Swifts Brook, a small headwater stream within the Grand River Watershed in Southern Ontario, originates, during baseflow periods, in a spring with a high nitrate concentration. The spring, which is the principle source of summer streamflow for the brook, has peak flow rates of up to 5.5 l sec 1 in March and April and minimum flowrates as low as 2.8 l sec in August or September. The nitrate concentration in the spring is highest during periods of highest flowrate and lowest in October or November. The mean annual concentrations of NO3 ~*-N in the spring outflow, weighted by flowrate, were 4.3 mg SL * in 1975 and 4.1 mg l in 1976.

During dry summer conditions a two km reach of the main channel, beginning at the spring, receives no surface flow contributions from ephemeral tributaries. Stream processes in this portion of the stream channel were studied in detail (Robinson et al. 1977). The location of the spring and study reaches are shown in Fig. 1. For this reach, the stream water surface width varied from 0.7 to 3.8 meters and the water depth and average velocity vary from 0.08 to 0.23 meters and 0.004 to 0.05 m sec respectively.

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