Assessing Feasibility and Benefits of Gravel Augmentation in a Dammed Southern Oregon River

Gravel augmentation below dams is growing in popularity as a watershed rehabilitation technique to replace degraded salmonid spawning habitats. Here we examine the potential for introducing gravel below Applegate Dam in the mainstem Applegate River in southwestern Oregon. The Applegate River is the second largest tributary to the Rogue River and provides habitat for several salmonid species and other native fishes. The Applegate River was impounded by Applegate Dam, creating the 988-acre Applegate Reservoir in 1980. We describe the amount and quality of sediment trapped by Applegate Reservoir, historic and current gravel abundance and permeabilities below the dam, changes in average substrate size distribution, and fish spawning metrics. It was estimated over 2.4 million tons (1.3 million cubic yards) of sediment trapped in the reservoir were between 7 and 90 mm and could have been used for salmonid spawning.  Substrate surveys of the first five miles below the dam revealed gravel was 10 times less abundant in 2009 than in 1972 prior to dam construction; in downstream reaches there was about 75% of the gravel abundance observed in 1972 present in 2009.

In 2009, gravel was twice as abundant in surveyed potential spawning areas in the lower river than in sites close to the dam. Although spawning-sized gravel was relatively scarce in the upper reach of the Applegate River below Applegate Dam, permeability and fine sediment measurements were comparable throughout the river, suggesting the dam had little impact on spawning gravel quality. Chinook salmon spawning densities varied greatly and were related to gravel abundance and proximity to the dam. Immediately below the dam, densities exceeded 200 redds per mile; however these densities fell sharply to 8 per mile about 0.25 miles below the dam, staying low before normalizing at 35 redds/mile about 4 miles below the dam.

Habitat models predicted gravel augmentation would do little for coho salmon habitat in the mainstem Applegate River, but could have benefits to winter steelhead, and especially fall Chinook salmon habitat; possibly reducing redd superimposition below the dam. Gravel augmentation could have other ecological benefits including increased sinuosity and bar formation, and decreased summer water temperatures, a high priority in Applegate watershed restoration. We recommend targeting active gravel augmentation in several locations and researching passive augmentation for others. Mining accumulated reservoir sediment deposits could provide gravel for augmentation and commercial needs, but is currently prohibited under current policy, which should be examined and reevaluated.