Field investigation of three-dimensional flow structure at stream confluences: 1. Thermal mixing and time-averaged velocities

Stream confluences are characterized by complex patterns of three-dimensional fluid motion. This paper examines the three-dimensional time-averaged flow structure at three concordant-bed confluences in east central Illinois. Two of the junctions have symmetrical planforms, whereas the other has an asymmetrical planform. Similarities among the sites include (1) pronounced convergence of flow at the upstream end of the confluences, (2) a region of stagnated fluid near the upstream junction corner, (3) a well-defined thermal mixing interface between the converging flows that persists downstream of each confluence, (4) a downstream velocity field characterized by two zones of maximum velocity separated by an intervening region of low velocity centered on the mixing interface, (5) convective acceleration of flow within the mixing interface leading to increasing uniformity of the downstream velocity field in the downstream direction, and (6) lateral deflection of flow by the dominant tributary. Prominent helical motion occurs at the asymmetrical confluence, whereas weak helicity is detectable only at one of the two symmetrical confluences. The downstream persistence of a well-defined mixing interface at the two symmetrical confluences and the disruption of this interface at the asymmetrical confluence suggest that helical motion enhances patterns of mixing at confluences.