University of California Water Resources Center

To determine whether large wood (LW, ≥1-m length, ≥10-cm diameter) plays a role in Chinook salmon redd (i.e. egg nests) placements in a regulated, medium-sized, Mediterranean-climate river, characteristics of 542 large wood pieces, locations of 650 redds, and habitat unit delineations (riffle, run, glide, pool) were collected during a spawning season along a 7.7 km reach directly below Camanche Dam on the Mokelumne River (average width 31 m). Large wood was regularly distributed across the study reach with an average of 70 LW pieces km-1. Some LW clustering was evident at islands. Chinook spawners built 75% of observed redds at spawning habitat rehabilitation sites, and 85% of redds were within one average channel width of large wood. At the hydraulic scale of ~10-1 channel widths, redds were within a 10 m radius of large wood 36% of the time. These results suggest that spawners had the opportunity to utilize large wood as cover and refugia. In the lower 4.7 km where marginal habitat was prevalent, redds were within a 5 m radius of large wood 21% of the time and within a 2.5 m radius 10% of the time, indicating use of the hydraulic properties of instream large wood structures. Results from randomized tests indicate that large wood-redd interactions systematically occurred at a greater rate than by random chance alone in the lower 4.7 km, but not in the upper 3 km, which implies that large wood aids spawning in marginal habitats.