The role of internal climate variability on future streamflow projections

•The impact of internal variability and climate change on streamflow is studied in 133 catchments of different sizes.•Internal climate variability is important for masking anthropogenic climate change, especially in the near future.•The size of the catchment used to have a significant impact on the timing of the emergence of the extreme flow.•Smaller catchments experience floods (droughts) at earlier (later) times.•Instead of KS-test and standard error of the mean (SEM), signal-to-noise (SNR) approaches are too conservative to detect the timing of emergence.