ALBERT

Description: ABSTRACT 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. Much of the tributary activity occurs in flash floods and debris flows during summer thunderstorms, which often load the channel with sand and deposit coarser material at the mainstem confluence. In contrast, mainstem channel flow is dominated by snowmelt runoff. To examine tributary controls, we systematically collected cross-section elevation and bed sediment data up and downstream of 26 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 bed 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. The data were analyzed and compared to models of channel organization based on lateral inputs, such as the Network Variance Model (Benda et al., 2004) and the Sediment Link Concept (Rice and Church, 1998). 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 more continuous coarse bed material along the mainstem, and thus, limiting the contrast with tributary confluences. 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 the surface geology in tributary watersheds, morphology of the Rio Chama at the junction (i.e., bends, confinement), and the relative magnitude and location of past depositional events. This article is protected by copyright. All rights reserved.