Experiment in Stream Restoration

Abstract

Aquatic habitats in a deeply incised sand-bed channel were modified by adding 1,380 t of stone and planting dormant willow posts. Restoration structures (groin extensions and longitudinal toe protection) were designed as complements to existing channel stabilization works. Prior to restoration, base-flow aquatic habitats were characterized by uniform conditions, little woody debris or riparian vegetation, shallow depths, and sandy bed material. The stage-discharge relationship, channel geometry, and bed material size were unaffected by restoration, but the average depth of scour holes adjacent to extended groins increased from 32 cm to 72 cm, and pool habitat in the lower half of the study reach increased from 2.9% to 14% of water surface area. Median water depth at base flow increased from 9 cm to 15 cm. Woody vegetation cover on one side of the channel increased from 38% to 78%. Fish numbers tripled, median fish size increased by 50%, and the number of species increased from 14 to 19. Groin extensions experienced partial failure due to erosion of sand from underneath stones.