Rates and patterns of thermal mixing at a small stream confluence under variable incoming flow conditions

Quinn W. Lewis, Bruce L. Rhoads

Research output: Contribution to journalArticlepeer-review


Confluences are important locations for river mixing within drainage networks, yet few studies have examined in detail the dynamics of mixing within confluences. This study examines the influence of momentum flux ratio, the scale of the flow (cross-sectional area) and the density differences between incoming flows on thermal mixing at a small stream confluence. Results reveal that rates and patterns of thermal mixing depend on event-specific combinations of the three factors. The mixing interface at this confluence is generally distorted towards the mouth of the lateral tributary by strong helical motion associated with curvature of flow from the lateral tributary as it aligns with the downstream channel. As the momentum flux from the lateral tributary increases, mixing is enhanced because helical motion from the curving tributary flow expands over the width of the downstream channel. The cross-sectional area of the flow is negatively correlated with mixing rates, suggesting that the amount of mixing over a fixed distance downstream of the confluence is inversely related to the scale of the flow. Density differences are not strongly related to rates of mixing. Results confirm that mixing rates within the region of confluent flow interaction can be highly variable among flow events with different incoming flow conditions, but that, in general, length scales of mixing are short, and rates of mixing are high at this small confluence compared with those typically documented at large-river confluences.

Original languageEnglish (US)
Pages (from-to)4442-4456
Number of pages15
JournalHydrological Processes
Issue number20
StatePublished - Sep 30 2015


  • Confluence
  • Mixing
  • Temperature

ASJC Scopus subject areas

  • Water Science and Technology

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