SMALL-SCALE GLACIAL LAKE OUTBURST FLOOD DISCHARGE AND SUSPENDED SEDIMENT CONCENTRATION VARIABILITY, LEMON CREEK, SOUTHEASTERN ALASKA

An interconnected system of small supraglacial lakes on Lemon Glacier near Juneau, AK was studied throughout summer 2001 to better understand downstream implications of glacial lake outburst flood (GLOF) on Lemon Creek water quality. Through processes of supraglacial lake expansion, increasing volumes of glacially stored water are discharged over time. Determining the effects of current GLOF discharge on Lemon Creek water quality will aid in anticipating impacts of larger volume GLOF discharges in the coming decades. Mechanisms governing supraglacial lake filling and drainage specific to Lemon Glacier are only hypothesized at this time as englacial or subglacial blockage disturbed by a moderate to extreme storm event triggering release in late summer. Glacially stored water is eventually discharged from beneath the glacier terminus into Lemon Creek, a proglacial stream supporting several resident and anadromous fish species that is currently listed by the EPA as impaired for sediment and turbidity. Daily sediment samples collected throughout summer 2001 at a USGS discharge gauging station 4 miles downstream from the glacier terminus were analyzed for suspended sediment concentration (SSC). No discernible discharge peak is associated with glacier lake outburst flooding on Lemon Creek, as glacial drainage events are triggered by, and tend to "piggyback" moderate to extreme storm events. However, the SSC increase generated by storm events was relatively minor when compared to the increase in SSC generated by outburst flood events, suggesting a “flushing” of subglacial sediment and debris unique to subglacial drainage.