Data relating to the mixing rate of the Liard and Mackenzie Rivers were collected in June 1968, for a distance of 300 miles (~482.8 km) downstream from their confluence. Samples were collected from 11 cross sections, spaced about 30 miles (~48.3 km) apart. At each cross section, 5 surface samples and 3 subsurface samples from a depth of 20 ft (~6 m) were collected, to give a total of 88 samples.The temperatures at the 88 sample points were measured in place. Secchi disk transparency readings were taken for the 55 surface sites. Water samples collected at the 88 sites were laboratory tested for their turbidity, conductivity, and sodium and chloride concentrations.Analyses of the results show that the Mackenzie and Liard waters were not fully mixed some 300 miles (~482.8 km) downstream from their confluence. Some overflow and underflow, due to density differences, probably occurred.Quite possibly the long distance required for mixing may be more common, in large broad rivers, than is generally realized.