Process-based river restoration near cities: Effects of levee removal on habitat-forming river dynamics at the confluence of the Tolt and Snoqualmie Rivers, Washington State

Background/Question/Methods

In the lowlands of Puget Sound, large rivers near cities are being restored in an effort to recover Chinook salmon. Instead of engineering artificially stable habitat features, projects are adopting ‘process-based’ designs that aim to re-establish normative rates of channel migration, wood recruitment, and flooding to produce a dynamic habitat mosaic. For example, in 2009, a 600-m levee was removed to restore river dynamics at the mouth of the Tolt River near Carnation, Washington. Evidence of project effectiveness is needed, both to improve restoration practice and to predict the consequences of future projects more accurately. The goal of our study was to determine whether levee removal restored habitat-forming processes and increased habitat area. Accordingly, lidar, orthophotos, and bathymetry were used to measure changes in the rate of channel migration, bar and side-channel formation, pool scour, and forest undercutting. Field surveys quantified changes in wood storage. Low-velocity edge habitat area, suitable for juvenile Chinook rearing, was mapped with a GPS and flow meter across a range of flows.

Results/Conclusions

Levee removal allowed the channel to migrate, to form bars, backwaters, and side channels, to scour pools, and to recruit wood, resulting in substantial rearing habitat for Chinook salmon. Between 2009 and 2011, the channel migrated laterally 14 m into the former levee position, eroded 6200 m² over 430 m of bank, and formed a 180-m side channel complex. Depositional bars doubled in number and bar area increased by a factor of five. Pool frequency tripled, though residual depths were unchanged. One-third of new pools were scoured by trees felled by the migrating channel. Though wood storage declined by 34% – as pre-existing piles disintegrated – logs over 16 m long became twice as common as more than a dozen mature cottonwood trees toppled into the river. Ultimately, low-velocity edge habitat at median daily flow levels increased by a factor of 2.7 – from 3.6 m² m^-1 to 9.7 m² m^-1. Preliminary findings suggest levee-removal was effective at restoring key habitat-forming processes in the Tolt River. These processes, in aggregate, have nearly tripled the quantity of rearing habitat for Chinook salmon. Further work is needed to determine whether these early responses accurately represent future conditions.