Water quantity is both the amount of flowing water in rivers and streams, and the amount of water stored in the ground, lakes, and wetlands.  Land use changes, increases in groundwater withdrawals, climatic change, and other human impacts can adversely affect the quantity of water available to humans and ecosystems. As measured by snow water equivalents, there is decreasing snowfall in the Cascades.  In addition, extensive development can substantially alter the hydrologic regime of streams, lakes, and wetlands.  Land use practices in the Central Puget Sound region have resulted in unprecedented flooding due to increased water volumes in our streams and rivers, while at the same time decreasing groundwater supplies due to reduced infiltration and increased stormwater runoff.

Water quality is closely tied to water quantity.  Over one-third of the streams in King County sampled in 2006 by King County’s Department of Natural Resources were given a "high concern" rating, almost double the number from 2000.  On the other hand, lake water quality in King County has improved, showing lower levels of phosphorous (a common pollutant from detergents), fertilizers and septic constituents.  Rising water temperatures due to changes in land use are also a major culprit in degrading water quality. 

Although urban sprawl continues to degrade our water supplies, a wide range of water conservation efforts in the Puget Sound are proving to be successful.  Using less water is the easiest and most cost-effective way to ensure a sufficient water supply.  Despite population growth, per capita and total water consumption has dropped over the past two decades.  A 2007 report shows that Seattle-area residents are consuming 20 percent less water than they did 20 years ago.  In addition, programs that encourage wastewater reclamation and reuse have bolstered supply.  

Still, there are considerable challenges to preserving the quantity and quality of our water supplies ahead, most significantly, climate change.  Rising temperatures are causing glaciers and snowpack to recede to higher levels everywhere, our region being no exception.  Because snowpack functions as a natural reservoir for water in the Central Puget Sound region, any decrease or changes to the snowpack will result in changes to water availability in the lowlands. For wildlife species that are closely adapted to flow patterns, even small changes in hydrology can result in serious disruptions to lifecycles. Lower flow levels also affect water quality, as less water is available to dilute contaminants.

Water pollution in Puget Sound due to increased human development has led to numerous restrictions on recreational and commercial activities.  For example, pollution over the past 25 years has reduced the area available for commercial shellfish harvesting in Puget Sound by nearly 30,000 acres.  During the summer of 2005, 24 of 65 Puget Sound beaches violated water quality standards for fecal coliform bacteria 

These dangers to business and recreation are also reflected in marine life. Since 1992, seven salmon stocks have become extinct in the Puget Sound region, and the number of stock considered healthy has declined from 93 to 81.  Dead fish and other marine mammals surfaced on the beaches of Hood Canal in 2002, having suffocated from a lack of dissolved oxygen in the water. 

Drinking water quality, however, remains generally good in the region, largely due to the protection given to our water supply watersheds.