Linking Marine Ecosystem Response to Shoreline Armor Removal and Large Dam Removals in the Elwha River and Nearshore, Washington, USA

Shaffer, J.A.; Oxborrow, B.; Parks, D.S.; Maucieri, D.G., and Michel, J., 0000. Linking marine ecosystem response to shoreline armor removal and large dam removals in the Elwha River and nearshore, Washington, USA.

Large in-river dams and shoreline armor have a significant negative effect on coastal hydrodynamic and ecosystem processes. Armor removal (AR) is a well-documented shoreline restoration tool, and removal of large dams is proving to be an extremely effective tool to restore riverine ecosystem processes. However, nearshore ecosystem restoration associated with dam removals (DRs) is incomplete when shoreline impediments, including shoreline armoring and lower river alterations, remain, and linkages between dam and shoreline ARs are not well understood. In this study, nearshore ecosystem processes and function restoration response to large DRs and shoreline AR are assessed. Two nearly century-old large dams in the Elwha River watershed in the NW United States were removed during 2011–14, which liberated upward of 18 million tonnes (Mt) or approximately ∼9 million m³ of silt, sand, and gravel to sediment-starved, armored, and unarmored shorelines. Within 1 year of the initiation of DR, unarmored shorelines in the drift cell broadened, flattened, sediment fined, and large woody debris (LWD) volumes significantly increased. Armored shorelines continued to be steep and coarse grained. In 2016–17, approximately 4700 m³ of large riprap (shoreline armor) was removed from more than 650 m of the armored Elwha River east delta reach drift cell. Following AR, previously eroding shorelines broadened, sediment fined, LWD volumes increased significantly, and beach wrack metrics resembled non-armored beaches. These changes followed AR and did not occur at unarmored DR or control treatments. Invertebrate communities also responded to dam and armor removal (DAR) and showed increasing trends every year for 3 years after the project. It is concluded that only partial nearshore ecosystem restoration occurs from large DR when shoreline armoring that impairs nearshore hydrodynamic processes remains and that full ecosystem restoration of the nearshore associated with large DRs is obtained by restoring impaired shorelines along with DRs.