North–south ((a), (c)) and east–west ((b), (d)) velocities (ma<sup>−1</sup>) estimated from GPS measurements collected during the drainage of Lake Half Moon ((a), (b)) and Lake Ponting ((c), (d))

<p><strong>Figure 5.</strong> North–south ((a), (c)) and east–west ((b), (d)) velocities (ma<sup>−1</sup>) estimated from GPS measurements collected during the drainage of Lake Half Moon ((a), (b)) and Lake Ponting ((c), (d)). Note that scales on the <em>y</em>-axis are different for the four panels.</p> <p><strong>Abstract</strong></p> <p>Supraglacial lake drainage on the Greenland ice sheet opens surface-to-bed connections, reduces basal friction, and temporarily increases ice flow velocities by up to an order of magnitude. Existing field-based observations of lake drainages and their impact on ice dynamics are limited, and focus on one specific draining mechanism. Here, we report and analyse global positioning system measurements of ice velocity and elevation made at five locations surrounding two lakes that drained by different mechanisms and produced different dynamic responses. For the lake that drained slowly (>24 h) by overtopping its basin, delivering water via a channel to a pre-existing moulin, speedup and uplift were less than half those associated with a lake that drained rapidly (~2 h) through hydrofracturing and the creation of new moulins in the lake bottom. Our results suggest that the mode and associated rate of lake drainage govern the impact on ice dynamics.</p>