Lake Sacajawea Water Quality Report
If you’ve spent time at Lake Sacajawea lately, you’ve noticed its water quality is not great. The north end isn’t too bad and the south end is fairly clear, but the middle section near the footbridge and Martin’s Dock is downright murky. Why is the water level so high, and why does it resemble chocolate milk? City staff did some checking to answer these questions.
In summertime, flushing water is pumped from the Cowlitz River and delivered to the north end of the lake via CDID Ditch 6. But last July, flushing water had to be turned off early in order to rebuild the Cowlitz River intake structure. To keep the lake level up while flushing water was off, the outlet weir was raised to hold more water in. The high water level allows flexibility to simulate flushing action by adjusting the weir elevation to create flow. To keep the lake as clean as possible through winter, the inlet gate and all lobe drain valves remained closed to minimize stormwater flows into the lake. Now that water quality in Ditch 6 has improved, the inlet gate can be opened to let cleaner runoff water into the lake; however, the City is not permitted to pump any flushing water from the Cowlitz River until May 15th.
One might expect lake water quality to suffer equally along its length, or progressively from one end to the other. Instead, the middle of the lake is predisposed to poor water quality, a tendency created by water depth and groundwater flow. Average lake depth is 7½ feet. Areas in the south lobes range from 14 to 22 feet deep but the middle and north lobes are shallow and susceptible to heavy rain that disturbs sediment and nutrient rich mud closer to the surface. From past sampling efforts, we also know iron levels in Lobe 4 between Hemlock and Louisiana are much higher than elsewhere in the lake, and similar to iron levels in groundwater near the lake. When groundwater enters the lake, the iron in the groundwater reacts with oxygen in the lake water and forms iron oxide, a brown/rust colored particulate that causes the water to discolor. Because Lobe 4 is shallow and receives much more groundwater flow than any other lobe, the potential to form iron oxide is high and the opportunity for the particulate to settle out is low, giving way to its mud puddle appearance.
As spring unfolds, City staff will continue managing the lake and doing what we can to improve water quality. Last week, the outlet weir was lowered 12-inches to drop the lake level and increase flow through the lake. This week, the inlet gate was opened to allow better quality water from Ditch 6 to flow into the lake to dilute and displace dirtier water. Water quality, level and flow are being monitored daily and adjustments will be made as needed to reflect changing conditions. If the condition of the Cowlitz River improves, lake flushing will begin on May 15th, the first day our permit will allow us to pump from the river.