Sediment budget for salmonid spawning habitat rehabilitation in a regulated river

Abstract

Bed elevation, feature adjustments, and spawning use were monitored at three Chinook salmon ( Oncorhynchus tschawytscha) spawning habitat rehabilitation sites to measure project longevity in a regulated river. Sites enhanced with 649–1323 m 3 of gravel lost from 3–20% of remaining gravel volume annually during controlled flows of 8–70 m 3/s and 2.6–4.6% of placed material during a short-duration (19 days) release of 57 m 3/s. The oldest site lost ∼50% of enhancement volume over 4 years. Of the mechanisms monitored, gravel deflation was the greatest contributor to volumetric reductions, followed by hydraulic scour. Spawning, local scour around placed features, and oversteepened slopes contributed to volumetric changes. As sites matured, volumetric reductions decreased. Sites captured as much large woody debris as was lost. While complexity is an extremely important aspect of ecological function, artificial production of highly diverse and complex habitat features may lead to limited longevity without natural rejuvenation.