Changes in the meander belt and channel pattern in the middle-low segment of the Cuarto river: Córdoba. Argentina

Abstract

The Cuarto River is an important watercourse in the Córdoba Province, Argentina. Its middle-low segment crosses the eastern plains of this province, one of the most populated and (agricultural-livestock) productive regions of the country. In this segment, the channel and fluvial belt morphometric parameters exhibit notable changes downstream. Understanding the morphodynamics of the Cuarto River allows the characterization and definition of channel reach with tendencies towards migration, aggradation and overflow. In this way, it is possible to identify critical channel reaches, a fundamental aspect for effective planning. This work aims to characterize the downstream changes in the fluvial belt, meanders geometry, channel pattern and channel slope, along the middle-low segment of the Cuarto River and link these modifications with authigenic (sediment supply and transport regime) and allogenic controls. Five channel reaches (R1-R5) were selected according to geological-geomorphological criteria and different geometric parameters (channel width, channel slope, braided index, meander geometry, confinement degree) were determined for each of them. Empirical equations were used to establish relationships between these geometric parameters. Although most parameters showed a generally downward trend towards downstream reaches, marked changes were observed. Particularly, in the R1-R2 reaches, the fluvial belt widens and is unconfined, and the channel width and braided index also increases. The relationship between meander length/channel width (Lm/w) and radius of curvature/channel width (rc/w) obtained were within the ranges for free meanders (8–14) and high mobility (2–3), respectively. These parameter values are associated with alluvial channels and suggest limitations in transport and conditions for sedimentation. However, in R3, R4 and R5, the fluvial belt narrows and becomes confined, with high sinuosity, a reduction in the width of the channel and the braiding index. This pattern is expected for reaches characterized by predominance of transport and a tendency to a mixed or suspensive load. Moreover, both groups of reaches also exhibit an abrupt contrast in bedload size (from very coarse sand/gravel in R1 and R2 to fine/very fine sand in R3, R4, and R5). The similarity between the exponents of the predictive functions (Lm/w, rc/w, and Lm/rc) and those obtained for the Cuarto River, in addition to the satisfactory regression coefficients of the measured parameters suggest an acceptable balance between meander geometry and channel width regime of sediment transport. The middle-low reach of the Cuarto River shows notable changes, which can be linked to regional-scale allogenic factors such as morphostructures and lithology (R1-R2 with elevated morphostructure, R3-R4 are transitional reaches towards depressed plains, while R5 with subsidence environments). Nonetheless, the river demonstrates the capacity to regulate these factors at the reach level through authigenic processes. Thus the variability of channel width, channel slope, braiding index, meander geometry and degree of confinement are explained mainly by downstream changes in sediment supply and transport regime (authigenic controls) and secondarily by changes in regional slope and banks lithology (allogeneic controls).