MIXING processes at river confluences have an important bearing on problems such as pollutant dispersal and the management of river systems, but remain poorly understood. Previous studies(1-11) have indicated that flow mixing downstream of a confluence is a slow process, typically completed at distances greater than 100 times the channel width. Bed morphology is now emerging as a critical factor : flume experiments have shown that the width to depth ratios(12) and height discordance(13,14) between the confluent channels should influence mixing rates(12-14). But the magnitude of these effects in real rivers is not known. Here we report measurements from three river confluences of moderate size which show that bed discordance can markedly increase mixing rates. For the rivers studied, mixing is complete at distances five to ten times shorter than those reported previously.