This dataset was produced as part of an investigation into the sources of mercury (Hg) in streams of the lower Mackenzie River Basin (MRB), carried out between 2018 and 2020 by scientists from Uppsala University, the Swedish University of Agricultural Sciences, Stockholm University, and Trent University in Peterborough, Ontario. Sampling in 2018 was carried out between 8-30 June at 19 sites between latitudes 59° and 68° N. Between 14 June 2019 and 3 March 2020, additional samples were collected by community partners at 2-3 month intervals from 4 rivers. Only basic water properties and dissolved Hg levels are reported here. Additional parameters, including stable isotopes of Hg, C, O and H, as well as radiocarbon, are reported and archived elsewhere.