Lake Refill and Water Quality

Lake Refill and Water Quality
Water Quality - 2/21/2018


As we approach the end of February the lake has just filled past 96 feet above sea level, leaving a bit over 2.5 feet remaining until full. This may seem slow considering we started refilling December first, but we did not get much rain in December, and this month we have had less than 1.5 inches rain. The last time we did a rain-only refill it required 16 inches of rain to completely fill the lake and we have received a bit over 12 inches so far. After a wet January I was anticipating being full by the end of this month, but it does not look like we will get enough rain to make that happen. Unless March is very dry we should have a full lake by April 1.

It also seems like the recovery has stalled recently but the refill rate always slows as we get to the four foot drawdown level. At that depth water starts flooding Lakewood Bay, West Bay and Blue Heron Bay. Once those areas are flooded any rain we receive has to fill the entire 400 acre lake. During a drawdown the exposed sediment is considered part of the watershed and water runs off into the abbreviated lake that still holds water. An inch of rain during full drawdown brings the lake level up much more than an inch of rain when the lake is full.

This graphic shows how we track the refill progress. The red columns show how much rain we get each day, with the scale on the left Y axis. Monthly rainfall totals are shown below the x axis. The blue line shows the lake level taken at noon each day, with the scale on the right Y axis. The orange horizontal lines show the full target at 98.7 feet and the drawdown level at 86.6 feet. The blue dashed line assumed the January fill trend was maintained in February. (Click on the image to make it larger).

Water Quality

We have had a few sunny days this month that have promoted some cyanobacteria growth. The lake had a fair bit of the cyanobacteria Microcystis last fall before drawdown and it has persisted over winter. Cyanobacteria can regulate their buoyancy and can float to the surface and get moved around with the wind. There were some wind blown pockets of buildup earlier this month that looked like a bloom was happening, but most of that was duckweed, a floating plant shown in the image below. Duckweed grows in many ponds throughout the watershed and when we get heavy winter rain the plants wash out into the lake. They are not harmful and are a valuable food source for birds.

We are also modifying our alum protocol starting this summer to better control phosphorus in Oswego Lake. We will continue our testing protocol and apply only when high phosphorus is feeding a bloom, but conditions over the past few years have shown we need to count on applying every summer. Therefore we are planning to do a mid to late summer alum application to reduce phosphorus enough to complete the summer without a major bloom. The lake will still be green, but our goal is to keep cyanobacteria from blooming and causing potential health issues.
The past few summers we have had very hot summers and long days of sunshine. Up to about 10 years ago it was common to have foggy weather in the morning that limited the amount of sun that hit the lake. This was helpful in slowing algae and cyanobacteria growth because the lake stayed cooler and there was less sunlight to feed a bloom. The past few summers with record hot temperatures we had few days with morning fog.
As the climate warms this is predicted to be the normal condition, with morning fog being the exception. Hotter summers means more irrigation from the lake, more evaporation and more water brought in from the Tualatin river. Water intake is an important player in our algae blooms because phosphorus in the river is four to five times what it is in the lake.