Water Quality
Lake Twelve faces several interconnected water quality challenges that have worsened in recent years. King County's long-term monitoring data paints a clear picture — and points toward the restoration work now underway.
Rising Phosphorus
Total phosphorus is trending upward — the primary driver of algal blooms and excessive plant growth. Sources include lawn fertilizer runoff, stormwater, and decomposing aquatic plants releasing stored nutrients each winter.
Invasive Aquatic Plants
Eurasian milfoil and fragrant water lily now cover an estimated 50% of the lake's surface, degrading habitat and impeding recreation. As they die each fall, they release nutrients that fuel regrowth each spring.
Garden Loosestrife
A Washington State Class B regulated noxious weed identified along the shoreline. Left unchecked it degrades riparian habitat, causes bank erosion, and increases nutrient runoff into the lake.
Nitrogen & Clarity
Nitrogen concentrations are decreasing and water clarity has shown modest improvement — signs that the lake's ecosystem remains resilient with proper stewardship.
2025 Monitoring Summary
| Parameter | Status | Trend | Concern |
|---|---|---|---|
| Total Phosphorus | Elevated | ↑ Increasing | High |
| Nitrogen | Moderate | ↓ Decreasing | Low |
| Chlorophyll-a | Moderate | ↓ Decreasing | Moderate |
| Water Clarity (Secchi) | Fair | ↑ Improving | Low |
| Invasive Plant Coverage | ~50% surface | ↑ Expanding | High |
| Toxic Algae Blooms | None (2025) | — Stable | Monitored |
Source: King County Lake Stewardship Program, 2025. Volunteer monitors: Jill and Ken Eide.