On the tongue of Gornergletscher, Valais, Switzerland, a supraglacial lake forms every spring at the confluence with Grenzgletscher.To investigate the ice-dynamical and hydrological properties of the glacier, an extensive field campaign has been carried out over the last years.Predictions of the future response and behavior of the glacier as well as the lake are the ultimate goal.Ice-penetrating radar surveys were used to map ice thickness and determine bedrock topography at 3 and 40 MHz.The 40 MHz data displays a thick layer of low backscatter in the center of the glacier.Based on data sets of borehole temperature measurements and glacier dynamics, we interpret this as a core of cold ice, advected from the accumulation region of Grenzgletscher into the tongue of Gornergletscher.As has been demonstrated for other glaciers, the position of the cold-temperate transition surface (CTS) is a long-term integrating indicator of changing climatic conditions.On Gornergletscher, along our profiles the CTS can be found in a maximum depth of about 180 m and extends laterally about 300-400 m, about 1/3 of the whole width.The migration of the CTS depends on surface ablation, heat flow related to temperature gradients, and advection of cold ice from upstream regions.We analyse the current state of the CTS, laying the baseline for future observations, and provide estimates for the expected future development.