The White River and Lake Tapps are part of a hydropower system completed in 1911–12. The system begins with a diversion dam on the White River that routes a portion of White River water into the southeastern end of Lake Tapps, which functioned as a storage reservoir for power generation. The stored water passed through the hydroelectric facilities at the northwestern end of the lake and returned to the White River through the powerhouse tailrace. Power generation ceased in January 2004, which altered the hydrology of the system by reducing volumes of water diverted out of the river, stored, and released through the powerhouse. This study conducted from May to December 2010 created a set of baseline data collected under a new flow regime for selected reaches of the White River, the White River Canal (Inflow), Lake Tapps Diversion (Tailrace) at the powerhouse, and Lake Tapps.
Three sites, one on the White River at Headworks, one on the White River at R Street, and one on the Tailrace, were equipped for continuous recording of water-quality data, and three sites (Headworks, White River Canal Inflow, and Tailrace) were sampled for discrete water-quality data. Nine lake sites were measured for physical and water-quality properties and samples were collected for analyses of nutrients, suspended solids, and fecal-coliform bacteria concentrations. Samples from the lake also were analyzed for concentrations of chlorophyll a and organic chemicals.
Discrete samples indicated that water from the White River, White River Canal Inflow, and Tailrace sites generally was turbid, warm, chemically dilute, and well-oxygenated. Exceptions occurred at the sites when flow to the White River Canal was suspended or when little or no flow was released from the lake into the Tailrace. The quality of physical properties and concentrations in water measured continuously at the three sites generally was good and met the freshwater criteria designated by Washington State Department of Ecology for recreational and aquatic-life uses, with several exceptions. The 7-day average of daily maximum temperatures (7–DADMax) was greater than the freshwater aquatic life criterion of 16 degrees Celsius (°C) for core summer salmonid habitat on 6 days at the Headworks site and 37 days at the R-Street site during the study. The 7-DADMax temperatures were greater than the 13°C criterion for spawning, rearing, and incubation on 6 days at the Headworks site and 20 days at the R-Street site. The freshwater aquatic life criterion for dissolved oxygen of 9.5 milligrams per liter (mg/L) for core summer salmonid habitat was not met at the Headworks and R-Street sites for periods during July and August 2010. Exceptions also occurred at the Headworks site for measurements of pH, which were greater than the aquatic life upper limit of 8.5 pH units during July 2010. Aquatic life pH criteria were not met at the Tailrace site during June, July, and August 2010, when pH was greater than 8.5 pH units, and during August 2010 when pH decreased to less than 6.5 pH units.
Lake Tapps water near the surface was relatively clear, warm, and well oxygenated. The clearest water of the nine lake sites was at the Deep site with a median Secchi disk transparency measurement of 6.05 m (meters), which represents a two- to six-fold increase over historical measurements of transparency at this location. Median water temperatures were 18.2–18.9°C and maximums were from 22.9–25.0°C. Median dissolved oxygen concentrations were greater than 8.42 mg/L and minimums generally were not lower than 7.4 mg/L.
By early July 2010, weak thermal stratification developed at most lake sites into at least a warm surface layer overlying a small thermocline. A well-defined hypolimnion developed below the thermocline only at the Deep site. With the development of thermal stratification, hypolimnion water became anoxic at several sites (Deep, Tapps Island, Snag Island, and Lake Outlet). By late September 2010, an anoxic layer about 15 m thick had formed in the hypolimnion of the Deep site. Mixing during autumn overturn i