The coastline of the area covered by the Elwha-Dungeness Watershed Plan is approximately 79 miles in length. This includes just the true marine (saltwater) coastline from the west side of the mouth of the Elwha River to the drainage divide on the northern coast of the Miller Peninsula that separates the Sequim Bay watershed from the coastal drainage flowing to the Strait from the Peninsula.
WRIA 18's nearshore environment encompasses roughly 300 square miles of onshore and offshore land and water under a complex ownership of federal, tribal, state, and local governmental jurisdiction as well as significant private ownership. It is an integral part of the larger Strait of Juan de Fuca and Puget Sound Estuary ecosystem.
The WRIA 18 Limiting Factors Analysis (Haring, 1999) describes that the nearshore and estuarine habitat of the Strait and WRIA 18 especially "have additional physical features which make them critical to marine and anadromous species. In particular, their abundant food supply, wide salinity gradients, and diverse habitats make them areas particularly valuable to anadromous fish for rearing, feeding, and osmoregulatory acclimatization during transition between freshwater and marine habitats."
- Elwha-Dungeness Watershed Plan