Cross-river temperature profiles were run in mid-June 1971 at approximately 20 km intervals from the confluence of the Great Bear and Mackenzie Rivers, at Fort Norman, N.W.T., to the Mackenzie Delta, a distance of 650 km. As the Great Bear River was cold, the Mackenzie River warm, and as temperatures were read to better than ±.01 °C in the field, the cross-profiles have provided a record of the lateral mixing of the two rivers. A flow distance of 500 km was required for nearly complete mixing. The 1971 cross-river temperature profiles and aerial infrared imagery taken in 1969 show a good agreement in the mixing pattern. It is suggested that where water temperature contrasts exist, temperatures, which can be read easily with a resolution of better than ±.01 °C, may serve as one of the easiest and most economical of the tracers suitable for mixing studies.