Seasonal precipitation seeps into the ground and recharges the region's aquifers. Wetlands throughout the watershed provide important habitat for birds, fish, animals and plants. Wetland areas store surface water during the rainy season, which reduces flooding, and release water slowly to groundwater and streams during droughts, helping to maintain flow rates and replenishing aquifers. Rivers and streams enter Puget Sound through deltas. These deltas can range in size from widened beds of sand and gravel at the mouths of small streams, to vast networks of branching channels, mudflats and tidal marshes at the mouths of larger rivers.
Deltas provide unique habitat at the point where fresh and salt water mix.
The drainage basin for a river, lake or estuary is called a watershed. Rivers, streams, storm drains and seepage carry on average 140 billion cubic feet of water per year, and discharge large amounts of dissolved and particulate material to Puget Sound. The character of material supplied to the Sound depends on the land use in a particular watershed. The shoreline of Puget Sound dodges in and out as it wraps around many islands and inlets, punctuated here and there by streams and rivers. There are more than 2000 miles of beach habitat in Puget Sound where the tides wash the shore.