nine paired sites were studied. A paired site consisted of an area associated with structure and an adjacent area lacking structure. At each paired site surface water velocity and hyporheic flow was measured, dissolved oxygen, intergravel water temperatures, pH, and electrical conductivity measurements were taken, and egg incubation tubes containing Chinook salmon embryos were buried.
Results indicated that structure significantly increases the range of surface water velocities relative to sites lacking structure. The larger differences in surface water velocity around sites containing structure created pressure differences that resulted in significantly stronger vertical hydraulic gradients within the subsurface. These results further support the idea that structure provides habitat heterogeneity within the surface water and hyporheic environments. Structure did not significantly increase the development, growth, and survival of salmon embryos, relative to sites lacking structure. However, at low gradient downstream sites, structure improved the survival, development, and growth of embryos, relative to sites lacking structure. When spawning habitat is limited, management and enhancement plans should consider the benefits structure provides to incubating salmonid embryos in reaches containing marginal habitat. Adding structure in suboptimal spawning reaches will create some locations adequate for salmonid incubation, which would otherwise be considered unsuitable.