CONTROLS ON DOWNWASTING AND RETREAT, BERING GLACIER, ALASKA

Measurements of ice front retreat, annual downwasting, and summer rates of daily glacier surface lowering have been under investigation along the eastern Bering piedmont lobe since 1998. GPS mapping, basic survey techniques, and the morphology of deglaciated foreland terrain have led to the recognition of several controlling factors to complement weather and climate. Our data indicate that retreat is slowest (15-40 m/year) on gently inclined lee slopes where the ice front is relatively dry. Conversely, retreat is most rapid and variable (50-175 m/year) from wet, stoss slopes that support low discharge, ice-contact melt water streams that meander along the retreating ice front. Furthermore, highly fractured, thin ice will retreat by calving at rates on the order of 100-600 m/year in areas of slow to passive movement into ice-contact lakes that exceed 30 m in depth. Rate of calving retreat is controlled by many factors, and is highly variable. Meter-long PVC pipes placed vertically within the glacier surface at multiple sites were in 2009 gradually exposed by mid summer downwasting at rates that varied between 7.7 and 8.6 cm/day, well within a multi-year database of 4.4 to 10.9 cm/day. As anticipated, sunny and dry weather results in greater rates then does overcast and rainy conditions. However, a consistent wind from ice-free terrain accelerates rates under all weather conditions. Yearly ice surface surveys extending 1 km upglacier from the terminus show that the ice surface is lowered by ablation an average of approximately 10 m/year. This rate appears to be linked to regional climate conditions.