An experimental study of stranding of juvenile salmonids on gravel bars and in sidechannels during rapid flow decreases

Juvenile salmonids can become stranded over gravel bars or trapped in off-channel habitat during rapid flow decreases that often occur in regulated rivers. In a stream channel experiment that simulated stranding over a gravel bar, more juvenile chinook salmon (Oncorhynchus tshwytscha) were stranded when the water was 6°C compared with 12°C. The rate of flow decrease was not a significant factor in the incidence of stranding. In contrast, the number of chinook and coho (O. kisutch) salmon juveniles that became trapped in side channels built in the stream channel increased with increasing rate of dewatering, and for coho salmon, more fish were trapped at night compared with the day. Even at the slowest rate of flow decrease some fish remained in the side channels after the channels became disconnected from the main flow. My results suggest that mortality as a result of trapping in side channels or pot-holes will be decreased, but may not be eliminated, by flow ramping, although this result needs to be corroborated with field studies.