Multi‐annual contemporary flood event overbank sedimentation within the vegetated lower Orinoco floodplain, Venezuela

Abstract

Multi‐annual contemporary flood event overbank sedimentation rates were quantified on the World's third largest river in terms of discharge, the tropical lower Orinoco. We discuss the role of variables at the basin and reach scales that contributed to the complexity of spatio‐temporal overbank sediment deposition patterns. Monitored in situ plots were characterized by distance to the main channel, hydroperiod, different geomorphological units, and vegetation cover. Flood event sedimentation rates showed a high spatial variability ranging from the absence of sediment deposition up to 225.46 kg m‐2 yr‐1. Banks and levees received relatively high amounts of sediment (39.6 kg m‐2 yr‐1), whereas observed mean sedimentation rates on the more distant floodplain and backswamps tended to be lower (17.7 kg m‐2 yr‐1). Significant differences in sedimentation rates were observed in two major vegetation types: dense herbaceous and shrubby vegetation (42.2 kg m‐2 yr‐1) and floodplain forest (12.7 kg m‐2 yr‐1). However, overbank sedimentation patterns also reflected imbricated hydrosedimentary and biogeomorphological vegetation feedbacks that co‐construct fluvial landforms. The incidence of an El Niño–Southern Oscillation–La Niña episode during the study period on sediment availability and floodplain sedimentation suggests that within whitewater rivers, where suspended sediment concentrations are naturally high, hydrological connectivity seems to be more important for floodplain sedimentation than variations in suspended sediment concentrations. These results may provide a good basis for future biogeomorphological investigation projects using complementary methodologies, in order to better anticipate global change and fluctuations in the occurrence, strength or duration of El Niño–La Niña episodes in the tropical zone and their consequences for flood discharge and sediment dynamics during channel–floodplain exchanges.

Changes in selected physico-chemical properties of floodplain soils in three different land-use types after flooding

“…Medians ± median absolute deviations (MAD) of basic parameters of the topsoil and the subsoil for the plots situated along the cross-sectional transect of the park (Figure 3), (0.00 = < 0.005) and 93-95%, respectively. Thus, the accumulation processes of various grain sized sediments in the park were affected both by local geomorphology and by the type of vegetation, as also published in other studies (Thonon et al 2007, Rodríguez et al 2019.…”

Vegetation characteristics control sediment and nutrient retention on but not underneath vegetation in floodplain meadows

“…In the same three plots the sedimentation, especially underneath the vegetation, was still reasonably high (19.65 -66.85 mg m -2 [overall median 60.55 mg m -2 ]), which is in contrast with other studies that found exponential decreasing sedimentation rates on horizontal lines in the floodplain (70,71). Also other studies found decreasing amounts of sediment with increasing straight distance from the river (27,29,40,72), with increasing flow path (42) and with decreasing hydrological connectivity (15,43,44). Our result show the substantial role of shallow sites, such as abandoned meander and depression within the floodplain for sediment retention.…”

“…Such a measure for the true 'hydrological distance' may thus better represent the topography of the floodplain. Some studies used other terms to describe a similar measure such as the flow path (41,42) or the hydrological connectivity (15,43,44). The aim of this study was a holistic analysis putting vegetation and topography control in perspective by first disentangling sedimentation on and underneath the vegetation under in situ conditions, second by quantifying the relative importance of vegetation characteristics in relation to topographical parameter and third, by investigated the effects of additional vegetation characteristics (species diversity and leaf surface structure) on sedimentation within a vegetation patch.…”

“…Such a measure for the true 'hydrological distance' may thus better represent the topography of the floodplain. Some studies used other terms to describe a similar measure such as the flow path (41,42) or the hydrological connectivity (15,43,44).…”

Professor Geoffrey Petts (1953–2018): An outstanding interdisciplinary river scientist

“…Tabacchi et al () explore the biogeomorphological succession driven by interactions between plants and physical processes in river corridors, specifically highlighting the importance of species composition and plant traits. Rodríguez et al () also consider interactions between plants and sediments that give rise to patterns of overbank sedimentation within the lower Orinoco floodplain. Finally, Javernick and Bertoldi () present the results of experimental flume research designed to investigate how gravel‐bed braided rivers naturally control vegetation encroachment through morphological processes and the impact of both naturally‐occurring and induced avulsions on those processes.…”