We propose two new indices able to quantify the sub-daily thermal alterations induced by the release of hypolimnetic water for hydropower production (thermopeaking). They are derived from the analysis of water temperature data for a given gauged station and quantify the i) sub-daily thermal rate of change and ii) the frequencies of oscillations contained in the thermal signal. The indices are independent from many of the physiographic characteristics of the catchment, thus allowing for a comparison among different gauged stations from different locations and catchments. We analysed the data from two different thermal datasets (Italyand Switzerland) for a total of 48 stations with 10 minutes time resolution of temperature data. The stations were grouped according to the absence of upstream hydropeaking releases (reference group) and the presence of upstream hydropeaking, hence potentially impacted by thermopeaking (impacted group). Using a simple statistical approach, based on a non-parametric definition of outliers, we identified the range of variability of the two indices for the reference group. The range measures the "unaltered" subdaily thermal variability of the indices. The upper limits of such ranges are the thresholds beyond which the subdaily thermal regime is classified as "altered". In the space of these two indices is possible to identify three different classes of alteration: absent or low, medium and high. We showed that among the hydropeaked stations (potentially impacted), 32% are moderately altered and 20% are strongly altered.