Estimation of Surface and Intragravel Water Flow at Sockeye Salmon Spawning Beaches in Iliamna Lake, Alaska

A new methodology for estimating water flow rate was modified and tested at two island beaches in Iliamna Lake, Alaska, that are used as spawning sites by sockeye salmon Oncorhynchus nerka. Hydrated plaster of Paris, which dissolves at a rate closely correlated with water flow and temperature, was fabricated into cylinders (35-cm2 surface area) and deployed for approximately 48-h intervals underwater at the gravel surface and 15 cm below the gravel surface to water depths of 3.5 m before and during the 3 week sockeye salmon spawning period in August. Water flow was significantly greater in shallower water, both at the gravel surface and within the gravel. Water flow also varied across these spawning beaches at 1.5-m isobaths, and flow was strongly correlated with wind conditions during the deployment period. Use of plaster of Paris cylinders was instructive in identifying relative water flow rates within and among sites that would have been difficult to measure with conventional methodologies but that may be critical to spawning site selection by sockeye salmon and incubation success at these island beaches.