Transitioning to Winter Conditions

Water Quality - 10/28/2018

I think it is safe to say our algae season is finally over. This is welcome news after a long and dry summer with only 1.81 inches of rain from June to September (which is what we consider the summer season for our lake monitoring program). In July and August we received only a trace of rain and had 27 days where the high air temperature was at or above 90 degrees. Needless to say this was prime cyanobacteria growing weather.

Summer weather data from June to September 2018. Temperature range is shown in grey, lake temperature in red circles, and daily precipitation in blue bars. Orange vertical bars separate the months, and the horizontal orange bar represents 90 degree air temperature. (Click to enlarge)

The proactive alum application the end of July was intended to provide us with a good summer, and it did for the most part. Conditions during most of August were good and if it were typical summer weather the algae growing season would have slowed by mid to late September. That did not happen this September as we had less than an inch of rain and temperatures above normal for the month, which extended the growing season into October.

Controlling algae in Oswego Lake in summer hinges on our use of Tualatin River water. Phosphorus tends to increase in June when we start using river water to keep the lake full. Since it was a dry spring this year we had to open the headgate several weeks earlier than we like, which meant importing phosphorus earlier than normal. The phosphorus concentration was climbing and exceeded our target level by mid-July. The alum application reduced the concentration below the target level for a few weeks.

Then, by late September the phosphorus concentration had climbed again, which led to a cyanobacteria bloom that peaked and started dying off rapidly in early October. Conditions were the worst the week of October 8th with surface scums that were decaying and producing some very colorful mosaics of algae death. I suppose it could have led to an artistic photographic exercise, but unfortunately it was not something we wanted to encourage. A few days after “peak scum” the entire bloom crashed and the lake started clearing again.

We try not to apply alum in late September because fall weather typically takes care of the bloom. Weather this September and October did not follow the normal pattern, which extended the growing season. We will spend our winter evaluating treatment protocol, and in light of our shifting weather patterns brought on by climate change may have to schedule a second, light alum treatment in early September.

Lake Level

You likely noticed the lake was a few inches below normal last week thanks to a dry October. We did not open the headgate because the wastewater treatment plants discontinue their summer phosphorus removal after October first. This means phosphorus in the river is at least twice what it is during summer, and summer concentrations are almost five times what we want in the lake. Rain this weekend filled the lake and it looks like it is going to be wet the coming week as well. 

I guess this is a good time to remind lake users that we tend to let the lake level fluctuate during winter, meaning it gets lower than summer norm and can get quite a bit higher than summer norm. This is intended in order to allow us to more effectively capture rainfall with hydro generation. Our turbine capacity only allows 110 cfs to pass through when fully opened, but there are times during heavy rain when water coming into the lake exceeds that volume. If we see a big storm coming we will create storage by lowering the lake a few inches and while generating the lake may rise a few inches above normal before the hydro can catch up. These excursions from normal are brief, and after a few days the lake level will be back to normal.

During these high points the lake will not be flooding, but just a few inches above normal. This scenario can be troubling if you look out your window during heavy rain and watch the lake come up. But rest assured we constantly monitor conditions and can tip the spillway if rain starts filling the lake too rapidly.

One final note about lake level this winter. If you store your boat on a lift it is a good idea to raise the lift as far as possible so high water does not float your boat off the lift. During summer the lake level only varies a couple inches so boats typically do not move. However, during winter the lake may fluctuate six to nine inches, which may be enough to make boats light on their lifts. A good practice is to tie off the front of your boat while on the lift so if it does float free it will not go far.


I am at the national NALMS conference this week in Cincinnati Ohio where I will be installed as the Region 10 representative for Oregon, Washington, Idaho and Alaska. I look forward to working with the group to further issues related to lake management in the Pacific Northwest.