Event sequence and sediment exhaustion in the lower Panuco Basin, Mexico

Abstract

Although large tropical river systems transport the majority of the world's runoff and sediment, few studies have examined the dynamics of suspended sediment transport in this setting. This includes Mexico, which contains two of the larger river basins draining into the Gulf of Mexico. This study provides a detailed analysis of discharge (Q) and suspended sediment transport dynamics for the lower Panuco basin (98,227 km 2) of eastern Mexico, including the lower Moctezuma, lower Tamuin, and Panuco rivers. Daily values of discharge and suspended sediment concentration (SSC) are examined for three stations, spanning from the late 1950s to early 1990s. Although the Panuco is a large basin, the relationship between discharge and suspended sediment concentration represents a departure from the oft-reported study by Heidel [Trans. - Am. Geophys. Union 37 (1956) 56]. Instead of sediment lagging discharge, the sediment wave precedes discharge by an average of 1.7 days, displaying pronounced clockwise (positive) hysteresis. This may be due to the sources of runoff and sediment, which are derived primarily in the lower reaches of the basin because of precipitation patterns, geology, and basin morphometry. The distinct differences in lithology between the Tamuin and Moctezuma basins, which join in the coastal plain to form the Rio Panuco, allow the influence of geologic setting to be considered. Q–SSC relations display positive hysteresis where the sediment is derived from erodible Tertiary shale deposits in the Moctezuma basin. In contrast, Q–SSC relations in the Rio Tamuin, which transports less suspended sediment because of its karstic setting, are more linear due to these relations being dependent on the entrainment of bed material. Consideration of the sequence of events during the rainy season suggests that much of the hillslope-derived sediment is flushed through the system by early events, which would occur because of exhaustion of hillslopes sediments. Q–SSC relations are increasingly linear during later events in the Rio Panuco, which is likely due to these events transporting a higher proportion of sediment entrained from the channel. This is manifest by a reduction in lead times, as the number of days between peak SSC and peak Q for early and late events significantly decreases from 2.6 to 0.9, respectively. Large events (>750 m 3/s) in the latter part of summer and early fall having high suspended sediment concentrations could be due to the replenishment of hillslope sediments, or due to the contribution of sediment from source areas that were not previously exhausted. The findings from this study are likely common to most drainage systems having seasonal bimodal streamflow regimes, whereas the seasonal signature of Q–SSC relations is likely enhanced in those settings where agriculture is a major component of the landscape.