Hansen Creek alluvial fan and wetland restoration project

Habitat restoration was undertaken in 2009-2010 on lower Hansen Creek, Washington. The project converted 140 acres of isolated floodplain into 53 acres of alluvial fan and 87 acres of flow-through wetlands.

Hansen Creek Alluvial Fan and Wetland restoration project (Poster #1)
Hansen Creek Alluvial Fan and Wetland restoration project (Poster #1)

Executive Summary

(exerpt from Hansen Creek Alluvial Fan and Wetland restoration project report, prepared by the Upper Skagit Indian Tribe Environmental Planning Department)

The Hansen Creek Alluvial Fan and Wetland Restoration Project is located just east of SedroWoolley in Skagit County, WA (Map 1). This project has reconnected a significant portion of the historic floodplain alluvial fan and riverine wetland to the main stem channel encompassing 140 acres of Skagit County Park land. This project is unique in its size of freshwater floodplain restoration. Its location within a natural resource park created the ideal opportunity to undertake a cornerstone watershed process restoration. The floodplain restoration goals incorporated the restoration of natural hydro-geomorphic processes to support fishery restoration, water quality restoration and flood reduction to downstream agriculture. It further supports the reestablishment of native floodplain habitat that will support a wide array of wildlife, an increase in groundwater recharge in a water-limited basin, and improve flow contributions to the stream in the dry season. Lessons from this project can advance freshwater floodplain restoration throughout the Puget Sound region, where WDFW estimates up to 90% of the lowland freshwater riparian systems have been lost to development (WDFW 2005).

Final design called for large woody debris (LWD) grade control structures in the main stem channel to facilitate sediment deposition in the upper project reach main stem, constructed side channels to facilitate flow in the floodplain, a significant number of LWD structures for side channel, alluvial fan and wetland habitat initiation, hummock islands to add complexity, and a diverse planting plan to institute habitat forming structure. The dikes that confined the main channel were notched open where historic side channels existed in the upper alluvial fan habitat, and notched open again in the lower riverine wetland habitat to support return flow, while the mid project section supported lowering of the dikes due to the almost exclusive invasion of Himalayan blackberry. The existing dikes were left in place where riparian canopy was established. The design allowed for low banks and opened notches to distribute high flows across the broad floodplain, creating floodwater holding capacity in the wetland and allowing sediment to settle across the fan and wetland.

The first major rain event of 2009 sent water flowing through the new side channels and carving additional channels. The first two wet seasons ultimately dropped approximately 30,000 cubic yards (CY) of material through the historic main stem channel and project site. The 2010 wet season continued to see new channel formation, flood water retention and sediment capture. The historic main stem channel has filled with sediment and in the lower wetland reach the main stem was abandoned in 2009 while in the upper fan reach the historic channel was abandoned 2 in 2010, moving the main stem into the east fan and wetland floodplain. The overall stream length through the project has significantly increased as a result of multiple braided channel formations. Downstream agricultural flooding reduction has been achieved with flood water detention and sediment capture within the project. Holding capacity of the newly restored wetland is effectively lowering downstream flood height, duration and frequency. Fish habitat was greatly increased with scouring of pools around many of the LWD structures, gravel sorting initiation and an establishing macroinvertebrate food web. Although canopy formation has not yet had time to occur, vegetation that was added was composed of native species and planted in a complex planting plan that mimicked natural vegetative communities (Figure 1).

Download the full project report

Previous report

Initial biological responses at a restored floodplain habitat, Hansen Creek, Washington – Final report to Upper Skagit Indian Tribe

Jeffery R. Cordell, Lia Stamatiou, Jason Toft, and Elizabeth Armbrust – University of Washington Wetland Ecosystems Team – School of Aquatic and Fishery Sciences

Download the full report to Upper Skagit Indian Tribe