Current Conditions

as of July 19, 2024 11:12pm
0.0 in | 72.7° F | 80.5° F | W 0.6 MPH gust to 0.6 MPH | 98.77' above sea level

Water Quality News for November 21, 2023

We are well into our drawdown and plan to start refilling the first half of December. It has been an active drawdown so far with the city busy replacing a sewer line in Blue Heron Bay and LOC staff testing a dredge option in Oswego Canal. Our water testing goes on although it is limited to the Main Lake only, but it is important to capture conditions during our drawdown.

We had a late season cyanobacteria bloom that was fairly pronounced earlier this month. I was going to write that cooler weather has reduced the activity because the area near the office was looking pretty clear. However, after going on the lake I noticed areas where it continues to be concentrated, which speaks to how the bloom moves around with the wind. Also, as noted in the most recent water quality update, some remnants of the bloom have washed to shore, leaving a blue residue on the sediment. This is a result of the blue pigment commonly found in cyanobacteria (also called blue-green algae) that helps it adapt to low light conditions. I would recommend keeping your dogs away from these areas so they don’t pick up the cyanobacteria on their paws.

Blue residue from cyanobacteria.

Cyanobacteria during the drawdown is understandable because erosion from exposed sediment is likely carrying phosphorus into the lake (we will know once we receive test results). However, we had cyanobacteria throughout this summer despite meeting our phosphorus target for much of the growing season. It may be due to the elevated phosphorus from Oswego Canal which was quite a bit higher than our target, but not as high as the concentration in the Tualatin River due to our alum injection at the headgate. There is a lot of organic material and high phosphorus sediment in the canal that water has to go through before it gets to the lake. We will work on ways to remove phosphorus closer to where water leaves the canal to enter the lake in the near term, and to remove sediment in the upper canal as a long term goal.

Phosphorus data from Oswego Lake during summer 2023. Blue line is total phosphorus from the Main Lake station in the deepest part of the lake. The sample is a composite of 1, 3, and 5 meters. The green dashed line is the phosphorus target. The yellow squares at the bottom indicate our alum applications. The red line is total phosphorus in the Tualatin River and the olive line is total phosphorus in Oswego Canal collected at the Bryant road bridge. The headgate was opened from June 29 to September 1.

City Sewer Project

As many of you know, the city is replacing the sewer line in Blue Heron Bay. They have placed a road from South Shore to the upstream end of the bay that parallels the existing sewer line. They use this road to excavate around the pipe and dig a trench adjacent it to lay down the new pipe. The laterals will be connected later. To find out more about this project visit the city website.

City sewer project in Blue Heron Bay showing the haul road traveling away from South Shore road.

During the excavation process they are using our dredge permit to remove soil that would have originally been sidecast and then replaced to restore the original contours. Between our permit and what the city is removing there will be a reduction of around 900 yards of sediment from the bay. This will help to slightly deepen the bay, but additional dredging will be necessary in the future to remove sediment that comes in from storm drains in this area.

City sewer project in Blue Heron showing the pipe trench with remnants of the old pipe on the right and the new plastic pipe on the left.

Oswego Canal Dredge

We are trying a new hydraulic dredge option for Oswego Canal that involves pumping mud into a dewatering box. Due to the limited access and wet material it is very difficult to use conventional equipment to remove material efficiently. Progress is slow even with this method, but we are learning a lot about how the equipment works and the best methods for removing material. We may need to dredge in years where we do not draw down so hydraulic pumping will be the best method. We are also saving money by doing the work ourselves and not relying on a contractor.

Dewatering box with about two feet of mud. The water mud line on the side of the felt curtain show how much dewatering occurred.

This area is prone to collecting a lot of material because it is where the canal widens in the easement area. Water from the upper canal slows as it leaves the narrow canal and deposits suspended material before passing under the bridge. This can include sediment from the Tualatin river, organic material from trees adjacent the canal, or trash that has washed in from storm drains. I don’t think this area has been dredged in many years so it is good to provide some relief for boaters.

Bryant Park easement where our dredge activity is taking place. The mud in this area is over two feet deep.

Fish Exclosures

Our fish exclosure test to see if carp were the reason we were not seeing any plants in the lake ended up being inconclusive. The intent was to place cages to exclude carp from certain parts of the lake that historically grew plants. If we saw vegetation inside the cages but none outside the assumption is carp were eating the plants. We did not see any plants around most of the exclosures, but nothing inside either, leading us to believe there were no viable plants in these areas. The only location we found plants was near Lost Dog creek, and the amount growing inside the cages was similar to that growing outside.

There has been very little growing in the lake the past few years, which is not ideal for fish and cyanobacteria control. Juvenile fish use plant beds to hide from predators, and plants compete with cyanobacteria for phosphorus. There are plants in some isolated areas, but not as widespread as they have been in the past. Whether this is a bad thing depends on how you use the lake. What would you think of a few more plants in the lake, particularly in areas where they can grow without reaching the surface?

One of the fish exclosures tipped over showing nothing inside.

As we approach the end of November we will be finishing our lake projects in preparation for refilling. Hopefully we will have a wet December and the lake quickly refills.

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