The Tribe is concerned about climate change impacts on salmon resources in Totten and Little Skookum Inlets, home to numerous salmon bearing streams. A primary focus of this work is to develop high resolution streamflow and stream temperature modeling for two key watersheds of interest to the Squaxin Island Tribe: Skookum and Kennedy Creeks. This work builds off of a project with the Suquamish tribe, in which a similar approach was applied to the Chico Creek watershed.

Another concern of the Tribe is sea level rise. The Tribe’s headquarters, many residences and other important infrastructure are located at relatively low elevations near Skookum Inlet, and could be at risk of flooding due to sea level rise. We quantified future extreme coastal water levels, accounting for both sea level rise and storm surge, to evaluate both changes in the magnitude and frequency of coastal floods.

We simulated historical and future streamflow on Skookum and Kennedy Creeks using the Distributed Hydrology Soil Vegetation Model (DHSVM, Wigmosta et al. 1994), and stream temperature on Skookum Creek using the River Basin Stream Temperature Model (RBM, Yearsley 2009). We calibrated both models through both adjustments to the meteorological inputs and the model parameters (e.g. soil properties, assumed channel dimensions).

For both models, we used dynamically-downscaled projections as inputs (Mass et al. 2022). Recent research indicates dynamically-downscaled projections are needed to accurately estimate changes in precipitation, particularly extremes (Salathé et al. 2014).

To evaluate changes in coastal flooding due to sea level rise, we first had to construct a time series of local water elevations for Skookum Inlet. We did this by combining the “non-tidal residuals” – all of the weather-driven variations in water levels – at the Seattle tide gauge, with the predicted tides at Arcadia, just outside of Skookum Inlet. We then used that time series to evaluate high water extremes under both current conditions and with added sea level rise based on the recent projections for Washington State (Miller et al. 2018). To facilitate planning, our evaluation focused on 92 locations of interest identified by the Squaxin Island Tribe, providing changes in the frequency of flooding for each location based on its elevation.

Pictured: Candace Penn, Squaxin Natural Resources. Photo credit: Erica Marbet

Sea level rise is projected to dramatically increase the frequency of coastal flooding during extreme storms. Many of the 92 locations surveyed by the Squaxin Island Tribe are either currently at risk of flooding or will be at risk in the future due to sea level rise. Even small amounts of sea level rise can lead to large changes in the frequency of flooding; we found that a sea level rise of just 14 inches, which all studies agree is highly likely over the coming decades, would result in a 50-fold increase in the frequency of today’s 100-year flood event.

Future changes in streamflow on Kennedy and Skookum Creeks are likely to further contribute to flooding and to disrupt salmon habitat. By the 2080s, we expect that peak streamflows, typical of winter months, will be considerably higher. The model also projects that the lowest streamflows, typical of late summer and fall, will be even lower. Water temperature in Skookum Creek is projected to be more than 7ºF (4ºC) warmer in summer, on average, by the 2080s.

We recommend further analysis, and possibly refining the model calibrations, to further ensure the accuracy in these projections. In particular, we found that calibration of the stream temperature model was difficult, and should be revisited to ensure the projections are sufficiently accurate. Although our calibration is quite good for low flow conditions, low flows are also difficult to model and could be sensitive to small discrepancies between the observations and the model.

Pictured: Erica Marbet, Squaxin Natural Resources. Photo credit: Joe Puhn

This study was funded by the Squaxin Island Tribe through grants from the Bureau of Indian Affairs (BIA) and the Northwest Climate Adaptation Science Center (CASC). Data collection by the Squaxin Island Tribe was the foundation for this work, including streamflow, stream temperature, and survey elevations. The authors would also like to thank Kevin Hanson at Kitsap County for his help identifying and obtaining streamflow and stream temperature observations, and the Mason Conservation District for their work to obtain the survey elevations used in the sea level rise analysis.

Projected Changes in Streamflow and Water Temperature in Chico Creek, Kitsap County. We provided projections of future streamflows and water temperatures for use in assessing the impacts of climate change on salmonids in the Chico Creek Watershed. This information is intended to help the Suquamish Tribe prioritize conservation and restoration actions.

Projected Sea Level Rise for Washington state – a 2018 Assessment. Using new science, we provide projections of likely sea level rise for various communities around the state, under multiple greenhouse gas scenarios. These projections are designed for direct application to risk management and planning in coastal habitat restoration, community and land-use planning, and infrastructure design and operations.

Climate Change and Flooding in Snohomish County: New Dynamically-Downscaled Hydrologic Model Projections. We produced new projections of future streamflow, with a particular emphasis on flooding, for the Snohomish and Stillaguamish Rivers. Funded by Snohomish County, the data will support the County’s efforts to incorporate climate change impacts in floodplain management decisions.

Heavy Precipitation Projections for Use in Stormwater Planning. Through a series of projects, we developed an online tool and online repository of data and other information from dynamically-downscaled projections. Results are designed to support both municipal and rural stormwater planning.

Effect of Climate Change on Flooding in King County Rivers: Using New Regional Climate Model Simulations to Quantify Changes in Flood Risk. We produced a new set of projections of 21st century climate, developed using a regional climate model. A key feature of these projections is that they provide hourly estimates of future weather conditions (temperature, precipitation, humidity, wind, etc.).