Presentation Abstract

The Elwha River Restoration Project was the largest US dam removal project to date, both in dam height and sediment released. During dam removal in 2011–2014, ~18 Mt of sediment washed downriver, and macroalgae virtually disappeared from the adjacent nearshore ecosystem. The link between current benthic light availability and sediment delivery and transport has been investigated in order to understand conditions during dam removal. Seven instrument platforms were deployed on the 10-m isobath along a 16 km transect centered on the river mouth for seven fortnightly periods in 2016 and 2017 to monitor near-bed photosynthetically available radiation (PAR), suspended sediment, wave climate, current velocity, temperature, and salinity. Water-column profiles, bed sediment, and water samples were collected during deployments. Seasonally variable chlorophyll-a and colored dissolved organic matter did not contribute substantially to light attenuation compared to suspended sediment. Along the 10-m isobath within 1.5 km of the river mouth, the greatest light attenuation occurred when wave events coincided with or followed periods of high river discharge. However, discharge events lasting attenuation; energetic tidal currents promote rapid sediment export out of the nearshore environment. In the buoyant plume, maximum light attenuation occurred within 1 m of the surface, reducing light through the rest of the water column. Benthic PAR varied more during spring tides when plume location was more variable. Alongshore 1.5 to 8 km from the river mouth, light availability was not directly coupled to river discharge. Light attenuation occurred throughout the water column, influenced by resuspension due to strong currents and wave events. This subsurface attenuation would not be captured by remote sensing. Predicting benthic light availability over event, tidal, and seasonal timescales will improve management strategies designed to limit ecosystem damage during other dam removals or sediment delivery events.

Session Title

Water Quality and Hydrodynamics

Keywords

Elwha river, Sediment transport, Light availability

Conference Track

SSE1: Habitat Restoration and Protection

Conference Name

Salish Sea Ecosystem Conference (Seattle, WA : 2018)

Document Type

Event

SSEC Identifier

SSE1-171

Start Date

4-4-2018 2:45 PM

End Date

4-4-2018 3:00 PM

Type of Presentation

Oral

Contributing Repository

Digital content made available by University Archives, Heritage Resources, Western Libraries, Western Washington University.

Geographic Coverage

Salish Sea (B.C. and Wash.)

Rights

This resource is displayed for educational purposes only and may be subject to U.S. and international copyright laws. For more information about rights or obtaining copies of this resource, please contact University Archives, Heritage Resources, Western Libraries, Western Washington University, Bellingham, WA 98225-9103, USA (360-650-7534; heritage.resources@wwu.edu) and refer to the collection name and identifier. Any materials cited must be attributed to the Salish Sea Ecosystem Conference Records, University Archives, Heritage Resources, Western Libraries, Western Washington University.

Type

text

Language

English

Format

application/pdf

Share

COinS
 
Apr 4th, 2:45 PM Apr 4th, 3:00 PM

Light availability controls in the benthic nearshore ecosystem of the Elwha River

The Elwha River Restoration Project was the largest US dam removal project to date, both in dam height and sediment released. During dam removal in 2011–2014, ~18 Mt of sediment washed downriver, and macroalgae virtually disappeared from the adjacent nearshore ecosystem. The link between current benthic light availability and sediment delivery and transport has been investigated in order to understand conditions during dam removal. Seven instrument platforms were deployed on the 10-m isobath along a 16 km transect centered on the river mouth for seven fortnightly periods in 2016 and 2017 to monitor near-bed photosynthetically available radiation (PAR), suspended sediment, wave climate, current velocity, temperature, and salinity. Water-column profiles, bed sediment, and water samples were collected during deployments. Seasonally variable chlorophyll-a and colored dissolved organic matter did not contribute substantially to light attenuation compared to suspended sediment. Along the 10-m isobath within 1.5 km of the river mouth, the greatest light attenuation occurred when wave events coincided with or followed periods of high river discharge. However, discharge events lasting attenuation; energetic tidal currents promote rapid sediment export out of the nearshore environment. In the buoyant plume, maximum light attenuation occurred within 1 m of the surface, reducing light through the rest of the water column. Benthic PAR varied more during spring tides when plume location was more variable. Alongshore 1.5 to 8 km from the river mouth, light availability was not directly coupled to river discharge. Light attenuation occurred throughout the water column, influenced by resuspension due to strong currents and wave events. This subsurface attenuation would not be captured by remote sensing. Predicting benthic light availability over event, tidal, and seasonal timescales will improve management strategies designed to limit ecosystem damage during other dam removals or sediment delivery events.