A study was conducted of water-quality conditions that may affect aquatic life during periods of low streamflow on the North Fork Shenandoah River, Va. Monthly mean streamflows in July 1999 at three streamflow-gaging stations were the lowest measured during the historical record on the river. Daily extremes of dissolved-oxygen concentrations were measured, along with pH, specific conductance, and water-temperature values, at 52 sites along 80 mi of the North Fork Shenandoah River from Cootes Store, Va., to its confluence with Passage Creek, near Strasburg, Va.

Dissolved-oxygen concentrations ranged from 2.1 to 16.4 milligrams per liter (mg/L). Dissolved-oxygen concentrations were equal to or less than the State water-quality minimum of 4.0 mg/L at 18 of 52 monitoring sites; all 18 sites were in the upper and middle portions of the river, where more than half of the first 34 sites had minimum dissolved-oxygen concentrations equal to or less than 4.0 mg/L. There were large variations from minimum to maximum dissolved-oxygen concentrations, with concentrations fluctuating as much as 10 mg/L per day; and typically 5 mg/L per day during the study period.

pH ranged from 7.6 to 9.6, with pH values frequently greater than 9.0 in the downstream portion of the river. Specific-conductance values ranged from 178 to 856 microsiemens per centimeter (μS/cm), with values greater than 600 μS/cm only measured at a group of five sites in the upstream portion of the river. Air temperatures ranged from 21.0 to 37.0 degrees Celsius (ºC), and water temperatures ranged from 17.00 to 30.14ºC. Along the length of the North Fork Shenandoah River, longitudinal variation in water-quality parameters was small. Groups of sites that differed from the general pattern define reaches where increased monitoring may help determine the factors that affect water quality at those sites.