Estuary Technical Group reports on tidal wetland restoration in Tillamook

By Laura Brophy
December 2020

The Southern Flow Corridor project in Tillamook County is one of Oregon’s largest tidal wetland restoration projects, with 443 acres of tidal marsh and swamp restored in 2016. Since 2013, the Estuary Technical Group has teamed with Oregon State University, the Confederated Tribes of Siletz Indians, the University of Oregon, and the Tillamook Estuaries Partnership to monitor pre-restoration conditions and post-restoration recovery of ecosystems at this amazing site. The team is currently finishing its final report, and we've highlighted some results below. Look for the full report on the ETG reports page soon!

The Southern Flow Corridor is located at the south end of Tillamook Bay on the northern Oregon coast. Tidal marsh reference sites (purple and orange areas) are located nearby.

Before restoration, the Southern Flow Corridor site was diked agricultural land. Much of the site was formerly pasture and had heavy growth of non-native, invasive reed canarygrass (foreground and middle ground of photo above). The 10-foot-tall white poles in the photo mark Estuary Technical Group monitoring locations.

Restoration at the Southern Flow Corridor included removal of dikes (not shown), excavation of tidal channels to bring back tidal flows (left), and installation of setback dikes and tide gates to protect neighboring property (right).

After restoration, reed canarygrass, a non-native invasive plant species, died back across much of the Southern Flow Corridor (left). Overall, non-native plant cover decreased across most of the site after restoration (right panel, blue dots), while non-native plant cover remained about the same at reference sites (right panel, green dots).

Native tidal marsh species returning to the Southern Flow Corridor site include Baltic rush (left), Pacific silverweed (center), and Lyngbye's sedge (right). Photos by Chris Janousek.

Vegetation change at Southern Flow Corridor was driven mainly by the restoration of tidal flooding and salinity. Salinity across the site increased after restoration (left panel, blue dots); salinity increased the most at lower elevations (right panel) where tidal inundation was most frequent.

The most dramatic changes in vegetation were seen on the lower parts of the site, which are flooded most frequently by the tides.

There was less vegetation change on the higher and less salty parts of the site, where invasive reed canarygrass still hangs on (above). Photo by Chris Janousek.

The Confedered Tribes of Siletz Indians monitored fish use of the restoration site by seining in the channels.

Tidal wetlands provide important habitat for young salmon, providing them with rich food sources and a sheltered environment for rapid growth before they enter the ocean. Fish monitoring showed that after restoration, twice as many juvenile chinook used the Southern Flow Corridor compared to pre-restoration, and juvenile chum salmon were six times more abundant than before restoration. Coho salmon used the site both before and after restoration.

This project was funded by the National Oceanic and Atmospheric Administration (NOAA), U.S. Fish and Wildlife Service (USFWS), and the Oregon Watershed Enhancement Board (OWEB). Many partners collaborated with related research projects as well as in-kind contributions of labor and materials. We are grateful for the opportunity to study the results of restoration at Southern Flow Corridor.