Seismic depth soundings on Nioghalvfjerdsfjorden Glacier (NFG), NE Greenland, reveal an overdeepened trough under the floating glacier. The maximum depth of the trough reaches more than 900 m below sea level. Mass balance calculations indicate considerable ice loss due to strong subglacial melting with a mean melt rate of 8 m/a. The geometry of the cavity and water mass characteristics from CTD measurements suggest the existence of a well defined regional circulation system. Warm, saline and rather dense water follows the inward inclining basal slope through the deep valley of Dijmphna Sund towards the grounding line. Shallow ridges at the eastern glacier front prevent this water mass entering from that direction. The comparatively cold, fresh and less dense melt water follows the subglacial ice topography leaving the cavity through the gaps towards the east. The abundance of subglacial melt water east of NFG is most probably one of the main reasons for the semi permanent sea ice cover in this region. Cold water masses upwelling in the Northeast Water Polynia and detected by satellite remote sensing are very likely influenced and modified by the subglacial melt water production.