Presenter/s: Laura Brown
Symposium Session: 2023 - 12 All that glitters is not Gold: Restoring Dredged Rivers
Topics covered: adaptive management and monitoring, dredged rivers, fish-salmon, lessons learned, riparian, temperature, and tidal
In 2019, WDFW and partners carved a ¼ mile channel through dredged material to restore off channel habitat along the lower Columbia River. Since the 1880s, anthropogenic impacts to the lower Columbia River have led to a reduction of habitat for salmonids. In the early 1900s, piles were placed along the mainstem and dredging of sand was already underway to maintain a navigation channel. These two actions began to transform the mainstem Columbia and impact salmonids, as critical shallow water habitats were targeted for dredge spoils placement. At RM 90, just upstream of the confluence with the Lewis River, over 1.5 million cubic yards of dredged spoils were placed within the mainstem along Bachelor Island by the USACE. This effectively covered nearly all existing shallow water habitat, leaving a groundwater connected floodplain lake in the center.
Starting in 2016, WDFW worked with DNR and USFWS to identify viable alternatives for a multi-species restoration project, with a particular focus on juvenile Fall Chinook and Chum salmon, waterfowl, and turtles. In 2018 and 2019, the South Bachelor Island Restoration Project excavated approximately 120,000 cubic yards of historically placed dredge material to create a 2,300 ft channel that reconnected a remnant 40 acre open-water wetland to the Columbia River mainstem. Key for waterfowl and turtles was an ability to retain water in the wetland year-round, despite significant seasonal changes in the mainstem. Additionally, it was important to design a self-maintaining channel, despite being carved through sand with a slow revegetation rate. Unique to this project was the placement of excavated material downstream of the channel along the riverbank to be slowly released back into the channel over time. Another unique component was the modification of the remnant pile-dike structure that crossed the wetland, in an effort to improve tidal exchange through the channel.
Since construction, physical and biological monitoring has been conducted. Annual topographic and shoreline surveys show that changes to channel form and shoreline embankments are occurring. Fish monitoring to date has shown that juvenile salmonids are using the newly created habitat, and that main stem and off-channel wetland water temperatures impact fish movement. Continued monitoring is crucial as this project serves as an example of what can be anticipated when attempting to restore shallow water habitat impacted by dredge material placement. As we continue to manage rivers to provide multiple benefits, determining innovative ways to restore habitat within an altered system will be critical to salmon recovery.