The region enjoys a modified Mediterranean climate, with relatively cool dry summers and mild wet winters. Warm moisture-laden air masses from the Pacific Ocean keep air temperatures fairly even throughout the year and provide moderate to heavy rainfall from November through April. The Cascades and Olympics contain many glaciers and snowfields, deep valleys and lake-filled basins. Streams that begin at glaciers and snowfields have the year-round flows that provide proper habitat for fish and other wildlife. In the lowlands and foothills are many streams of non-glacial origin, with streamflows and lake levels fluctuating according to the seasonal patterns of rainfall and replenishment from ground water.
Some smaller streams go dry in late summer and early fall.
In the foothills and mountains, some groundwater is found in crevices and fracture zones in the rock. 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.