Shoreline Stewardship

IV. B. 5. Subestuaries (River Mouths and Deltas)

Subestuaries consist of mouths of those streams and rivers that enter a larger estuary (e.g., Puget Sound) (Figure IV-6).  Subestuaries are numerous in Puget Sound, although they are often very small where watersheds are short and stream flows are low. Taken in total, subestuaries are an important component of Puget Sound ecosystems.  They can form deltas where organic matter accumulates, and flats where shorebirds and fish feed (Table IV-5).

Wetlands are associated with subestuaries, which further slow peak flows.  These wetland areas also filter runoff, improving the quality of the water before it enters the estuary. Salt marshes in subestuaries, although often small, are important rearing areas for juvenile salmonids, providing refuge and food before they outmigrate.  Eelgrass can form in subestuaries. Birds use subestuaries for bathing and drinking, particularly in the late summer months when freshwater is more limited (Norman 1998).

Resources and functions of subestuaries are similar to those for tidal marshes and flats, because they can contain both habitats. Stressors on subestuaries are primarily from upstream and shoreline development, which alters stream flow, surface and groundwater flows, riparian functions, and water quality.  Alterations of hydrology from filling and diking can affect the functioning of subestuaries also.

Small deltas are evident in locations such as Lynwood Center and Murden Cove, where small streams meet the surrounding waters of Bainbridge Island.  Subestuaries are recipients of upstream organic matter, nutrients, and large woody debris (LWD), and undergo salinity variation important spatially and temporally to nearshore fishes.