The objective of this study was to identify relations between reservoir flushing rate and water quality and test if these relations could be used to provide assessment of water-quality conditions and trends. The data set used in this investigation is from four reservoirs located in the White River Basin of northern Arkansas and southern Missouri: Beaver, Table Rock, Bull Shoals, and Norfork lakes. Data were collected from 17 mainstem sites along the downstream gradient by the U.S. Geological Survey for the U.S. Army Corps of Engineers between 1974 and 1995.

Flushing-rate and water-quality relations were systematically identified at various upstream and downstream locations within each reservoir. Annual flushing rates were prepared for each sampling date by dividing average inflow or outflow of various averaging periods by reservoir volume. Inflow or outflow flushing rate averaging periods that provided the best fit with water-quality constituent concentrations or values were identified and used to evaluate water-quality conditions and determine flushing-rate adjusted concentrations or values. Flushing-rate adjusted trends were compared with unadjusted trends to better resolve water-quality changes over time.

Of the 17 sites examined on the 4 reservoirs, water-quality constituent concentrations or values at the upstream site on Beaver Lake and the James River arm on Table Rock Lake were most affected by variability in flushing rate. Relations between concentration or value and flushing rate existed for every water-quality constituent tested at the upstream site on Beaver Lake. Negative relations indicated that point sources dominated loading. Fewer relations existed at the upstream White River mainstem site on Table Rock and Bull Shoals lakes due to upstream flow regulation. In general, flushing rate averaging periods related to concentration or value were relatively short at upstream sites and increased downstream. Transition from inflow to outflow relations occurred further downstream for many constituents.

Results from this study showed that better water-quality and trend assessment could be achieved if relations between reservoir flushing rate and water quality constituent concentrations or values were determined. As a result of relations developed between reservoir flushing rate and water quality at sites on these four reservoirs over the 22-year period, it appears that water-quality conditions upstream in Beaver Lake and in the James River arm of Table Rock Lake improved over the respective periods of record. Results also indicated that these sites were impacted by activities upstream in the respective basins. Norfork Lake appeared to be impacted less than the upper end of Beaver Lake or the James River arm of Table Rock Lake. Flow regulation upstream in the White River may have affected the water quality in the White River mainstem of Table Rock Lake and Bull Shoals Lake by reducing the magnitude and variability of water-quality constituents loaded into these reservoirs, allowing more stable, higher quality conditions.