Trend detection in the temperature of Quebec (Canada) rivers

Trend detection in environmental data subjected to natural fluctuations can prove difficult. With river temperature, the difficulty also lies in the typically limited lengths of the time series. In this work, three different river temperature data sources (continuous and spot instream measurements, and satellite thermal images) were combined to build 30- to 44-year-long time series at 116 locations, distributed throughout most of the Quebec Province territory. Mann-Kendall tests and a non-linear model were used to detect and quantify trends. Uncertainty and biases due to the different data sources and uneven samplings were evaluated and included in the assessment of the trends’ quantification and reliability. Overall, most trends detected in the temperature of Quebec rivers are positive, with higher warming occurring in summer. Annual maxima are found to have increased by 0.023°C/year (median value) between 1979 and 2022, and this maximum appears to occur 2.6 days (median value) later.