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. Plain Language Summary: As debris‐covered glaciers melt, they develop a complex, "hummocky" surface that is often attributed to variable thickness debris. Thousands of enclosed depressions pit the hummocky topography. An enigmatic quality of the depressions is that most do not contain meltwater ponds. To better understand how depressions form and grow, we examined the size distribution and geometry of depressions on the Ngozumpa glacier, located in the Everest Region of Nepal. These analyses suggest that the depressions undergo positive feedback growth. With a simple model, we demonstrated that positive feedback growth produces distributions of depressions similar to the Ngozumpa Glacier. Variable thickness debris cannot explain positive feedback depression growth because downhill debris movement and pond overflows would inhibit growth. However, englacial drainage is widespread on debris‐covered glaciers, and karst sinkholes undergoing positive feedback growth develop distributions similar to the depressions. Englacial drainage makes positive feedback depression growth possible by focusing incision and debris removal within the depression, limiting the influence of negative feedbacks. Sustained depression growth primes the glacier for the rapid development of glacial lakes, which increase melt rates and can pose outburst flood hazards. Our results show that englacial drainage is an important driver of hummocky topography development on debris‐covered glaciers. Key Points: Power‐law size distribution and geometric scaling relationships suggest positive feedback depression growth on the Ngozumpa GlacierSimulations showed that only strong positive feedback growth produces comparable power‐law size distributionsEnglacial drainage enables positive feedback growth by regulating water storage in ponds and removing debris from depressions [ABSTRACT FROM AUTHOR]