Using Global Fiducials High Resolution Imagery, Landsat Imagery, and Sentinel Imagery to Monitor Cyclic Subglacial Draining of Berg Lake, Alaska: 2014 - 2019

The USGS National Civil Applications Center analyzed Global Fiducials Library (GFL) imagery derived from US National Imagery Systems, Landsat, and Sentinel satellite imagery to monitor recent dramatic changes in the size and level of Berg Lake, Alaska. Berg Lake is the ~25 km² ice-marginal lake located at the terminus of the Steller Glacier, the western tributary of the Bering Glacier System.

Early 20^th century mapping and photography show that small ice-marginal lakes developed adjacent to five lobes of the Steller Glacier terminus. By 1966, terminus retreat resulted in the coalescing of the lakes (Berg Lakes 1-5) into a single larger water body. Post-1938 aerial photography and post-1960 satellite imagery document subsequent Berg Lake growth. Since 1948, the terminus thinned ~ 120 m. Through the early 21^st century, Berg Lake drained through a gorge into the upper Bering River. Several 20^th century floods, the last in 1994, resulted from ice-dam collapse at the outlet causing downstream flooding. Continued 21^st century thinning lowered the lake surface below this outlet.

Between 2014 and 2018, Berg Lake drained and refilled six times. These subglacial drainage events, each lasting less than a week, were observed in 'before-and-after' images spanning periods of 6-42 days. Each drainage event resulted in the flooding of an ~100 km² downstream area between the Gandil River and Bering Lake.

Stream gauges on Bering River provided data on the timing and magnitude of individual events. Analysis of 2018 event imagery shows that lake drainage began a few days before significant gauge flow increases. The most severe flooding occurred within a week of the onset of draining. In all observed cases, the inundation lasted 2-4 weeks.

We propose that rising lake levels float the thinning Steller Glacier terminus. This allows the lake to quickly drain through sub-glacier channels and conduits. Lake water enters along the western side of the Steller Terminus at the deepest part of Berg Lake and travels 8 km south, reappearing at the northern end of Lake Ivanov. As Berg Lake drains, the terminus lowers and seals the exit.

Monitoring the rapidly changing dynamics of Berg Lake demonstrates the value of long term (decadal) land-change studies using GFL imagery time-series and other publicly available satellite imagery.