Channelized, distributed, and disconnected: subglacial drainage under a valley glacier in the Yukon

The subglacial drainage system is one of the main controls on basal sliding, but remains only partially understood. Here we use an 8-year dataset of borehole observations on a small, alpine polythermal valley glacier in the Yukon Territory to assess qualitatively how well the established understanding of drainage physics explains the observed temporal evolution and spatial configuration of the drainage system. We find that the standard picture of a channelizing drainage system that evolves towards higher effective pressure explains many features of the dataset. However, our dataset underlines the importance of hydraulic isolation of parts of the bed. We observe how disconnected portions of the bed systematically grow towards the end of the summer season, causing the drainage system to fragment into progressively more distinct subsystems. We conclude with an adaptation of existing drainage models that aims to capture the ability of parts of the bed to become hydraulically disconnected due to basal cavities of finite size becoming disconnected from each other as they shrink.