Low Flow Variability in Groundwater-Fed Streams

Accurate low flow measurements are important in groundwater-fed streams in order to assess in-stream flow requirements and the nature of stream interactions with groundwater, as well as the potential impacts of climate variability and climate change. Variability in stream discharge was measured over the low flow season in Bertrand Creek, Pepin Brook and Fishtrap Creek, which drain the Abbotsford-Sumas aquifer in southwest British Columbia and northwest Washington State. Discharge was measured to quantify variability: (1) along the length of the stream and relate those variations to differing groundwater contributions over the low flow season, and (2) across a series of cross-sections, both repeating across a middle cross-section (at-a-station measurements) and along consecutive sections from upstream to downstream (downstream measurements), to assess the repeatability of the low flow measurements. Downstream increases in discharge in Fishtrap Creek and Pepin Brook are attributed to groundwater discharging into the channels along these sections, as supported by lower dissolved oxygen concentrations and stable water temperatures. Along Bertrand Creek, stream water interaction with groundwater appears to be variable, with the stream recharging the aquifer along some reaches, and discharging in others. Over the summer season, discharge decreased in all streams; on average the decrease was in the range 0.01 to 0.05 m3/s. As the flow decreased, the coefficient of variation of the mean discharge tended to increase when the flow was very low (i.e., <0.1 m3/s), although the trends for Fishtrap were weak. Overall, the variations in measured discharge are of sufficient magnitude that subtle changes in streamflow caused by some real effect may be difficult to detect.