Non-contemporaneity of flow events as a control on the morphology and sedimentary character of ephemeral tributary junctions

River confluence geometry, dynamics, and sedimentology have predominantly been studied from the confluence downstream. Having also identified a paucity of literature on confluences of ephemeral streams, we concentrated our observations on the topography, sedimentary character, and morphologic changes that occurred during two monsoon seasons at ephemeral confluences with the Rio Grande in central New Mexico, USA. We quantified how trunk and tributary water discharge modified the junction and lowermost reach of the tributary through deposition and subsequent erosion. By excavating the sedimentary deposits, we found evidence of recirculatory deposition in the confluence mouth when only the trunk confluent flowed, and coarse bar deposition when flow was dominated by the smaller, ephemeral confluent. Trunk sediments deposited in the ephemeral branch exhibited parallel as well as cross stratification, with thicknesses decreasing upstream from the junction. Tributaries with wide junction mouths at the confluence hosted more extensive low-energy flow zones that promoted deposition of trunk river sediment, diverting and temporarily storing sediment during high trunk flows. Larger flow events in the tributaries then dissected the prior confluence junction deposits. The nature of these eddy and slackwater deposits and their resultant topographies are ephemeral, but with expected hydraulic and ecological consequences.