Do the river catchment models assume no change in anthropogenic activities in the catchments? Has there been work on how this impacts uncertainty estimates?

True, we assume that anthoropogenic activities remain constant. E.g. Arheimer, B., Donnelly, C. and Lindström, G. (2017). Regulation of snow-fed rivers affects ﬂow regimes more than climate change. Nature Communication 8(62). http://doi.org/10.1038/s41467-017-00092-8