Maybe so, but for arguments sake here is a statement from an 'official' state site about Lake washington:
Quick facts on Lake Washington's Status
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.