Water Quality - April

Water Quality - April
Water Quality - 4/2/2021
This is my favorite time of year. The days are getting longer, trees are starting to bud, we actually have occasional sunny and warm days, and the lake is going through the “clear water phase” where lake clarity is its best all year. During this time of year, the lake has warmed from its winter sleep to activate an expanded population of aquatic insects known as zooplankton. These critters are beneficial because they keep the algae population in check and provide food for juvenile fish. During spring they become so abundant that they eat almost all algae in the lake, so much so that the algae that are left cannot support the large population of zooplankton. The result is a mass die-off of zooplankton, and since the algae are already gone it leaves very little in the lake to restrict visibility.

To measure clarity we lower what is called a Secchi disc into the water until it cannot be seen anymore. On March 17th the disc was visible to a depth of nearly 20 feet, but there were still many zooplankton so I expect clarity will increase in the next few of weeks. This improvement assumes there will be no large rainstorm that washes sediment into the lake since that would reduce visibility. Historically our best Secchi reading was in late April 2009 when visibility was over 30 feet. Maybe we will get there this year if the lake stays cool and there are no large storms.

I would encourage you to take advantage of the warm days to peer into the lake to see if the bottom is visible. You may also see all the zooplankton scurrying around looking for algae to eat. Oh, and while you are looking, try to see if any carp schools are visible. More on that later.

Water Conservation
For at least the past 20 years there has been an area near our powerhouse where water flows out of the hillside. Historically it was not very much water so not much of a concern, but after the long drawdown for LOIS we noticed the volume of water increase substantially. We never measured it before the project, but anecdotally it seemed like a lot more water. On our next drawdown we noticed the flow of water stopped when the lake level was at our normal drawdown elevation. This confirmed the water was coming directly from the lake, and possibly some pile driving associated with the LOIS project opened additional passages for water to exit the lake.

In anticipation of our most recent drawdown, we started looking at options for plugging the leaks at the lake end or capturing water where it came out at the powerhouse end. We could try to plug as many leaks as possible, but there was no way of knowing where they were without considerable effort and cost. The only guaranteed method of recovering the water was to build a catchment where the water comes out of the hill, pumping the water back into the flume line, sending it back to the lake.

In order to know how large a pump to use we set up a system to measure flow coming out of the hill. During the dry summer months, we were losing around 900 gallons per minute or 1.3 million gallons per day. Over the course of the summer, we were losing more water than the country club uses to irrigate their golf course. The project was designed last summer and we started earthwork last October when the lake was drawn down. This year the mechanical and electrical work was completed, with final programming taking place this month. We will only operate the pump during summer when rainfall is not adequate to keep the lake full.

So why is it necessary to capture this water? This project helps our ongoing efforts to improve water quality in the lake by reducing summer algae blooms. This is achieved by reducing phosphorus that comes into the lake. During summer, the best way to limit phosphorus input is to decrease our reliance on the Tualatin River. Our goal is to control phosphorus to a level that limits cyanobacteria growth while still allowing beneficial algae to grow. For Oswego Lake this is a concentration of 20µg/L, but the Tualatin River has a phosphorus concentration of between 100-150 µg/L. Therefore, it is in our interest to keep this water out of the lake.

Spring rainfall is what determines when we open the headgate and allow river water into the lake. On dry years we have had to open it as early as March. During wet years we can wait until July. Our goal this year, thanks to our pump-back project is to not use the river until August, while being able to get off the river in September. This would be a huge win for the lake because we would not have to use as much alum to capture phosphorus and our overall cyanobacteria population should be greatly reduced.

We are starting the first phase of a project to census and control carp in the lake. Carp are not native to Oswego Lake and can pose significant water quality problems. The species of carp we have are not the flying silver carp, or the grass carp, but what are known as common carp. Common carp are omnivorous, eating insects, worms, crayfish, or other organisms living in the sediment. The problem is they till up the sediment while they are foraging, which releases phosphorus into the water column. While they root around looking for insects they are uprooting aquatic plants, reducing the amount of shelter plants provide for other fish.

The first thing step in our project is find out how many carp are in the lake. To do this we will be netting and tagging fish to get an idea of their population and movement. With this information we will put together a capture plan to start removing them. This will be an ongoing process because carp lay 300,000 eggs in a single spawn and can spawn several times a year. Fortunately, we have a fish screen so no more will come in from the river, but there are many already in the lake and they lay millions of eggs a year.

The first phase of the project will be working with a fish biologist to help with the planning efforts. Next year we will be netting and tagging to get an idea of how many are in the lake, followed by a capture and removal project starting the following year.

While we will be doing a formal census next year you can help us now by taking advantage of the clear water to let us know if you see schools of carp near your dock, or anywhere on the lake. This will give us an idea of where to target our efforts. Please report any sightings to carp@lakecorp.com

Have a great spring and enjoy our clear water!