SEDIMENTOLOGY AND TOPOGRAPHY OF CONFLUENCE OUTLETS OF SELECT MID RIO GRANDE TRIBUTARIES

Prior confluence studies have concentrated on the hydrodynamics, topography and sedimentology of confluence areas located in the main stem, with limited information available on the tributaries. A focus on the tributary mouth is particularly important when the sediment or hydraulic regime of the tributary contrasts strongly with the mainstem. Semi-arid ephemeral streams (arroyos) exemplify such a contrast. They typically flow after localized high intensity rainfall events. The sediment they deliver to main trunk rivers, such as the Rio Grande in central New Mexico, is often much coarser than the mainstem bedload. The confluences forms an essential landscape of sediment and water delivery downstream, contributing to the Rio Grande’s bed morphology, which can locally impact agriculture and threaten infrastructure. Our aim is to quantify confluence characteristics of planform geometry, discharge ratio, bed discordance, and sediment stratification at the confluence outlet of three ephemeral tributaries of the Rio Grande.

Using drone-based photogrammetry with Structure from Motion software, orthophotos and Digital Elevation Models (DEMs) allows for multi-temporal comparison of topographic changes of three tributary confluences. Analysis of the DEMs of Difference will provide volume change estimates of sediment storage at the confluence outlets. Analysis of grain size and stratification based on trenching of tributary bed sediment provides a window into the hydrodynamic conditions during deposition of the morphological units at the tributary confluence outlet.

Preliminary findings show the Rio Grande basin experienced abnormally high flow stage due above average snowmelt in spring 2019. This allowed for the deposition of a well-sorted temporary sandbar at the mouth of the three tributaries, creating an asymmetric tributary mouth bar with preferential deposition occurring downstream from the junction corner. Cross-stratification of the bar deposits preserve evidence of flow separation along the tributary mouth and development of a recirculating eddy within the tributary during high flows on the Rio Grande that coincided with lack of flow (backwater conditions) in the tributary.