Length Scales, Rates, and Variability of Mixing Downstream of River Confluences

Past work on lateral mixing downstream of river confluences has focused on mixing at individual confluences, limiting general knowledge of this process. This study evaluates the average length scales, rates, and variability of lateral mixing downstream at 43 confluences for 150 events based on contrasts in gray-scale intensity of confluent flows captured by aerial images. Only 45% of the events exhibit complete mixing over the length of the imaged downstream reach. The average dimensionless length scale (s_d) of complete mixing is 7.4 times the downstream flow width and varies from s_d = 1.1–26.3, but not all flows completely mix. A nonlinear spatial pattern of mixing, consistent with relations derived from shallow flow theory, reveals that mixing rates are substantially higher immediately downstream of confluences with most mixing occurring over s_d < 15. The findings confirm that confluences are important locations of accelerated mixing in river networks.