SEDIMENT STORAGE IN AN ICE-MARGINAL LAKE, MATANUSKA GLACIER, ALASKA

This paper presents new data on the volume, sedimentology, and expected life-span of an ice-marginal lake at the Matanuska Glacier, Alaska. Outflow from a subglacial discharge vent switched drainage in May, 2003, due to the collapse of an ice-cored moraine on the stream’s west bank. Stream flow was diverted into an ice-marginal lake. The newly-formed lake inlet provided the opportunity to study sediment storage in the lake and to calculate the expected duration of the basin. A meltwater-fed stream flows into the lake from the east and the lake’s outlet discharges to the northeast, into the North Fork of the Matanuska River. The lake is approximately 250 meters long in the east to west direction and 100 meters wide north to south. Lake depths were measured by probing from a boat. The calculated lake volume is 83,200 m3. Hourly water samples were obtained over a 24-hour period from both the inflowing stream and the lake outlet to determine the amount of sediment entering storage in the lake and to examine the relationship between grain-size distribution of the suspended load and stream discharge. The suspended-sediment concentration of all samples increases with increasing discharge throughout the diurnal cycle. The lake was found to have an insignificant amount of water storage; thus, the discharge at the outlet was equal to the discharge at the inlet. For the lowest stream discharge value of 699 cfs, the sediment concentration values ranged from 1.70 g/L for the lake outlet to 1.91 g/L for the inlet. For the highest stream discharge value of 898 cfs, the sediment concentration values ranged from 2.53 g/L for the lake outlet to 2.80 g/L for the inlet. The total sediment flux into the lake is calculated to be 90 m3/day. Given the lake volume and the calculated sediment flux, the lake will fill with sediment in approximately 900 days. Preliminary results indicate no clear relationship between the grain-size distribution of the suspended-sediment load and the discharge.