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Water quality

On the whole, the water quality of Lake Washington is extraordinary for a large lake surrounded by urban development--perhaps the best in the world.

Key factors for the lake's excellent water quality include:

  • The cleanliness of the Cedar River, which provides half of its inflow;
  • A rapid flushing rate, with average water residence only 2.3 years;
  • The lake's depth, which causes waters in the lake to mix from top to bottom annually, oxygenating the lowest waters (which prevents the chemical release of phosphorus from the lake floor).

There are, however, water quality concerns in the lake, including:

  • A long-term trend of increasing alkilinity, the causes and effects of which are unclear;
  • Spring algal blooms in 1995 and 1996 that were the worst in more than a decade;
  • High fecal coliform counts in some localized areas.

Fish habitat

The size, quality and low elevation of the lake are critical for its providing excellent habitat for fish--particularly sockeye salmon, which rear in lakes rather than rivers and which grow to unusually large size in Lake Washington.

Despite this excellent habitat, survival rates for young sockeye in the lake appear to have been below normal ranges since the mid-1980s, the last period when adult sockeye returns to the lake were consistently high. Improving lake survival rates is critical to ensure that investments protecting and restoring habitat in lake tributaries provide the greatest return

The Lake Washington Studies will provide recommendations on how to raise lake survival rates for young sockeye, concentrating on the following issues:

  • Opportunities for structural and operational improvements at the Ballard Locks;
  • Policies and actions that could reduce sockeye predators, especially exotic species;
  • Actions that could respond to seasonal food shortages in the lake;
  • Potentially different recommendations for salmon and trout from the Cedar vs. the Sammamish.

For questions about Lake Washington, please contact Debra Bouchard, Water Quality Planner or Curtis DeGasperi, Lead Hydrologist, King County Science and Technical Support Section.