Recurrence of July Joklhlaup Flooding in the Mendenhall Glacier Watershed is Driven by Record Breaking Precipitation, Regional Warming, and the Collapse of a Tributary Glacier near Juneau, AK USA. Cathy L. Connor; Eran W. Hood; Michael Hekkers; Nathaniel Kugler
CONTROL ID: 1482861
TITLE: Recurrence of July Joklhlaup Flooding in the Mendenhall Glacier Watershed is Driven by Record Breaking Precipitation, Regional Warming, and the Collapse of a Tributary Glacier near Juneau, AK USA.
AUTHORS (FIRST NAME, LAST NAME): Cathy L Connor1, Eran W Hood1, Michael Hekkers1, Nathaniel Kugler1
INSTITUTIONS (ALL): 1. Natural Sciences, University Alaska Southeast, Juneau, AK, United States.
ABSTRACT BODY: During the summer of 2012, the U.S. Weather Service Station in Juneau, AK (located at 24 m asl and near the 1769 Little Ice Age terminal moraine of the Mendenhall Glacier), recorded the lowest daily average maximum May to July temperature of 12.2oC (54.9o F), [2.4oC (-4.4o F) below normal] over 69 years of record. This year’s summer temperature anomalies contrast with an overall Juneau trend of warming 1.6oC, (2.88oF) since 1943. The rising temperature parallels glacier ice reduction by thinning at a rate of >2m/yr and ice terminus retreat of 3.86 km between 1909 and 2011. Mendenhall Lake which began forming after 1930 has increased from 3.9 to 4.2 km2 in area and 0.05-0.09 km3 in volume between 2000 and 2011 as the glacier retreated. Since 2000, maximum lake depth has increased from 70 to 90m at the lakefront terminus.
Northeast and 3.6 km above the glacier terminus, the Suicide Basin Ice Fall no longer flows into Mendenhall glacier, which has created a large ice-marginal basin that can hold a substantial volume of water. Once rare, mid-summer flooding has recently been caused by abrupt subglacial releases of rain water stored in this tributary cirque basin. Large water volumes are lifting and flowing under the surviving main trunk of the Mendenhall Glacier. These glacial outburst floods have raised Mendenhall Lake levels and increased discharge into the outlet Mendenhall River. On July 19-22, 2011 an estimated subglacial discharge of 37,000,000 m3 (1,306,642,650 ft3) raised proglacial Mendenhall Lake level by 1.67m (5.5 ft) and increased discharge on the Mendenhall River from 79 to 453 m3/s (2,800-16,000 f3/s). Temperature sensor strings on buoys in the lake have captured lake bottom (-49m) temperature drops of ~ 1oC as cold waves of subglacially released water move at depth from the glacier base into the river. Lake temperature data from summer 2012 sensors will be presented at this meeting. During summer 2012, a repeat joklhlaup event occurred July 3-6, rising lake level from 1.37m to 2.54m (4.5-8.35 feet), just below flood stage at 2.74m (9 ft) and increased river discharge from 58 to 259 m3/sec (1,900 to 9,150 ft3/s). Over the 46 years of record of Mendenhall River hydrologic monitoring by the USGS Water Resources Division, only 5 events of mid-summer elevated water stages have been recorded with two of them in 2011 and 2012.