The drainage from a wetland was diverted in fall 1988 from eutrophic Pine Lake, a small (36-ha), thermally stratified (maximum depth 11.9 m) lake in the Puget Sound region. Although the wetland contributed only 20% of the lake's annual phosphorus loading, 90% of the phosphorus was soluble and entered the lake's lighted zone during winter-spring. Furthermore, the wetland drainage fueled a spring blue-green algal bloom, which was the lake's principal water quality problem. The diversion resulted in a reduction of 36-kg TP loading (86% of total external) and greatly improved lake water quality during spring. The spring blue-greenal gal bloom was eliminated in 1989 and 1990. Spring epilimnetic mean TP declined from 27 μg· L−1in 1980 to 16 μg · L−Jin 1990 and mean chladecreased from 18 to 6 μg· L−1, while transparency increased from 1.9 to 4.5 m. However, lake quality during late summer and fall worsened from 1980 to 1990 concomitant with a doubling in hypolimnetic TP. Metalimnetic populations of blue-green algae (AphanizomenonandAnabaena) formed surface scums following lake turnover in the late summer and fall of 1989 and 1990. Increasing development in the watershed (from 9% to >50% residential since 1976) probably contributed increased P loading to the lake and possibly increased hypolimnetic P and fall algal blooms.