2019 Water Quality Summary

2019 Water Quality Summary
Water Quality - 4/1/2020
Summer 2019 was a reprieve from the hot weather of the past three years, with mild temperatures associated with onshore flow. The morning fog layer that was mostly absent the last few summers was back, moderating daytime high temperatures. This led to only 10 days of 90 degrees and above in 2019, compared to 34 in 2018 and 30 in 2017.

Mild weather had many positive effects on water quality. Although the headgate was opened in early June, a rain event late that month allowed the gate to be closed from June 27 to July 18. Mild temperatures the rest of summer reduced evaporation and irrigation requirements, meaning less need for river water to make up for these losses. Finally, due to morning fog full sunlight did not occur until late morning, reducing the amount of solar input that warms the lake and feeds cyanobacteria blooms.

The result of all this was a very nice year for water quality on Oswego Lake. The target phosphorus concentration of 20 µg/L was maintained for most of the summer and phytoplankton volume was much lower than the past four years. 


Average monthly phytoplankton volume from 2016 to 2019 in µm3/mL. Overall volume in 2019 (blue bars ) was much lower than in the past four years.

In addition, cyanobacteria was not the dominant phytoplankton species during summer, which is not normally the case. They were dominant in mid-April, but the total phytoplankton volume at that time was so low the cyanobacteria present was inconsequential. Most of the summer the lake was overwhelmingly populated by diatoms, which are key algae species at the base of the lake food web. The first three months of 2019 had a huge bloom with nearly all the phytoplankton volume consisting of the diatom Stephanodiscus nigarae, a very common freshwater species.

The chart above and below differ in how they present phytoplankton volume. The "Average Phytoplankton Volume" chart above provides actual volume for all phytoplankton species for the four years from 2016 to 2019. The chart below lists the main categories of phytoplankton from 2019 by percent of the total volume.


Percent phytoplankton in Oswego Lake by volume. Stacked colors equal 100 percent, with individual colors representing different algal species. The lake was dominated by diatoms for most of the year as represented by the  color.

Maintaining a healthy lake is a matter of controlling phosphorus, and summer average phosphorus during 2019 was the lowest in 20 years. This can be attributed to timing of water intake, particularly during the hottest weeks of the summer. Late June is the period of longest daylight hours and if the sun is shining from sunup to sundown it means a lot of warming and photosynthetic energy feeding algae and cyanobacteria. If the headgate is opened during this period it leads to a trifecta of cyanobacteria growing stimulus; Warm water, lots of sunshine, and a steady stream of phosphorus to feed the growth. 


Average summer phosphorus concentration for the main basin of Oswego lake from 2000 to 2019. The average concentration of 16 µg/L was the lowest in 20 years. Phosphorus was much higher in the years before we started using alum in 2005.

In June this year there was one hot spell June 11 and 12, but that dissipated and the rest of the month was moderate. The result was cooler water in the lake during the June to July period, leading to a reduced average monthly water temperature in the lake, with the high temperature two degrees cooler than the past two years. 


Weather for the 2018/2019 water year. Grey shading shows the daily temperature range, solid black line is high temperature from the 2017/2018 water year. Blue open circles are lake temperatures near the bottom and red open circles are lake temperatures near the surface. Blue bars show daily rainfall. Rainfall axis is on the left and temperature axis is on the right. Total rainfall for the month is below the chart, with normal rainfall (based on the Oregon City weather station) below the monthly total.


Average daily water temperature during 2019 and range from 2001 to 2019. In past years the June-August water temperature was near the top of the range but during 2019 it was near the middle.

Tualatin River Flows
The headgate was opened for the season on June 10 due to a dry period from May 26 to June 26. The headgate was closed on June 26 after over an inch of rain fell in two days from June 26 to 27. The headgate was opened again July 18. 


It cannot be stressed enough how important it is to forestall opening the headgate as long as possible. While Oswego Lake has a goal of keeping phosphorus under 20 µg/L during summer, the Tualatin River is only required to keep phosphorus below 100 µg/L during that period. This means during summer the river is providing phosphorus to the lake that is up to five times what is necessary to contain algae and cyanobacteria blooms.


Phosphorus in Oswego Lake main basin (blue line) and the Tualatin River at Stafford bridge (red line). Bars on top show timing of headgate opening (green bar) and alum application (yellow bar). Our goal of 20 µg/L is represented as a dashed line.

Looking Ahead
It is looking like another mild summer this year so hopefully we can repeat the positive results. In light of our effort to delay opening the headgate, we will be keeping the lake high as we move into June. Normally we keep the water elevation around 98.70 during summer, but in spring we run it up to 98.8 which will delay opening the headgate by several days.

I am encouraged by the weather forecast for this summer, and if they are correct it should be a repeat of last summer. This will be very welcoming and should provide us with another year of good water conditions.