King County recently posted Lake Sawyer water quality test results for samples taken in June earlier this year. Like always, I would caution that we are looking for trends and cannot reach conclusions based on any sample above or below the "norm". Measuring lake water temperature is a good example where we can clearly see a trend - the lake is clearly getting warmer.
As we would expect, water temperatures in May/June are lower than they are August, but the trend is up over this ten year period, particularly near the surface. Since warmer water has less oxygen carrying capacity than does colder water, this trend is not positive for lake trout and salmon populations.
Sampling is typically taken each year in May and August. This year, samples were taken later in June. Secchi measures the depth at which a disk lowered into the water can no long be seen. The deeper the better - which means the clearer the water. Over the last ten years, the trend has been positive with somewhat better clarity. The two samples taken this year in June show reduced clarity. We'll see what future results have to tell us.
Total phosphorus is an important indicator of long term water quality. Lower phosphorus levels mean less "fertilizer" available to spur plant growth in the lake, such as algae, and the better lake water quality will be. Higher phosphorus levels are not good for the lake and can lead to lake eutrophication and the loss of fisheries. Essentially, as noted by King County, no trends here over the last ten years and we see lots of variability, particularly at deeper depths. Our concern over mining at the John Henry Mine has largely been driven by concern for higher phosphorus concentrations being discharged into the lake via the Rock Creek watershed.
More about water quality indicators in future posts.
Footnote: For those concerned about the potential adverse impact of the Ten Trails development on Lake Sawyer water quality, particularly phosphorus discharged to the lake via stormwater runoff, stormwater runoff from current plats under development do not flow into the Rock Creek/Lake Sawyer watershed and thus do not currently impact the lake.
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