LITHOSPHERIC FLEXURE SETS FLUVIAL MORPHOLOGY, DISSOLVED LOAD GEOCHEMISTRY, AND BASIN HYDROLOGY IN THE LOWLAND RIO BERMEJO, ARGENTINA

The interplay between tectonics and topography is well-studied in mountain ranges, but these interactions have received considerably less investigation in low-relief areas. Here, we use the Rio Bermejo, Argentina as a test case to examine links between lithospheric flexure and earth surface dynamics in a lowland setting. The Rio Bermejo transverses the east Andean foreland basin, where lithospheric flexure induced by the topographic load of the Andes creates a subsiding foredeep proximal to the mountain front, followed by an uplifting forebulge, and a distal, slightly subsiding backbulge. This flexure causes the Rio Bermejo to transition from an aggrading, braided channel that is super-elevated above the surrounding floodplain in the subsiding foredeep, to a narrow, meandering river that is incised up to 13 m below the floodplain in the uplifting forebulge. As the river traverses from the forebulge to the slightly subsiding backbulge, the channel further narrows and experiences a drastic decrease in lateral migration rate. We suggest that these flexure-induced morphologic changes also influence the hydrology and geochemistry of the Rio Bermejo. We observe a ~25% reduction in discharge moving downstream along the Rio Bermejo, and suggest that this is due primarily to the channel super-elevation in the foredeep, which limits the influx of groundwater to the channel, but allows surface water to spill out from the channel during floods and enter the groundwater reservoir. In the foredeep, incision of the channel into the surrounding floodplain allows an influx of solute-rich groundwater into the river, resulting in an ~20% increase in the dissolved load solute concentration. In the backbulge, the groundwater table intersects with the surface in multiple locations, causing the formation of spring-fed channels disconnected from the mainstem Rio Bermejo. Our work highlights how relatively subtle tectonic forcing can result in large scale changes in the geomorphic, hydrologic and geochemical expression of river systems.