Over the last several years, I have regularly posted historical and current water quality data sample results for Lake Sawyer. Thanks to the King County Lake Stewardship program and local lake steward Ken Docktor, we are able to see and track water quality trends for the lake. King County also prepares an annual Download 2020-Sawyer-2019-Lake-Stewardship-Monitoring-Report (1) that helps us understand what all this data mean. However good and helpful this data may be, we also need to understand the quality of water that enters the lake from primary sources. This means Ravensdale Creek (officially called Covington Creek) and Rock Creek. If there is a growing problem/risk, we need to know where it is coming from so that mitigation measures can be taken to protect the lake - ergo, our recent focus on the John Henry Mine, Oakpointe's Ten Trails Master Planned Development, stormwater treatment from new development along SR169, development around Lake Sawyer, etc.
Like many government programs, the crash of 2007 led to elimination of many of King County's stream monitoring efforts. Fortunately, at least a few of these programs were restarted in 2014 and we now have some trend sampling data for influent streams Ravensdale Creek and Rock Creek that are the lake's lifeblood. Here's a map showing current King County stream sample locations:
Based on data developed through studies undertaken in the early 1990's, we know that Ravensdale Creek historically provides over 60% of annual lake influent flows and contributes less than 30% of the lake's annual nutrient loading of phosphorus and nitrogen. For Rock Creek, it has been just the reverse and is the lake's primary source of "bad actors" entering the lake. What do recent trends look like?
Beginning with water temperatures that have been rising some over the last decade, what do each of these streams look like (latest samples taken through June 2020)?
Interestingly, monthly sample results show little if any change in stream water temperatures, except for seasonal variations. Both streams tend to fluctuate in the same cycle with the smaller Rock Creek fluctuating in temperatures with higher highs and lower lows throughout the year. But no clear trend upward over this period, which is good news.
How about levels of dissolved oxygen which are critical to our fisheries and salmon spawning?
Ravensdale Creek appears to be remarkably stable with high dissolved oxygen levels throughout the year with little if any change over this period. That's good news. Rock Creek continues a pattern of significant seasonal fluctuations throughout the year.
How about total phosphorus concentrations?
Clearly, Rock Creek has significantly higher levels of phosphorus than does Ravensdale Creek with neither stream showing an upward trend. And keep in mind that stormwater runoff from the Ten Trails development does not flow into the Rock Creek/Lake Sawyer sub-watershed and would thus not impact these numbers.
More sample data in future posts.