A framework for assessing restoration performance: a case study from the Nisqually River Delta

The restoration of the Nisqually River Delta represents one of the largest efforts toward re-establishing the ecosystem function and resilience of modified habitat in the Puget Sound, particularly for anadromous salmonid species. Here we use monitoring and applied studies to assess restoration performance within a tiered framework of Opportunity, Capacity, and Realized Function. Opportunity is evaluated with physical metrics for outmigrating salmon to access and benefit from the expansion of newly restored habitat. Capacity refers to specific functions of the restored habitats, such as the capacity to produce prey resources for outmigrating salmon. And realized function is the integration of Opportunity and Capacity metrics so that outmigrating salmon would benefit from accessing and foraging in newly restored habitats. We monitored a variety of physical parameters to measure changes in opportunity potential from historic, pre-restoration, and post-restoration habitat conditions at several sites across the delta. These parameters included channel morphology, water quality, tidal elevation, and landscape connectivity. We conducted fish catch surveys across the delta to determine if salmon were utilizing restored estuary habitat. Overall major channel area increased 42% and major channel length increased 131% from pre- to post-restoration conditions. Furthermore, the results of our tidal inundation model indicated that major channels were accessible up to 75% of the time, as opposed to 30% pre-restoration. Fish catch surveys showed that outmigrating salmon utilized newly-accessible habitat as quickly as one year post-restoration. The presence of salmon in restored tidal channels confirmed rapid post-restoration increases in opportunity potential on the delta despite habitat quality differences between restored and reference sites