The monthly water quality testing at Lake Cavanaugh continued to show expected trends in June. As the upper layer of the lake warmed up over the last month, the temperature gradient returned and the oxygen in the deep water started to decline. Currently there is about a 25 degree temperature difference between surface and 75ft depth, and the dissolved oxygen at 75ft dropped down to about 60%, which is still good.
Nutrient levels were also measured (Nitrate and Phosphate) and remain low or undetectable during dry weather, which is good. After rainfall, these levels spike as nutrients from soil wash into the lake and become available for plants and algae to consume during growth cycles. Please always avoid using phosphate fertilizers and soaps so the nutrient levels will remain low enough to keep algae growth in check.
Also, many may already be aware, but there were 2 studies of Lake Cavanaugh water and sediment quality done in 2013 by the WA State Dept of Ecology. In one study, they found that sediment at Lake Cavanaugh contained elevated mercury, but they never tested the fish tissue to see if it was also elevated. Just because sediment mercury is high, doesn’t mean the fish tissue will be high as well. In another study, they found that fish from Lake Cavanaugh had fairly high levels of HCB (hexachlorobenzene), a pesticide that was banned in the 1980’s. So since 2018, Lake Cavanaugh has been listed as a Category 5 on the state’s impaired waterbody list for HCB for restricted consumption of largemouth bass, sockeye salmon and cutthroat trout.
Since there is no follow-up data on mercury, and it’s likely that HCB levels have dropped since 2013, the Dept of Ecology is willing to prioritize another fish tissue study this October, provided the Community can help catch the fish. This would help confirm if the sediment mercury is buried/inaccessible or available and accumulating in fish tissue. It might also confirm if the category 5 HCB impairment status at Cavanaugh could be lowered or removed. If you are interested in helping catch fish for this study during August 1 to Sept 26th, please contact Jeni Gaines.