Lake Sacajawea is several shades of brown these days, with a bit of green.
South of the Louisiana Street Bridge, the water is a bit murkier brown, while the south end of the lake is closer to an algae green, with varying shades in between. Not to mention, the lake’s level is higher than usual.
While the city has been able to drain extra water from the lake’s south end, and add fresher water at the north end, it won’t be able to pump any fresh Cowlitz River water from the city’s new river intake structure until May 15. Protections for endangered smelt prevent pumping of any sort between Oct. 31 and May 15.
High iron levels, stormy weather and natural shallowness all play a part in the lake’s odd display of colors, according to a lake water quality study the city released Tuesday. Some of the trouble can be traced back to the city’s early shutdown of lake flushing last summer in order to rebuild the $1.7 million Cowlitz River intake structure off Fisher’s Lane.
The city turned the normal flushing of the lake off July 31 last summer to complete construction on the river intake system, said city engineer Amy Blain. “After August we were able to get a week in (of flushing) or a day here and there but not a lot.” (In-river work can only be done in August to avoid harming endangered fish. )
Adding 3 million gallons of clean water from the Cowlitz River, or “flushing,” has been a key management tool for the past 40 years in order to maintain water quality and control ugly, malodorous algae blooms.
Over the past winter, heavy rain fall collected in the lake and all drain valves remained closed, allowing the water level to rise. The city wanted high water levels that would later allow some flexibility to simulate flushing by opening a weir, or barrier, at the south end of the lake to let water flow out. The city was able to begin this simulated “flushing” last week.
But some parts of the lake are more vulnerable to murky conditions than others, particularly the middle section between Louisiana Street and Hemlock Street. The average depth there is only about seven and a half feet, as opposed to between 14 and 22 feet at the southern end of the lake. Engineers say that because the lake bottom is closer to the surface, sediment and nutrients are easily disturbed by winds and heavy rains, which essentially stirs up the material.
Engineers also know that iron levels are higher in this section than anywhere else: about 23 milligrams per liter here versus 1.3 milligrams per liter at the very north end. As iron in the groundwater reacts with oxygen in the lake, iron oxide forms. The particulate hangs around in the shallow water, causing part of that brown color.
Luckily, Blain said, the rusty color is just an aesthetic issue and doesn’t pose a problem to the overall health of the lake.
Blain said that she hasn’t noticed much of an issue with the smell of the lake either, a source of complaints in the past.
Water quality, level and flow are monitored daily, according to the city. As for anticipating an unpleasant lake stench later this summer, “it depends on how hot of a summer we get and if we get some algae blooms,” Blain said. “But generally we’re able to send enough water through.”