Flood and Erosion Hazard Assessment for the Sauk-Suiattle Indian Tribe Phase 1 Report for the Sauk River Climate Impacts Study

Natural Systems Design, Inc.
Posted on: 3/07/2017 - Updated on: 1/14/2019

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The Sauk-Suiattle Indian Tribe initiated a pilot study to assess the impacts of anticipated climate changes to both tribal infrastructure and the Sauk river ecosystem that supports fish and wildlife critical to the tribe. The Sauk-Suiattle Indian Tribe’s homeland encompasses a broad area including the Sauk and Cascade River watersheds in northwestern Washington. The Sauk River is a large meandering alluvial river that flows north into the Skagit River. Sauk River Miles (RM) start at its confluence with the Skagit. The Tribe’s reservation is located on an alluvial terrace within the Sauk River valley at RM 14, five miles north (downstream) of Darrington and one mile south of the Suiattle River confluence (Figure 1). In the 1940s the main channel of the Sauk River flowed on the eastern side of the reservation, directly adjacent to current residential housing. This old channel was still active and clearly evident in 1954 (Figure 2). In the last 60 years the river’s main channel has been located on the eastern side of the valley but between 1989 and 2013 it has migrated back to the west, toward the reservation, at an average rate of 43 ft/yr (Figure 3). Un-interrupted at this rate, the river would reach housing within 25 years. This could potentially happen much sooner given the old 1940s channel could be rapidly re-occupied by the river (Snohomish County Surface Water Management 2009). The alluvial terrace underlying tribal housing, offices and community buildings is easily erodible and thus at serious risk given the river’s tendency to migrate (Figure 3). The Tribe wanted to know whether the warming climate could worsen flood and erosion risks, and whether changes could adversely impact salmon habitat. This report focuses on flood an erosion risks and how they may be impacted by climate change.

Natural Systems Design, Inc. (NSD) prepared this report for the Sauk-Suiattle Indian Tribe as part of the first phase of an interdisciplinary effort designed to contribute critical understanding of Sauk River ecosystem dynamics and sensitivity to climatic changes. The objectives of this report are to: (1) describe the hydrology and geomorphology the Sauk River near the reservation, (2) evaluate available information on potential for climate change to affect future flood flows in the Sauk River basin, (2) document historical changes in river channel and floodplain characteristics of the Sauk-Suiattle Reach, and (3) evaluate the near- term and future threats to tribal infrastructure posed by Sauk River streambank erosion and flooding.