Embracing disturbance along a populated river: Evidence that removing a levee and restoring the floodplain can replace a flood-prone trailer park with a refuge for salmon

Background/Question/Methods

Levees removal has become a central tool in salmon recovery since the listing of Puget Sound fall Chinook salmon. Chinook salmon inhabit mainstem rivers which often have regulated flows, armored banks, and levees. Floodplains contain agricultural, industrial, and low-density developments that pre-date contemporary regulations. To restore critical habitat, governments are purchasing intact or flood-prone properties and removing levees. The objective of this study is to test the effects of levee removal on the Cedar River in Washington State. This study addresses a regionally-significant question: Does ‘making room for the river’ allow natural processes to rebuild a patchwork floodplain that supports a productive biotic community, especially salmon? The project was completed in 2013. An armored levee was removed, logjams were built, deformable channels were excavated, tens of thousands of trees were planted. The primary indicator of project performance was low-velocity edge habitat, used by juvenile salmon for rearing and refuge. Habitat area was quantified using GPS surveys across a range of flows, before and after the project. Channel dynamics were measured from ground-surface models and orthoimagery. Time-lapse photography documented the effects of each flood. Large wood was quantified and classified by size. Fish use was quantified with snorkel surveys and redd counts.

Results/Conclusions

Habitat for juvenile and adult Chinook salmon improved within the first two flood seasons, as evidenced by increased edge habitat area and extensive spawning. A flood in March 2014 caused streambed aggradation and widened the channel by 2,000 m² over a 140-m section of river where the channel first encountered the former levee. Edge habitat area has increased by 87% from pre-project levels during high flows (at 35 cms), and similarly across the range of surveyed discharges. The greatest increases were in backwaters and side channels. Snorkel surveys demonstrate that backwaters and side channels support the highest densities of juvenile Chinook salmon (up to 2.8 fish m^-2). Adult spawners showed a striking affinity for the side channel, which, in 2014, contained 9% of all Cedar River Chinook redds, but only composed 0.3% of the potential spawning habitat in the entire river. These results imply that that levee removal, in combination with deformable side channels, can trigger geomorphic changes that increase critical habitats for juvenile and adult Chinook salmon. These changes may occur in spite of flow regulation; an important issue for restoration ecology in managed rivers.