Aquatic Macrophyte Encroachment in Chinook Salmon Spawning Beds: Lessons Learned from Gravel Enhancement Monitoring in the Lower Mokelumne River, California

We studied the effects of colonization of the spawning habitat of Chinook salmon Oncorhynchus tshawytscha by rooted aquatic macrophytes in a regulated central California river. At least seven species of plants invaded four spawning sites below the lowest nonpassable darn during an extended period of relatively low instream flows. The depths associated with aquatic vegetation were similar to those used by spawning salmon. Velocities were significantly lower where plants took root. As plants expanded over riffles, spawning females avoided those areas. We observed significant overall reductions in vegetation after flow events predicted to be of sufficient magnitude to mobilize substrates. However. the proportion of vegetation dislodged did not strongly correlate with the proportion of substrate that we predicted would mobilize. Even so. in areas where vegetation was reduced, use by salmon spawning subsequently increased. While shear stress may be pan of the overall predictive equation for macrophyte control at spawning sites, other factors must be studied before a specific management action can be recommended.