Why do lake levels seem to be getting lower each summer and higher in the winter? The following graphic, updated daily at Lake Sawyer Weather, shows how big the fluctuations have been in recent years.

As discussed in my recent post The Mysteries of Lake Sawyer Hydrology, there appear to be no easy answers to our many questions. We know that water enters the lake from three main sources, direct precipitation and stream flow from both Ravensdale Creek and Rock Creek, and exits the lake over the weir into Covington Creek, through lake evaporation and groundwater outflows. But where do we start to better understand these flows? The most obvious starting point is with precipitation and thanks to the weather station at Lake Sawyer Weather, we have great local data.

Seasonal data can be even more revealing.

What begins to complicate matters, however, is to remember that the Lake Sawyer watershed, which feeds Ravensdale Creek and Rock Creek, is a very large area - totaling over 8,120 acres. Thus, we also need to understand what is happening throughout the entire watershed.

That's why being concerned about the John Henry Mine and its impact on the Rock Creek sub-watershed is important. And that's also why we are concerned about stormwater from development within these watersheds.
Given all of this information at our disposal, we should then be able to understand, for example, why water levels in Lake Sawyer reached such a low point of nearly - 40" below the weir in 2015. Right? Well, not so easy.
First, we need to recognize that 2015 is not the lowest point we have on record. That dates back to 1994 when water levels below the weir fell to -40.41".

So what caused 2015 water levels to fall so far and then to rise so dramatically above the weir in such a short time? As it turns out, it was simple in this case - beaver dams. Water levels in Ravensdale Creek had backed up and risen so high that WSDOT became concerned about the safety of the SR 169 bridge where it crosses the creek that crews were brought in to remove some of the dams, causing a gush of water to run into the lake raising water levels in a very short period to over 15" above the weir.
Rock Creek faced similar problems, though not as dramatic. A local group of volunteers called the Rock Creek Restoration Team obtained a hydraulic permit from State DOE and removed the Rock Creek beaver dams during the summer of 2016. Thank you all!
Beaver dams are likely to be a recurring problem that we should monitor. All you have to do is take a walk along the trails of Ravensdale Creek and you will still see lots of them.
In addition to beavers, the timing of rainfall appears to be a very large factor in how low water levels become. For example, 2017 rainfall totaled 51.64", the highest annual level we saw in the last 10 years, yet lake water levels still reached a low point of -23.16". However, little of this rain fell in drier summer and early fall months. When it did begin to rain in October, the lake filled quickly, rising from -23.16" on October 12 to +0.79" on November 15. This last year in 2019, rainfall totaled just 35.1" for the year, yet the lake level fell to a low of just -20.7". That's because more rain fell in summer/early fall months in 2019 than it did in 2017.

This still doesn't answer the question of whether Covington Water District or Kent Water District wells, or lakeside irrigation pumps are draining the lake and by how much, but enough for today. I'll address these questions in a subsequent post.
P.S. Thanks again to Bob Edelman for all your work in maintaining the Lake Sawyer Weather website and weather station. It's invaluable.