THE INFLUENCE OF TRIBUTARY CONFLUENCES ON CHANNEL FORM AND SEDIMENT TRANSPORT: RIO CHAMA, NM

Numerous morphological changes can occur where two channels of distinct sediment and flow regimes meet, including abrupt shifts in channel slope, cross-sectional area, planform style, and bed sediment size along the receiving channel. Along the Rio Chama between El Vado and Abiquiu Dams, northern New Mexico, arroyo tributaries intermittently deliver sediment from erodible sandstone and shale canyon walls to the mainstem channel, where flow is dominated by snowmelt runoff. Much of the tributary activity occurs in flash floods and debris flows during summer thunderstorms, which often load the channel with sand and leave coarser material at the mainstem confluence. Our prior work along the Rio Chama suggested sediment delivery at these confluences influences the pattern of channel narrowing associated with upstream dam operations. To examine tributary controls, we systematically collected cross-section elevation and bed sediment data up and downstream of 25 tributary confluences along a 17 km reach. Data from 203 cross-sections were used to build a one-dimensional hydraulic model for comparing estimated channel parameters at bankfull and low-flow conditions at these sites As compared to intermediate reaches, confluences primarily impact gradient and sediment size, reducing both parameters upstream of confluences and increasing them downstream. Cross-section area is also slightly elevated above tributary confluences and reduced below. Major shifts in slope and bed sediment size at confluences appear to drive variations in sediment entrainment and transport capacity and the relative storage of sand along the channel bed. At a larger scale, hillslope-channel coupling increases in the downstream third of the study reach, where the canyon narrows, resulting in steeper slopes and larger bed material along the mainstem, and potentially limiting impacts of tributary inputs and altering the sediment transport regime. However, channel form and sediment characteristics are highly variable along the study reach, reflecting variations in the size and volume of sediment inputs related to tributary watershed parameters (i.e., slope, size, lithology, sediment storage), morphology of the Rio Chama at the junction (i.e., bends, confinement), and the relative magnitude and location of past depositional events.