Temporal variability of water chemistry in flowing waters of the northeastern United States:: does river size matter?

The seasonal variability of discharge, water chemistry, suspended sediment, and suspended algal abundance was examined in temperate rivers of the northeastern United States to evaluate the accepted (but rarely tested) assumption that streams are more temporally variable than large rivers. Temporal variance of discharge and water-column characteristics was determined for 108 stations on 98 rivers with data measured between May to October of 1978 by the US National Stream Quality Accounting Network (NASQAN). Temporal variance of water-column characteristics increased in relation to the mean estimate of the variable (r(2) = 0.69-0.93, p < 0.001) with the exception of pH (p > 0.05). River size, estimated by median discharge, had little effect (partial r(2) = 0.01-0.02, p < 0.05) or no effect (p > 0.05) on the temporal variance of water-column characteristics after accounting for differences in the mean. In contrast, hydrological variability (measured by the coefficient of variation of daily discharge, flood frequency, number of months with floods) was negatively related with median discharge (r(2) = 0.25-0.38, p < 0.001). When rivers were separated into regional drainage basins, hydrological variability was strongly related to median discharge in some regions (r(2) = 0.45-0.76, p < 0.001) and not in others (p > 0.05). Temporal variance of water-column characteristics was not affected by river size within individual drainage basins. The intensity of upstream impoundment (measured by the number of impoundments and total impoundment storage volume) also negatively influenced the seasonal variability of discharge (r(2) = 0.08-0.24, p < 0.01) but had little effect (partial r(2) = 0.01-0.03, p < 0.05) or no effect (p > 0.05) on the temporal variance of water-column characteristics. Power functions are provided as guidelines for sampling design to estimate the number of samples required to obtain a mean with a given level of precision for each of the water-column characteristics. Nutrient status may be more important to consider than river size when determining sampling frequency for monitoring programs. On average, water chemistry is as variable in large rivers as in small ones.