Rising ELA and expanding proglacial lakes indicate impending rapid retreat of Brady Glacier, Alaska M. Pelto, 1 * D. Capps, 2 J. J. Clague 3 and B. Pelto 4 1 Nichols College, Dudley, MA, 01571, USA 2 Denali National Park and Preserve, Denali Park, AK, 99755, USA 3 Centre for Natural Hazard Research, Simon Fraser Univ., Burnaby, British Columbia, V5A 1S6, Canada 4 University Massachusetts-Amherst, Amherst, MA, 01003, USA Abstract: Brady Glacier is a large Alaskan tidewater glacier that is beginning a period of substantial retreat. Examination of 27 Landsat and MODIS images from the period 2003 to 2011 indicates that Brady Glacier has a mean equilibrium line altitude (ELA) of 745 m and accumulation area ratio (AAR) of 0.40. The zero balance ELA is 600 m and equilibrium AAR 0.65. The negative mass balance associated with the increased ELA has triggered thinning of 20–100 m over most of the glacier below the ELA from 1948 to 2010. The thinning has caused substantial retreat of seven calving distributary termini of the glacier. Thinning and retreat have led to an increase in the width of and water depth at the calving fronts. In contrast, the main terminus has undergone only minor retreat since 1948. In 2010, several small proglacial lakes were evident at the terminus. By 2000, a permanent outlet river issuing from Trick Lake had developed along the western glacier margin. Initial lake development at the terminus combined with continued mass losses will lead to expansion of the lakes at the main terminus and retreat by calving. The glacier bed is likely below sea level along the main axis of Brady Glacier to the glacier divide. Retreat of the main terminus in the lake will likely lead to a rapid calving retreat similar to Bear, Excelsior, Norris, Portage and Yakutat glaciers. Copyright © 2013 John Wiley & Sons, Ltd. KEY WORDS glacier lakes; glacier retreat; transient snow line; equilibrium line altitude Received 24 August 2012; Accepted 15 May 2013 INTRODUCTION Brady Glacier is the largest glacier in the Fairweather Range of southeast Alaska and northwest British Columbia; it has a length of 51 km and an area of 490 km 2 (Armstrong et al., 2012). The glacier flows south and terminates on a large outwash plain that transitions into a tidal delta complex in Taylor Bay. Brady Glacier and Taku and Baird glaciers are the only land terminating glaciers in southeast Alaska that did not significantly retreat between 1950 and 2000 (Molnia, 2008). Taku Glacier has maintained a generally positive mass balance (Pelto et al., 2008), while Baird Glacier is thinning and appears poised to begin a retreat (Molnia, 2008). Brady Glacier is unique among these glaciers because it presently dams at least ten proglacial lakes, each greater than ≥1 km 2 , seven of which are examined here. The lakes are in different stages of evolution: incipient, stable and non-draining, and periodically draining. Brady Glacier occupies a deep valley that extends from Taylor Bay on the south to near the north end of Glacier Bay (Figure 1). Ice-penetrating radar measurements near the main longitudinal axis of the glacier indicate a bed at least 200 m below sea level and maybe a fjord if the glacier and outwash plain are removed (Barnes and Watts, 1977). Approximately two thirds of the ice in the valley flows SSE towards Taylor Bay and one third flows NNW into Lamplugh and Reid glaciers and Glacier Bay (Bengtson, 1962; Derksen, 1976). The divide between south-and north-flowing ice lies at approximately 820 m a.s.l., based on the 2000 Shuttle Radar Topography Mission (SRTM) digital elevation model (DEM), which has a 30 m spatial resolution and 6 m vertical resolution (Capps, 2011). Brady Glacier was first mapped by Captain Vancouver in 1794; at that time, the glacier was calving icebergs into Taylor Bay (Klotz, 1899). The glacier ceased calving and advanced approximately 8 km during the 19 th century (Bengtson, 1962). Its secondary distributary termini also achieved their maximum extent at that time (Capps et al., 2011). After 1870, the outwash plain began a rapid expansion and, by 1977, extended more than 6 km *Correspondence to: Correspondence to: M. Pelto, Nichols College, Dudley, MA 01571, USA. E-mail: mspelto@nichols.edu HYDROLOGICAL PROCESSES Hydrol. Process. 27, 3075–3082 (2013) Published online 11 June 2013 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/hyp.9913 Copyright © 2013 John Wiley & Sons, Ltd.