Confluence of Ephemeral Tributaries With The Mid-Rio Grande

River confluences have been studied for the past half century, concentrating on confluence hydrodynamics (e.g., momentum ratios, turbulent and coherent flow structures), confluence channel size and shape, sediment characteristics and their dependence on confluence angle, symmetry, sediment texture and the relative water/sediment discharges of tributary and trunk. Recent studies have also directed attention at engineering methods to decrease depth of confluence scour holes, and increase tributary width, thereby intensifying habitat heterogeneity.

Few studies have concentrated on the tributary channel upstream of the confluence, with exceptions concerning (i) situations where the scour hole progresses upstream into the tributary, (ii) the non-coincidence of flows in trunk and tributary streams giving rise to temporally varying confluence deposits, and (iii) the dating of slackwater deposits located in tributaries.

We studied the sedimentary character and the temporally varying morphology of three ephemeral confluents of the mid-Rio Grande close to Socorro, NM: Arroyo de la Parida, Arroyo de los Pinos, and Arroyo de las Cañas, focusing on these tributaries upstream of the confluence. Trenching the confluent deposits, we identified bedding type and thickness. Using repeat drone flights and applying Structure from Motion (SfM), we generated orthophotographs and Digital Elevation Models (DEMs), from which we determined the temporally varying tributary morphology and volumes of sediment storage from DEMs of Difference. We applied and compared three methods of change detection, choosing the optimal 95% confidence interval threshold for a Fuzzy Inference System.

Topographic changes took place in these tributaries depending on their width, confluence angle and upstream stable section, as detailed hereafter. The arroyo de la Parida was widened at its confluence by the Bureau of Reclamation. No topographic change was detected when only small events occurred, but several 2020 flash floods deposited sediment wedges up to 1 m thick, particularly in the southern, downstream edge. The Arroyo de las Cañas confluence comprises two outlets separated by a vegetated bar. Considerable (ca 1 m) deposition occurred in the northern, upstream outlet, whereas half as much deposition took place in the southern, downstream outlet, the latter having a smaller confluence angle with the Rio Grande, thereby facilitating the transfer of tributary sediment to the trunk. The Arroyo de los Pinos experienced a large flood on July 2018, generating extensive, 1 m scour throughout the lower 200 m, depositing large bars in the center of the Rio Grande. The eroded channel slowly aggraded in its upstream reach, with local erosion in the downstream corner of the confluence.

The sedimentary deposits in the Arroyo de los Pinos confluence comprise lower clay overlain by fine sands and medium cross laminated sands, the latter overlain by slackwater and bank collapse clays. This succession is an indication that the Rio Grande's overbank deposition was generated by recirculatory flow. A similar succession was laid in the Parida confluene, with sandy-gravel at the bottom of the sequence and as much as 7 intercalations of clay laminae overlain by cross-bedded sands. These may represent sequential deposition by slight changes in generally increasing Rio Grande water levels.