While glacial erosion rates are becoming better understood, how eroded sediment is transported under and out of a glacier remains poorly quantified. Field data show that coarse sediment export from temperate glaciers is ‘on–off’ suggesting that it is hydraulically controlled through the effects of changing subglacial discharge upon the excess of bed shear stress over critical shear stress. If sediment export is ‘on–off’ it is likely that for some distance upstream of the snout margin there are diurnal cycles of erosion and deposition in subglacial conduits. Borehole imagery and speleological investigations of such conduits have confirmed that the channels have soft, unfrozen sediment beds. As such, conduits might better be viewed as both rivers under ice and channels eroded into ice, as the presence of eskers post glacier-recession might also suggest. We still know very little about the geometry of such conduits. Quantifying sediment flux through such conduits is challenging because they are inaccessible. One alternative is to model sediment flux, as this is also highly dependent upon knowledge of conduit geometry, but geomorphological reasoning questions this domination. Here, we present the results of a very-high-resolution, three-dimensional GPR survey of a large temperate Alpine valley glacier, the Glacier d’Otemma in the south-western Swiss Alps. By looking at spatially dense grids of GPR measurements (with a line spacing of between 1 and 2 m), we are able to identify the locations and geometry of sub-glacial conduits below the glacier tongue, for ice thicknesses between 20 m and 80 m. We test these data by comparing conduit geometry and location inferred in year x but then revealed following glacier recession and snout collapse in year x + 1. With these data we are able to produce high-resolution glacier bed-topography maps of the glacier snout margin, showing that in this case the glacier bed comprises mixed soft sediments and bedrock outcrops.