Juvenile salmonid utilization of floodplain rearing habitat after gravel augmentation in a regulated river

Background/Question/Methods

Gravel augmentation is used in sediment-starved streams to improve salmonid spawning habitat. As gravel is added to river channels, water surface elevations rise in adjacent areas, activating floodplain habitat at lower flows and floodplains inundate more frequently, potentially affecting the quantity and quality of juvenile salmonid rearing habitat.  We analyzed five years of juvenile Chinook salmon Oncorhynchus tschawytscha and steelhead O. mykiss data from snorkel surveys before and after gravel augmentation in a regulated river in California’s Central Valley.  We measured the quality and quantity of rearing habitat (current velocity and areal extent of inundated riparian vegetation) following gravel placement and tested whether these factors affected juvenile abundance.

Results/Conclusions

Gravel augmentation increased floodplain extent, and rearing habitat conditions improved significantly.  Juvenile abundances increased significantly for both species following augmentation; however, the strength of the relationship between abundance and habitat variables was greater for smaller fish.  These results suggest that, in addition to enhancing salmonid spawning habitat, gravel augmentation can improve rearing habitat where channel incision and/or regulated hydrographs disconnect floodplains from main river channels.