Geomorphology and evolution of the late Pleistocene to Holocene fluvial system in the south-eastern Llanos de Moxos, Bolivian Amazon

Abstract

In the Bolivian Amazon several paleochannel generations are preserved. Their wide spectrum of morphologies clearly provides crucial information on the type and magnitude of geomorphic and hydrological changes within the drainage network of the Andean foreland. Therefore, in this study we mapped geomorphological characteristics of paleochannels, and applied radiocarbon and optically stimulated luminescence dating. Seven paleochannel generations are identified. Significant changes in sinuosity, channel widths and river pattern are observed for the successive paleochannel generations. Our results clearly reflect at least three different geomorphic and hydrological periods in the evolution of the fluvial system since the late Pleistocene. Changes in discharge and sediment load may be controlled by combinations of two interrelated mechanisms: (i) spatial changes and re-organizations of the drainage network in the upper catchment, and/or (ii) climate changes with their associated local to catchment-scale modifications in vegetation cover, and changes in discharge, inundation frequencies and magnitudes, which have likely affected the evolution of the fluvial system in the Llanos de Moxos. In summary, our study has revealed the enormous potential which geomorphic mapping and analysis combined with luminescence based chronologies hold for the reconstruction of the late Pleistocene to recent fluvial system in a large portion of Amazonia. AbstractIn the Bolivian Amazon several paleochannel generations are preserved. Their wide spectrum of morphologies clearly provides crucial information on the type and magnitude of geomorphic and hydrological changes within the drainage network of the Andean foreland.Therefore, in this study we mapped geomorphological characteristics of paleochannels, and applied radiocarbon and optically stimulated luminescence dating. Seven paleochannel generations are identified. Significant changes in sinuosity, channel widths and river pattern are observed for the successive paleochannel generations. Our results clearly reflect at least three different geomorphic and hydrological periods in the evolution of the fluvial system since the late Pleistocene. Changes in discharge and sediment load may be controlled by combinations of two interrelated mechanisms: (i) spatial changes and re-organizations of the drainage network in the upper catchment, and/or (ii) climate changes with their associated local to catchment-scale modifications in vegetation cover, and changes in discharge, inundation frequencies and magnitudes, which have likely affected the evolution of the fluvial 2 system in the Llanos de Moxos. In summary, our study has revealed the enormous potential which geomorphic mapping and analysis combined with luminescence based chronologies hold for the reconstruction of the late Pleistocene to recent fluvial system in a large portion of Amazonia.

Alluvial plain dynamics in the southern Amazonian foreland basin

“…The results of this study, combined with other published data (Guyot et al, 1996;Lombardo, 2014;do Nascimento Jr. et al, 2015;Plotzki et al, 2015), suggest that these small rivers are highly active and play a dominant role in shaping the SAFB alluvial plains. These rivers, and in particular the Sécure, Isiboro, Moleto, Maniqui, Piraí and Grande rivers, show extremely reduced meander migration rates downstream from where the crevasses opened up (for example Fig.…”

“…Stratigraphic cores performed across the alluvial plain have shown that, since the mid-Holocene, distributary fluvial systems formed by the Mamoré's tributaries (Fig. 1) have deposited thick layers of sediments over the southern and central part of the LM (Lombardo, 2014;Plotzki et al, 2015). This region hosts one of the most important collections of pre-Columbian earthworks in Amazonia, including monumental mounds, raised fields, ring ditches, fish weirs, canals and causeways (Lombardo et al, 2011;Prümers and Jaimes Betancourt, 2014).…”

Holocene land cover change in south-western Amazonia inferred from paleoflood archives

“…A different situation is found on the eastern side of the Mamoré River, the furthest away from the Andes. Most of this area was covered with a sedimentary lobe deposited by the Río Grande (Lombardo et al, 2012;Plotzki et al, 2015). All the samples that indicate a vegetation shift from SF savannah to Forest are located here.…”

“…The southern movement of the South American Summer Monsoon caused an increase in precipitations at about 4 to 3 kyr BP and the southward expansion of the Amazonian rainforest into north-eastern LM at the expense of the savannah (Carson et al, 2014;Mayle et al, 2000). Large scale river shifts, such as those of the Beni River (Dumont, 1996), the Mamoré (Plotzki et al, 2013), the Grande River (Lombardo et al, 2012;Plotzki et al, 2015) and several other tributaries of the Mamoré (Lombardo, 2014;Lombardo, 2016), formed fluvial distributary systems that covered most of the CSM (Lombardo et al, 2018). During the late Holocene, the combined action of strong winds and waves formed the hundreds of geometric and oriented lakes that dot the LM landscape (Lombardo and Veit, 2014).…”

Holocene floodplain soils along the Río Mamoré, northern Bolivia, and their implications for understanding inundation and depositional patterns in seasonal wetland settings

“…Alluvial surface soils in the LM show abundant pedological evidence for dominantly hydromorphic conditions (Boixadera et al, 2003). Recent studies on the type and chronologies of the paleochannel systems in the LM inundation savannah have documented large-scale changes in inundation patterns and/or frequencies, likely as the result of the interplay between environmental and tectonic drivers (Plotzki et al, 2011;Lombardo, 2014;Plotzki et al, 2015). In this context, periods of reduced flooding intensities in the southeastern LM seem to be expressed in the development of regional-scale paleosols, which underlie mid-to late Holocene fluvial and lacustrine sediments (Lombardo et al, 2012), or are exposed in cut-banks along the outer meanderbelts of most major rivers in the LM (Iriondo, 1999).…”