A survey of supra-glacial lakes on the western margin of the Greenland Ice Sheet reveals a seasonally-driven hydrological system, culminating in widespread lake drainage in late summer. We used satellite imagery to study the evolution of 292 lakes across two sites totalling 22 000 km2 in area. During 2001, the lakes combined area increased to 75 ± 5 km2 by the beginning of July. Over the following 25 days, an area totalling 36 ± 3.5 km2 drained from 216 lakes. At one study site, we used meteorological data and a positive degree day model to calculate the volume of water generated by melting in the lake catchments. Based on this estimate, the mean depth of filling lakes surveyed rose from 1.5 ± 0.7 m on 7th July to 3.9 ± 1.1 m on 1st August, in agreement with a value for one lake of 4.4 ± 0.9 m we have derived from airborne altimetry. During this 25 day period, we estimate that 38 ± 18 × 107 m3 of water drained from the surface at this site, and that there was an average water flux of 1.3 ± 0.3 m3 s− 1 passing through each lake that drained completely.