Evaluating Ecological and Geomorphic Responses to Stage 0 Restoration

Recently there has been a growing application of river restoration techniques concentrating on restoring geomorphic processes that encourage deposition and habitat development over time. The approach is well described in the literature by Cluer and Thorne (2014) as restoration to a Stage 0 condition of the Stream Evolution Model. These projects are expected to result in more diverse, resilient, and biologically productive river systems. However, ecological and geomorphic responses to Stage 0 restoration have yet to be fully documented, and traditional monitoring strategies are largely ineffective due to the scale and complexity of the projects. This Evaluating Ecological and Geomorphic Responses to Stage 0 Restoration Monitoring Project is a multi-disciplinary study to examine short-term responses to restoration on the South Fork McKenzie River, apply new monitoring approaches, define the Stage 0 practice, and synthesize existing data from similar restoration sites in Oregon. Due to COVID-19 and the Holiday Farm Fire, partner cooperation was limited to just the Oregon Department of Fish and Wildlife. Adult salmonid monitoring was incidentally reported and was never intended to be a component of this project.

The Upper Willamette River Conservation and Recovery Plan for Chinook Salmon and Steelhead (Recovery Plan) (ODFW and NMFS, 2011) identify McKenzie River Chinook as the “genetic legacy population” of the Upper Willamette River (UWR) Evolutionarily Significant Unit and one of four core populations. The McKenzie River is considered one of the most important remaining areas for natural production in the Willamette Basin, and the recovery and sustainability of UWR Chinook will in large part be dependent upon the McKenzie River population. The proposed monitoring project focuses on the lower South Fork McKenzie River, which is of particular importance to UWR Chinook. Historically, the broad alluvial South Fork McKenzie River valley contained a series of complex, multi-thread channels with abundant gravel, pools and large wood. The South Fork McKenzie River watershed is contained within two geologic provinces, the High Cascades and Western Cascades. The High Cascades are characterized by young volcanic rock that allows snowmelt and rainfall to percolate deep into the ground before emerging through a series of springs in the headwaters for the McKenzie and South Fork McKenzie Rivers.

Streams within the Western Cascades are more heavily influenced by rain runoff and prone to landslides that deliver the large wood, gravel and other sediment needed to create complex habitat within rivers and floodplains. The combination of cool, clean water, and consistent flows emerging from springs and high levels of large wood and sediment originating highly dissected landscapes in the Western Cascades historically combined to create an excellent habitat for salmon in the South Fork McKenzie River.

While the South Fork McKenzie River has been significantly altered over the course of the last century, resulting in impaired habitat for salmon, significant investments are being made by the US Army Corps of Engineers to mitigate the impact of Cougar Dam on salmon and by the US Forest Service (USFS) and their community partners such as the McKenzie Watershed Alliance (MWA). In 2018 and 2019, the USFS and MWA implemented two significant floodplain restoration projects on 1.4 miles and 210 acres of the South Fork McKenzie River downstream of Cougar Dam. These restoration projects removed over 120,000 cubic yards of confining floodplain features (berms, old roadways, floodplain deposits) and aggraded over 1.2 miles of incised channels. Initial results and observations show that floodplain connectivity has increased dramatically, and that gravel and fine sediment have been restored to the system. The addition of nearly 4.000 pieces of large wood and whole trees throughout the restoration project area is sorting sediment and redirecting flows throughout the valley bottom, helping to create and maintain pools and complex habitat while aiding in the retention of nutrients and sediment within the system.

While encouraging, the initial results documented by project managers on the South Fork McKenzie River have not been fully vetted or quantified. The lack of a comprehensive monitoring approach has made it difficult to incorporate design elements used on these large-scale projects elsewhere in the Willamette River basin or throughout Oregon. The proposed monitoring project will benefit salmon and their habitat by evaluating the linked physical and ecological responses to the restoration projects. The intent is to quantify these responses to better understand the potential and/or limitation of applying the design approach used on the South Fork McKenzie elsewhere in the Pacific Northwest.