Shoreline Stewardship

IV. B. 4. Tidal Marshes

Tidal marshes include salt and freshwater marsh habitats that experience tidal inundation (Figure IV-5) (Table IV-4).  They generally occur at elevations from MHHW and above, and are located where sediment supply is relatively high and accumulation of sediment is facilitated by protection from waves and currents.  Marshes commonly develop on deltas of streams and rivers. The root mat created by marshes stabilizes sediment.  Marshes tend to prograde outward through time by accumulation of sediment and organic matter

Figure IV-5.  Tidal marsh in Eagle Harbor (© WA Dept. of Ecology 2000).

Primary production rates for regional tidal marshes are great, ranging from 529 to 1108 g of carbon per square meter per year (Thom 1981).  The organic matter enters the detrital food web in the fall and winter when growth of plants ceases and physical breakdown of marsh vegetation occurs.  Marsh plains are used by a variety of bird taxa as well as mammals for nesting and foraging including the American widgeon, black brant, bufflehead, and great blue heron.  Juvenile salmon reside in tidal marshes and forage on prey resources produced in and imported to the marsh system, where significant growth of juvenile salmon has been recorded (Shreffler et al. 1992).  Tidal marshes are believed to be one of the most important habitats contributing to juvenile salmon growth and survival (Bottom et al. 2001). Juvenile salmon access the marshes primarily during higher tides, when they are able to reach the productive edges of the marsh system.  The deeper portions of the channels afford refuge for the fish during low tides.  Studies have shown that salmonids feed on insects produced in the marsh, as well as on small crustacea often found associated with the edges of the marsh (Shreffler et al. 1992; Simenstad and Cordell 2000).  Juveniles can spend extended times in tidal marsh systems where they may transition from a freshwater  to a salt water physiology.   Marshes can serve as effective

sediment traps and locations of intense nutrient cycling; two processes that can facilitate improvement in water quality. 

Most of the tidal marshes in Puget Sound have been lost over the past 150 years (Bortleson et al. 1980).  Alterations in hydrology, sediment supply, sea level, or marsh plant production can affect the maintenance of the marsh.  Filling, dredging, and diking have destroyed large areas of tidal marshes in Puget Sound.  Similar to tidal flats, embayments with additional sediment input (i.e., stream discharge) are likely areas for tidal marsh development, such as Manzanita Bay, Fletcher Bay, Blakely Harbor, Eagle Harbor, Port Madison Bay, and Murden Cove.