Englacial drainage drives positive feedback depression growth on the debris-covered Ngozumpa Glacier, Nepal

The development of hummocky topography is a poorly understood aspect of down-wasting on debris-covered glaciers that is often attributed to variable debris thickness. Thousands of enclosed depressions pit the hummocky topography. To better understand depression growth, we examined the size distribution and geometry of depressions on the Ngozumpa Glacier, in the Everest Region of Nepal. The depressions exhibited a power-law size distribution, fractal perimeters, and power-law depth-area scaling, which suggest positive feedback growth. With a simple model, we showed that positive feedback growth produces similar power-law size distributions. Based on these findings, we propose a “sinkhole” hypothesis for the development of depressions. Drainage into englacial sink points removes debris from the depressions and inhibits ponds from overflowing, thereby enabling positive feedback growth via incision, increased sub-debris melt rates, and ice cliff retreat. By facilitating sustained depression growth, englacial drainage preconditions the ablation zone for the rapid growth of glacial lakes.