King County recently posted updated water quality test results for Lake Sawyer taken through August 2019. Once again, one cautionary note - we need to focus on trends. As the historical data shows, sample results can bounce around from sample event to sample event but it takes a period of years for significant issues, either good or bad, to emerge. Lake water temperatures are a good case in point. We have test data that goes all the way back to 1993 and trends here couldn't be more clear.
Unfortunately, lake water temperature is a variable over which we have least control, but the adverse impacts on our salmon and trout fisheries are also very significant. Higher temperatures increase plant growth, decreasing dissolved oxygen levels in the water which further adversely impacts trout and salmon.
Secchi is a measure of water clarity. A disk is lowered into the lake and when it reaches the point where it cannot be seen, that is the data point recorded. The lower the disk goes, the better it is. Current data shows continued positive trends.
Total phosphorus is an important indicator of long term lake health. The greater the level of phosphorus in the lake, the more plant growth we will see, the less dissolved oxygen there will be and over time, this is how lakes die - a process called eutrophication. This is also the variable over which we have the greatest control, with the principal source of phosphorus being surface water runoff such as stormwater into Rock Creek and Ravensdale Creek, with highway and development runoff into Rock Creek being the principal bad actor here.
On the positive side of the ledger, stormwater runoff from SR169 is currently untreated discharging directly into Ginder Creek which flows into Rock Creek and represents a significant share of current phosphorus discharge into Rock Creek. Thanks to the persistence of City staff and with the help of grant funding, we will soon have stormwater treatment capacity in place to mitigate this source. In addition, Master Plan Developer Oakpointe is required to meet a phosphorus anti-degradation standard from stormwater runoff from the development. In other words, they can have zero net impact on phosphorus discharge into Lake Sawyer.
On the other hand, there is now significant development in our city around the lake and elsewhere in town outside the MPDs with potential for significant adverse impact on phosphorus concentrations in the lake. In addition, stormwater runoff from mining and reclamation at the John Henry mine flows into Ginder Creek and on into Lake Sawyer and also posing significant risk (more about the status of the mine in future posts).
Identifying trends for phosphorus can be the most difficult given the variables of precipitation across the 500+ acre Lake Sawyer watershed, the season of the year and depth of water in the lake. Following are results from samples taken in May of each year at different lake depths.
Sample results in both May and August show a lot of variability from year to year, but not a clear trend.
King County will publish an annual water quality report for the lake after the first of the year. Will pass it along when its available.
Footnote 1: There is currently no stormwater runoff from Oakpointe MPD's flowing into Rock Creek or elsewhere in the Lake Sawyer watershed. This will change when development moves to other plats within the property that do drain into Rock Creek but not currently.
Footnote 2: The recent scare about potential toxic algae in Lake Sawyer is not substantiated by the facts. Intensive sampling by King County found algae levels in the lake to be very low and in many cases at nondetectable levels.
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