A 900 MHz ground-penetrating radar (GPR) profile, collected along 1.7 km of the centerline of a high Arctic glacier in Svalbard, is interpreted using shallow ice core data and thermomechanical flow modeling. Four distinct zones are visible in the image and correspond to areas of firn, recent and older superimposed ice, and ablation zone glacier ice on the ground. The areas of firn and superimposed ice are characterized by relatively steeply and gently dipping internal reflecting horizons (IRH), respectively, in the radar image. The IRHs arise from permittivity contrasts due to density variations, produced in firn by alternating firn and ice layers, and in superimposed ice by varying air bubble content. Recently formed and older superimposed ice can be distinguished in the GPR image. A three-dimensional flow model is used to indicate the long-term mass balance and flow history responsible for the spatial distribution and orientation of superimposed ice IRHs in the radar image. Results indicate that the IRH distribution can only be explained by invoking a down-glacier shift in mean 30 year equilibrium line altitude (ELA) by 20--30 m, compared with measured mean 30 year ELA. They also suggest that many of the superimposed IRHs are relict features, produced >100 years B.P., when the ELA was 100--150 lower than at present. This work advocates combined GPR and flow modeling as a unique tool for validating mass balance measurements and/or inferring the mass balance/dynamic history of a glacier beyond a recent measurement period.