Mapping the future of flood risk for the Stillaguamish and Snohomish River


Mauger, G.S., S.-Y. Lee, J.S. Won, 2018. Mapping the Future of Flood Risk for the Stillaguamish and Snohomish Rivers. Report prepared for the Snohomish Conservation District. Climate Impacts Group, University of Washington, Seattle.


Climate change is projected to lead to more frequent and severe flooding in response to sea level rise, declining snowpack, and heavier rain events. Most studies of future flood risk have focused on quantifying the amount of sea level rise, or the increase in streamflow during flood events, and have stop short of evaluating which areas are flooded and at what depth. The few studies that have modeled the extent and depth of future flooding (e.g., Hamman et al., 2016; Mauger and Lee, 2014) are limited in spatial coverage and are not easily comparable due to differences in the data and methods used. The purpose of the current project was to pilot a new inexpensive approach to developing a consistent set of flood projections over a much larger area. This allows for apples-to-apples comparisons of risk.

In this study, we quantified changes in the depth and extent of flooding for all but the uppermost reaches of the Snohomish and Stillaguamish watersheds. We developed the streamflow and sea level rise projections needed as input to the hydraulic model simulations, in both cases using the latest comprehensive set of projections for the region, and worked with consultants at Fathom, Ltd. ( to evaluate and optimize the flood simulations. In addition, we evaluated streamflow and sea level rise projections for use in a separate study of groundwater variations in the lower elevations of each basin.

Our projections give a middle estimate of 16-18 inches of sea level rise by the 2080s, and a middle projection showing a +42 to +46% increase in peak streamflow for the 2-year event and -3 to +20% change in the 100-year event. The hydraulic modeling results show a clear increase in flood depth and extent for the more frequent flood events (e.g., 2-year and 10-year); these results are described in a separate report by Fathom, Ltd. (Smith, 2018).

Changes are more ambiguous for the larger events (e.g., 100-year). There are likely two explanations for this: (1) flooding in much of the basin is currently valley wall to valley wall for the 100-year event, meaning that changes in flood extent may be relatively minor for these larger events, and (2) because it is a rare event, it is much more difficult to develop robust statistics for the 100-year event.

Overall, we find strong evidence for an increase in the frequency and extent of flooding for all events, with particularly large changes for the more routine flood events (e.g., 2-year and 10-year). These more frequent flood events are of particular importance for agriculture and ecosystems in each watershed.