Upstream factors affecting Tualatin River algae—Tracking the 2008 Anabaena algae bloom to Wapato Lake, Oregon

Significant Findings

• A large bloom that included floating mats of the blue-green algae Anabaena flos-aquae occurred in the lower 20 miles of the Tualatin River in northwestern Oregon between July 7 and July 17, 2008.
• The floating bloom was deemed a hazard to recreational users of the river due to the potential production of algal toxins (anatoxin-a and microcystin), and a public health advisory was posted for the lower 10.8 miles of the river by the Oregon Department of Human Services for July 12–25, 2008.
• The bloom caused nuisance taste and odor issues and required modified drinking-water treatment techniques where water was withdrawn for municipal uses in the upper reaches of the Tualatin River, some 46 miles upstream of the worst algae problems.
• Using water sample data from Clean Water Services and the Joint Water Commission, and continuous and discrete monitoring data from the U.S. Geological Survey, the source of the anomalous water-quality conditions and the likely source of the Anabaena bloom was traced upstream to discharges from the Wapato Lake agricultural area near Gaston, Oregon, in the upper part of the watershed near river mile 60.
• The Wapato Lake algae bloom occurred as standing water remained on the lakebed far longer than normal—into early summer. A failure of the levee on the edge of Wapato Lake in December 2007 caused by heavy rainfall and high water in the canal outside the levee inundated the lakebed to a depth of 7–9 feet, storing thousands of acre-feet more water than its normal winter volume. The water could not be pumped out until the levee was repaired or river levels receded, thus delaying drainage of the lake until summer and facilitating the bloom.
• In normal summers, the lower Tualatin River grows a moderate crop of algae that responds strongly to streamflow (residence time), light available for photosynthesis, and phosphorus concentrations. In 2008, however, inoculation of the river with phytoplankton and zooplankton discharged from Wapato Lake some 30 miles upstream of the lower, pooled reach of the river demonstrated the importance of upstream factors on plankton communities and water-quality conditions in the Tualatin River.
• The Wapato Lake algae bloom of July 2008 provided useful information and lessons for agencies managing public health, wetlands, agricultural activities, and water quality in the Tualatin River basin and similar river basins elsewhere.
• The results and insights derived from this study can be used to enhance future monitoring and data collection strategies designed to improve water quality and plankton models and better predict dissolved-oxygen concentrations in the lower Tualatin River.