Effects of Sea Level Rise in the lower Chehalis River and Grays Harbor estuary

In the eastern Pacific Ocean (including along the Pacific Northwest coast), sea level has declined in the short term but has risen in comparison with historical levels. Predicted sea level rise will move the shoreline landward along the coast, including both within and outside of the Grays Harbor estuary. For the southern Washington coast (including Grays Harbor), relative SLR is estimated to rise in the range of 3-45cm (1-18”) by 2050 and by 6-108cm (2-43”) by 2100. To better understand what these predicted changes in sea level rise (SLR) will mean for juvenile salmon habitat availability in Grays Harbor in the future, we applied three different scenarios of SLR to the tidal portions of the estuary. Preliminary SLR modeling was conducted using the Sea Level Affecting Marshes Model (SLAMM), utilizing data from 2009 Light Detection and Ranging (LIDAR) flights by the Federal Emergency Management Agency. We ran the SLAMM model using a 5m cell size and simulated the IPCC A1B maximum scenario, which is 59cm sea level rise by 2100, as well as scenarios of 75cm and 1 meter SLR to accommodate newer data suggesting the IPCC AIB maximum may be exceeded. Several trends in habitat availability were immediately obvious. In the central estuary and North Bay (and to a lesser extent South Bay), there will be extensive loss of low elevation tidal mud and sand flats (roughly 83% lost) under the A1B scenario predicted to occur by 2075; under the 75cm and 1 meter scenarios, by 2050. Both Goose and Sand Islands are submerged by increasing sea levels by 2100 (A1B and 75cm scenarios) or 2075 (1 meter scenario). In the inner estuary zone, the extensive mud flats around Moon Island (near the airport) and Rennie Island are submerged by 2075 in all three scenarios. The increase in salt water intrusion will result in a decrease in freshwater marsh habitat, with inland fresh water marsh declining to ~45% of 1981 levels and tidal fresh marsh declining to roughly 10% of 1981 levels. In the surge plain, an area of extreme habitat complexity offering refugia for juvenile salmon from high winter flows, the predicted changes in SLR will result in a rapid transition from forested tidal swamp to irregularly flooded marsh by 2025 even in the most conservative scenario (A1B); the net loss of forested area is predicted to be severe (~97% for the estuary as a whole). These changes will result in complex alterations in habitat availability and productivity for the estuarine food web that are difficult to anticipate. The preservation of estuarine habitats is essential for the species that depend on them for survival, so it is critical that as sea level rises, new areas of habitat are made available as the waterline migrates landward. Planning for land acquisition and protection of these sensitive areas, rather than disruptive alterations (e.g. shoreline armoring, dikes, and levees) will be essential in helping offset habitat loss