Physical and chemical characterization of a spring flood event, Bench Glacier, Alaska, USA: evidence for water storage

Previous studies of alpine glaciers have demonstrated that as water dischar ge increases through the summer, the predominant mode of subglacial drainag e shifts from a distributed system to a more efficient conduit drainage sys tem. We observed an early-melt-season speed-up and flood event lasting roug hly 2 days in a small, uncomplicated Alaskan glacier that appears to have r esulted from a sudden shift of the subglacial system in response to a signi ficant accumulation of meltwater within the glacier. Calculated meltwater i nputs exceeded discharge before the event; the implied change in storage ov er this 10 day period was equivalent to roughly 0.13 m averaged over the en tire glacier bed. The pattern of discharge and suspended-sediment variation s and the appearance of large ice chunks in the stream suggest that the spe ed-up occurred during a period of establishment of new subglacial conduits. A culminating flood and associated suspended-sediment pulse appear to have marked the final establishment of the new section of subglacial conduit. T he flood ended the episode of high sliding velocity, but released water wit h high solute concentrations that reflect relatively long contact time with sediments. Discharge of stored water, inferred from high solute concentrat ions and lack of diurnal variation in discharge, continued for at least 3 d ays. While events such as this must recur through the melt season as the co nduit system extends up-glacier and the locus of meltwater inputs shifts, t heir manifestations in the outlet stream will likely be more subdued later in the season.