Milltown Island, located in the South Fork Skagit River delta, historically consisted of equal parts estuarine and palustrine shrub vegetation (Collins and Montgomery 2001). Estuarine shrub habitat is rare in the Skagit delta and in Puget Sound; only 6% of this historical habitat remains in the modern Skagit delta, much less than other Puget Sound deltas (Hood 2002). Estuarine shrub habitat provides significant ecological functions to rearing juvenile salmon. Beaver living in tidal shrub habitat construct dams in tidal channels that provide low-tide pools which act as predation refuges for juvenile salmon during low tides (Hood 2002).
The Milltown Island site was historically diked for farming, which largely isolated a network of estuarine distributary channels on the interior of the island. Floods damaged the dikes in the 1970s, which were never repaired. Milltown Island (212 acres) was sold to Washington Department of Fish and Wildlife (WDFW) after farming was deemed impractical in this area. On-site tidal channel abundance is much less than in nearby reference areas.
The goal for this site is to implement restoration to recreate historic conditions and improve salmon rearing habitat. To that effort, several dike breaching efforts have been undertaken in 1999, 2004, 2006, 2007, 2011, and 2014. In 1999, a limited amount of dike demolition work was conducted by the US Navy in cooperation with the Army Corps of Engineers, SRSC, and the Washington Department of Fish and Wildlife. Subsequent efforts in 2004, 2007, 2008, and 2010 were carried out by SRSC. Because conventional construction techniques – using heavy equipment – were not a viable or cost effective approach to work on the island, demolition and excavation of new tidal channels were accomplished through the use of explosives.
Key objectives for the phased project include:
- Increase tidal flushing and riverine flooding of the site.
- Increase the development of tidal channels on the site.
- Restore native vegetation to the site.
- Provide low tide habitat refuge to salmon by encouraging beaver colonization of the site
Due to very difficult access for heavy equipment, it was decided that the use of explosives would best allow project objectives to be accomplished. Demolitions occurred during an accepted in-water work window during late summer. Actual blasting events were scheduled during daytime low-low tides to further limit any possible impact of the explosives on local fish populations. Crews coordinated work with neighboring property owners and emergency management officials.
Strategically selected levees were removed or perforated to reconnect tidal and riverine processes to the interior of the island. To prepare for demolition work, a series of six inch diameter holes were bored to a depth of 100 to 120 percent of the levee height (approximately six feet) with a hydraulic auger. The interior channels were bored to a depth of three feet with hand-held augers. The explosive packages were hand assembled, inserted into the prepared holes and backfilled, and then detonated by a local explosives contractor. Expended demolition materials were collected and properly disposed of.
From 2006 through 2007, 1100 feet of levee were removed and 3725 feet of channel were constructed. In 2011, 370 feet of levee were removed and 1200 feet of channel were constructed. In 2014, an additional 65 feet of levee was removed.
In addition to dike breaching and channel excavation, SRSC has been working to restore natural vegetative communities on the island. Work has included planting and invasive species control. Reed canary grass was abundant throughout the site. It outcompetes desirable shrubs and trees, but is shade intolerant. SRSC has installed native plants on Milltown Island in an effort to shade out the grass. Nine native plant species have been planted at the site. Additionally, some mechanical and chemical control of reed canary grass, cattail, and yellow flag iris have been implemented in selected locations at Milltown Island. The site continues to be monitored for vegetation, fish, channel development, and topographic changes.
Dike breaching and removal efforts and channel construction have been implemented in several years, including 1999, 2004, 2007, 2008, and 2011, with opportunities for additional work in the future. The site continues to evolve, and is being monitored for vegetation, fish, channel development, and topographic changes.
Primary Project Contact
Nora Kammer – Restoration Ecologist
Seattle City Light
Salmon Recovery Funding Board
Washington Department of Fish and Wildlife
Collins, B. D., and D. R. Montgomery. 2001. Importance of archival and process studies to characterizing pre-settlement riverine geomorphic processes and habitat in the Puget Lowland. In: J. M. Dorava, B. Palcsak, F. Fitzpatrick, and D. R. Montgomery, eds. Geomorphic Processes and Riverine Habitat. American Geophysical Union, Washington, D. C. pp. 227-243.
Hood, W.G. 2002. Sweetgale, beaver, salmon, and large woody debris in the Skagit River tidal marshes: an overlooked ecological web. Skagit River Tidings, Skagit Watershed Council, Mt Vernon, WA.
Wright, D.G., and G.E. Hopky. 1998. Guidelines for the use of explosives in or near Canadian fisheries waters. Can. Tech. Rep. Fish. Aquat. Sci. 2107: iv