Gene's Corner: Lake Water Quality 2025

Written September 2025, by Gene Welch, Professor Emeritus, Civil and Environmental Engineering, University of Washington

The lake’s water quality is judged from MLIRD’s sampling at six main lake sites twice per month during spring-summer, including lower Parker Horn, South Lake, lower Pelican Horn, and lower, middle, and upper Rocky Ford Arm. These six sites represent 91% of the total lake area, not including middle Pelican and upper Parker, which are also sampled, as well as inflows.

The lake’s water quality has looked rather good this spring-summer. The average May through August total phosphorus (TP) concentration at the six main sites was 22 ppb (parts per billion). That concentration indicates a mesotrophic state, not eutrophic (over 25 ppb) as in the 1970s and 1980s, and not hypereutrophic (over 100 ppb), as in the 1960s.

Total P through August this summer has averaged 16 ppb at South Lake (SL) and 23 ppb at

middle Rocky Ford Arm (MRFA). That is less than the recent very good water quality years of

2021-2023, with May-September average TP at 22 ppb and 38 ppb, respectively.

These relatively low TP concentrations so far this summer are the reason blue-green algae (or

cyanobacteria) averaged only 12% of total algal volume at SL and 14% at MRFA through mid-

September. That was less than in 2021-2023 with 20 and 23% of total algal volume. Blue-greens were 65% and 81% at those sites in 2017-2018 due to the high average TPs of 65 and 81 ppb.

A study of 99 lakes showed that to minimize the risk of blue-greens exceeding 50% of total algal volume, TP should be less than 30 ppb. The lake’s TP and %blue-greens have been well below those limits in 2021-2023 and so far this summer.

The very good water quality so far this summer, and in 2021-2023, was due to relatively high

inflows of low-TP Columbia River water (CRW) that averaged around 185,000 AF (acre-feet).

That inflow represents about 1.5 lake volumes. The low-TP CRW diluted inflow TP from Crab

Creek, Rocky Ford Creek, groundwater, and internal recycling from bottom sediments (internal loading). Inflow of CRW was much less in 2017-2018, averaging 105,000 AF.

What about water quality in 2024? It was similarly good until August when the canal breach

occurred and TP concentrations increased resulting in average TPs of 65 ppb at SL and 88 ppb at MRFA. Surprisingly, blue-greens averaged only 26% of total algal volume at both sites.

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