PALEODISCHARGE BASED ON CHANNEL MORPHOLOGY VS. ACTUAL DISCHARGE MEASUREMENTS FOR A RECENTLY-ABANDONED GLACIAL STREAM, MATANUSKA GLACIER, ALASKA

This paper presents a study conducted on a proglacial stream channel that was abandoned and exposed subaerially at the Matanuska Glacier, Alaska. The goal of this research is to compare paleodischarge calculations using channel morphology with historically-recorded discharge data collected by the USGS for the stream. In May of 2003, a stream that emanates from one of the northernmost subglacial-discharge vents of the glacier abandoned its original channel due to the melting and collapse of an adjacent ice-cored moraine; the moraine collapse diverted stream flow into an ice-marginal lake. The stream incised more than 1 m into its sediment-lined channel and into the underlying ice-cored bed. The abandoned stream channel (now a terrace) served as the basis for reconstructing maximum discharge values. Maximum stream discharge was estimated using the Manning equation. Values for the equation were determined from field measurements of multiple channel cross-sections (R), bed slope (S), and the average grain-size of bed sediment (n). The maximum discharge estimated from the Manning equation by averaging results for two cross-sections of the channel, one located immediately upstream and one located immediately downstream from the USGS-gaging station, was 7.20 m3/s. Data collected at two-hour intervals by the gaging station prior to channel abandonment show that maximum seasonal discharge generally occurs between June 22 and July 8. Discharge data recorded during the 2000 melt season are used for comparison purposes, as this is the most continuous recent data set available. If all daily data during the June 22 to July 8 window are considered (12 reading/d x 17 d), the average maximum stream flow is 5.61 m3/s. The average discharge using only the maximum discharge value for each day (1 reading/d x 17d) is 9.24 m3/s. Thus, the Manning equation closely estimates the maximum discharge of the abandoned proglacial stream channel.