Patterns of Gravel Scour and Fill after Spawning by Chum Salmon in a Western Washington Stream

The patterns of gravel scour and fill during the incubation period of chum salmon Oncorhynchus keta were studied in Kennedy Creek, a low-gradient alluvial channel draining into southern Puget Sound, Washington. In 1991–1992, scour occurred during two storms having estimated return intervals of 1.4 years and less than 1 year. Scour to median egg pocket depth (0.2 m) occurred at 20% of the monitored locations during the greater event and at 1.3% during the lesser event. Differences in depth of scour were observed between the two study reaches and among habitat units (riffles, lower riffles, glides, pool tailouts, and pool lateral bars) within reaches. Average depth of scour in a relatively simple, straight, and narrow reach of the creek was 0.075 m, whereas average scour was nearly twice as deep (0.140 m) at a more complex, sinuous, and wide reach. Scour to median egg pocket depth occurred at 28% of the monitors in the complex section compared with only 9% in the simpler section. Average scour depth and the percentage of monitors scoured to median egg pocket depth were greater in pool-associated habitats (pool lateral bars and pool tailouts) than in riffle-associated habitats (riffles, lower riffles, and glides). Chum salmon often used sites such as pool tailouts that would be suitable under moderate flow conditions but that would be prone to scour under higher flow events. The relatively widespread scour to egg pocket depths that we observed during a bank-full event indicates that scour can be a significant source of egg-to-fry mortality for salmonids spawning in low-gradient pool–riffle channels.