How Lake Sawyer Water Level is Measured

 

The Lake Sawyer Live Weather Station measures the level of the lake relative to the top of the NE side of the weir at the Covington Creek outlet. The data are posted daily.

 

Arial photo of Lake Sawyer weir

 

What is a weir and why is there one at Lake Sawyer?

A weir is a small dam, used to alter the flow of a stream. See Wikipedia article. The Lake Sawyer weir, like most weirs, allows water to spill over the top. It also has a fish ladder for the two salmon runs that cross Lake Sawyer and go up Ravensdale Creek to spawn.

 

Photo taken from the Covington Creek side looking ESE. The structure between the two concrete spill-ways is the fish ladder. The short spill-way on the Northeast side is one inch lower than the long spillway.

 

The story of the Lake Sawyer weir goes back to the early 50’s. There was a natural dam at the outlet to Covington Creek that probably had been built with the help of beavers. A developer, Vern Cole Realty, removed the dam to partially drain the lake and create more buildable lots. They found that it’s not a good idea to mess with a salmon run and/or lake-front property without permission. A lawsuit was filed by lake residents that resulted in Vern Cole Realty agreeing to build the weir. The court decree, dated August 5, 1952, stated:

 

      IT IS HEREBY ORDERED, ADJUDGED AND DECREED that the Maximum Lake Level of Lake Sawyer be, and the same hereby is established at 518.94 feet above mean sea level according to the United States Geological Survey datum; and

      IT IS FURTHER ORDERED, ADJUDGED AND DECREED that the Vern Cole Realty Co., a corporation, be and it hereby is authorized to proceed to have plans prepared for the construction of a suitable dam or  spillway in Covington Creed and when said plans are approved by the Washington State Departments of fisheries and Game and the Director of Hydraulics. As required under the statures of the State of Washington, said plans, together with evidence of their approval, may be presented to the court ex parte and without notice; and

      IT IS FURTHER ORDERED, ADJUDGED AND DECREED that any and all expense incurred for the preparation of plans for the construction of a dam or spillway in accordance with said plans, shall be born entirely by the Vern Cole Realty Co, a corporation; and

      IT IS FURTHER ORDERED, ADJUDGED AND DECREED that this court shall retain jurisdiction of this cause for the purpose of making future orders herein, and in particular for the approval of any plans and/or specifications submitted for the construction of a dam or spillway in Covington Creek, the construction of a dam or spillway in accordance with the plans submitted, and the regulation by the Director of Hydraulics thereof subsequent to the construction of any dam or spillway.

 

So Lake Sawyer became the only lake in the State of Washington to have an adjudicated maximum height. And you can take any complaints about the weir to the Director of Hydraulics or King County Superior Court.

 

Also see Craig Goodwin’s postings on the history of the weir here  and here on his Black Diamond Now blog.  

 

 

How is the level of the lake measured?

Measuring the level of the lake and logging the data is not particularly difficult but can be very expensive. There are a number of different types of gauges ranging from pressure sensing to ultrasonic transducers. Unfortunately, most are designed for measuring fluid levels in tanks. In addition, to be able to log the data automatically and remote from the sensor requires cable or RF transmission. I was looking for some type inexpensive and low maintenance device. The solution came from discussions with an engineer friend and long-time owner of a cabin on the lake, Jack Sperry. Jack was looking for a way to monitor the level of the lake from his winter home in Renton in case the lake gets above flood stage. We came up with the idea of a large float gauge that I could monitor with the same camera that’s used for weather photos.

 

Jack built a float gauge consisting of a sealed PVC pipe with an arrow attached, riding in a larger PVC pipe. He then attached the rig to a leg of my boat lift. Later he added a fixed arrow pointer near the top to be used as a reference. I developed the software to monitor the position of the float and convert that to lake level. Here’s the process: Every morning at 30 minutes after dawn a photo of the gauge is taken. (The time was selected to get both adequate light and calm water. A foggy morning is best.) The software then does image matching for the fixed reference arrow and the gauge arrow. The setup was calibrated so that the difference between the positions of the two arrows can be converted to lake level relative to the top of the NE side of the weir, which is one inch lower than the SW side. The attained accuracy has been far better than expected (or needed for that matter).

Image used to determine level. (The “third arrow” on the bottom is a reflection of the float arrow in very calm water.)

 

What is flood stage for Lake Sawyer?

Flood stage for Lake Sawyer is, by definition, the level where developed property begins to be inundated. This occurs on the lake when the level is approximately 10 inches above the weir. The worst flooding I have seen was on February 10, 1996 when the lake level rose to nearly 26 inches above the weir – we had ducks swimming in our yard.

 

2015 was an anomalous year. The lake level dropped to the lowest level in recent memory, bottoming out at 39 inches below the top of the weir on October 28. Suddenly, water started pouring in and reached 15 inches above the top of the weir on December 10. That change in level required about 400 million gallons of water.

 

The low level in 2015 was caused by beaver dams on both inlet streams to the lake during our “dry” season. When the dams were removed the water poured in from areas that had been flooded by the dams. I don’t have photos of the high and low for that year but here are two that give an idea of how a normal high event and the extremely low water event appear at the weir.

 

 

The photo on the left was taken on March 11, 2014 when the lake level had reached 10.5 inches above the weir. Coincidently, that is the historical mean high water level according to Jack Sperry’s analysis of historical data. The photo on the right was taken on September 30, 2015 when the lake level had fallen to 33 inches below the top of the weir. The high water marks on the side of the fish ladder are clearly visible in that photo.