TEMPORAL AND SPATIAL DYNAMICS IN RIVERBED CHEMISTRY - THE INFLUENCE OF FLOW AND SEDIMENT COMPOSITION

To characterize the nutrient pool in the riverbed of an unregulated river in western Canada and identify factors controlling spatial and temporal variability, nitrogen and phosphorus concentrations in open water, porewater, and sediment-exchangeable pools were monitored in the Pembina River, Alberta, at three sites that differed in sediment composition (cobble, sand, and fine sediments). Comparison of porewater chemistry for three years showed that interannual variation was related to discharge rate (22, 87, and 68 m3/s for May-November 1988, 1989, and 1990, respectively) and changes in the size composition of the bottom sediments. However, within-year variations in riverbed chemistry were not correlated with either flow or current velocity. These results suggest that large-scale interannual changes in flow act as a set-point mechanism, defining the particle size composition and chemistry of the riverbed; flow changes of short duration had no predictable effect on riverbed chemistry. Our finding on the highly dynamic nature of riverbed chemistry has important implications for benthic productivity and elemental fluxes between surface waters and the riverbed because dramatic changes in riverbed chemistry can occur in the span of a year even without a flushing scour.