Fluvial depositional architecture in an unconfined environment is governed by sediment dispersal across the alluvial plain through river-path switching by avulsion. Documented inter-avulsion periodicity from modern rivers ranges from tens to over a thousand years. In this study, a quantitative spatio-temporal reconstruction of avulsion history is presented of the non-vegetated and pristine modern Río Colorado dryland river system in the semi-arid Altiplano Basin (Bolivia), based on the integrated analysis of satellite imagery and absolute age dating using optically stimulated luminescence, complemented with sedimentological and geomorphological ground-truth data. This approach enables us to reconstruct the chronological order of channel belts of the Río Colorado, to determine avulsion recurrence time and inter-avulsion periodicity, to identify mechanisms for flow path changes, and to present a morphodynamic model for the spatio-temporal evolution of fluvial deposits in a semi-arid environment. In a maximum timespan of 12.71 ± 1.5 ka, successive avulsions of the Río Colorado created a sheet of interconnected fluvial deposits, consisting of diverging and juxtaposed alluvial ridges that formed by sediment aggradation in point bars, crevasse splays, levees, and on the channel floor. The ridges show lateral onlap and amalgamation as the result of repeated avulsion and compensational stacking, whereby the river avoided the positive alluvial-ridge relief of its precursors. The resultant morphology is fan-shaped, convex-up with a surface area of approximately 500 km ² and a maximum observed thickness of 3 m. The results show inter-avulsion periods of the river of up to 1.28 ± 0.34 ka. A paucity in fluvial activity around 2 ka BP, and at present, is interpreted as the result of low river discharge related to long-term dry periodicity in the El Niño Southern Oscillation circulation system. Each river path started as a low sinuous, single-thread channel in a narrow belt, and in time increased its width and sinuosity by point-bar expansion and rotation.

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Sedimentation at the terminus of low-gradient river systems in a semi-arid climate setting is characterized by thin- but laterally extensive amalgamated sand sheets. Analysis of absolute age dating with Optically Stimulated Luminescence measurements of fluvial sands in the Holocene Río Colorado (Altiplano Basin, Bolivia) aimed to (1) analyze the processes that created this sedimentary architecture, and (2) provide an analogue for thin but laterally-extensive sandstone reservoirs in an overall low net-to-gross setting such as the Rotliegend feather edge. The Río Colorado created a network of laterally-amalgamated fluvial sands by successive river-channel switching as the result of avulsion in a time period of only 4000y. The network formed by successive river avulsions with frequencies from 60 to 910y. The total area covered with these deposits approximates 500 km2; maximum thickness of the deposits is 2m. Fluvial sediment accumulated by vertical aggradation of sand in levees, vertical stacking and lateral amalgamation of crevasse splays, and vertical aggradation of channel-floor sand. This resulted in alluvial ridges with a positive relief on the floodplain. Subsequent river positions avoided the positive relief, and juxtaposition of successive river positions resulted in compensational stacking and amalgamation of fluvial sand deposits to a laterally connected extensive sand sheet. @en

The Geological Modeling Project GM2 under the Joule I research program was initiated in 1990. Two consortia were forrned, which concentrated on separate aspects of geological modeling. The TUD, CSIC, TNO consortium dealt with geological characterization of fluvial sediments as a tool for mathematica! reservoir modeling, whereas the DALUC consortium developed interactive sequentia! forward-modeling software for the investigation and delineation of hanging-wall traps. @en