FIGURE 6.21 Vertical land movements in the Puget Sound area based on interferometric synthetic aperture radar from 2002 to 2006. Surface movements in the radar line of sight range from -4 mm yr-1 (subsidence, blue) to + 4 mm yr-1 (uplift, red). Black lines are fault locations, and dashed lines are geophysical anomalies. SOURCE: Finnegan et al. (2008).
Opportunities for Restoration
Efforts to restore tidal marshes have focused on the deltas of the major rivers draining into the sound, where many of the marshes have been diked for agriculture. A recent assessment of restoration needs in the sound (Schlenger et al., 2011) noted that delta shorelines have been so altered in the Duwamish, Puyallup, and Deschutes areas that they are now classified as artificial shoreforms. Restoring the tidal hydrology and riverine freshwater and sediment input are key elements of a delta restoration strategy. Tidal hydrology and sediment input affect many delta processes, including distributary channel migration, tidal channel formation and maintenance, sediment retention, and exchange of aquatic organisms.
Clancey et al. (2009) identified berm or dike removal or modification as the most efficient method of rapidly restoring tidal flow processes. This action could be complemented by modifying channels and making minor topographic changes such as filling ditches and removing road fill. In some areas, the tidal floodplain has been extensively filled and restoration may require resculpting of the land surface to ensure appropriate flooding and drainage of river and tidal waters.
Areas where tidal action was recently restored through these measures include portions of the Nisqually Delta and the Skokomish River. In October 2009, after a century of isolation from tidal flow, a dike was removed to inundate 308 ha of the Nisqually National Wildlife Refuge (e.g., Figure 1.13). The Nisqually Indian Tribe restored an additional 57 ha of wetlands, making the Nisqually Delta the largest tidal marsh restoration project in the Pacific Northwest. Studies show more than 3 cm of sedimentation in the first year of restoration.2 A smaller scale restoration was carried out on the Skokomish River in September 2007, when tides were reintroduced to a 108-acre site for the first time in 75 years. For such tidal reintroduction projects to be successful, sedimentation (both mineral and organic accumulation) must both raise elevations to a level where marsh flora and fauna can flourish and maintain those elevations over time as sea-level rise increases relative water levels. Within Puget Sound, variations in vertical land motion (Figure 6.21) either increase or decrease the amount of elevation change required.
The supply of river sediment also is important for maintaining elevation of existing marsh. Dams or road crossings within a delta’s watershed may indicate that river systems may not provide enough sediment to sustain the elevation of restored habitats. Rates of sediment delivery from the Puget Sound watershed vary over time and place, depending on runoff patterns and land use changes. For example, the Skagit River carries more than 2 million tons of sediment per year, and streams draining the Olympic Peninsula (excluding the Skokomish) carry generally less than 15,000 tons per year (Figure 6.22). The spatial patterns of sediment delivery, combined with general trends in vertical land motion, can be used to identify areas where restored coastal marshes would most likely survive future sea-level rise. In general, areas with high fluvial sediment supply and low subsidence or marginal uplift