Model evaluation of Stage 0 river treatment on juvenile spring Chinook in the South Fork McKenzie River, Oregon

Abstract

Low-tech river habitat restoration techniques, including Stage 0 treatments, are increasingly applied but often lack robust evaluation of their effects and benefits. In 2018, 1 km of the South Fork McKenzie (SFMK) River, Oregon was modified to Stage 0 conditions for the benefit of ESA-listed Chinook salmon by raising the incised channel to the geomorphic grade line, reconnecting relic side channels, increasing floodplain connectivity, and distributing large wood throughout the reach. Field observations indicated depths and velocities were lower at the treated site than at the untreated site. To understand how these changes in physical habitat may translate into changes in juvenile Chinook length, abundance, and emigration timing, this study combined field observations with the individual-based model inSALMO. Model results indicated that the new habitat conditions can produce longer outmigrants, a result of rearing longer in the treated reach than in the untreated reach. The treated reach also produced more outmigrants at the end of the season in dry and wet water years, a result of more higher quality juvenile habitat conditions (i.e., lower velocities and depths, more cover) that favored fish remaining in the reach. No evidence of the treatment on spawning or incubation success was found. Numerical experimentation indicated that, under both treated and untreated conditions, outmigrant abundance was more sensitive to changes in temperature than to food resources due to the particularly low water temperatures released from an upstream dam impacting hatching success within this site. Mean outmigrant length was more sensitive to changes in food availability than to temperature. Collectively, the model results demonstrated that the Stage 0 activities at SFMK may increase juvenile length and number of reared individuals, though results should be evaluated across the diverse styles of Stage 0 projects. Further, the methods reflect the utility of moving beyond species-habitat indices as a tool in evaluating restoration practices.