Spatial patterns of floodplain sediment texture at the lower Orinoco

JOHANNES STEIGER; JEAN-LUC PEIRY; J. ROSALES; J. L. LOPEZ; A. LARAQUE; B. CASTELLANOS; ANA CASADO

Manaos

Conferencia; 2° International Conference on the Status and Future of the World's Large Rivers; 2014

BOKU - University of Natural Resources and Life Sciences

The objective of this investigation is to analyze spatial patterns of sediment texture across the river channel and the adjacent floodplain of the lower Orinoco river. Every year between May and November, the lower Orinoco river floodplain is inundated by a flood pulse of geomorphologic and ecological significance. The studied river section is located upstream of the Orinoquia Bridge (Isla de Mamo), where maximum annual flood magnitudes of 76 000 m3 s-1 and specific total suspended sediment fluxes of 89 t km-2 yr-1 are observed. The river channel runs along a geological fault and is laterally constrained by the Precambrian Guiana shield on its right bank. The floodplain mainly develops on the left bank and large, rather stable, vegetated islands divide the main channel. During field investigations in March 2009 (i) topographic profiles were determined along transects on the island and floodplain perpendicular to the main channel using a DGPS, and (ii) 75 subsurface sediment samples were taken along these transects within different geomorphic units and vegetation covers, as well as from the bottom of the main river channel using a dredge sampler. Grain size analyses were carried out at the laboratory and multivariate statistics were performed on the results obtained. Trends of spatial patterns of sediment texture are interpreted in relation to distance to channel, altitude, geomorphic unit and vegetation cover. Floodplain construction is largely determined by sediment supply during flood events. A high potential availability of sediment with a wide range of grain sizes was observed within the lower Orinoco river. However, preferential axes of flood flow, topography, and vegetation cover within the floodplain largely control the variable spatial pattern of its sediment textures. This pattern in turn contributes to structuring the geomorphic template which supports the high biodiversity of this unique tropical ecosystem.