Contribution of Geomorphology to the dynamics of small continental wetlands with temporary flooding: the case of the Pantanal of the Pandeiros River, Brazil

Wetlands represent an excellent example of the interface between lithosphere, atmosphere, and hydrosphere. Despite being distributed throughout the planet, the pace of their destruction has increased, especially since the 1970s. In tropical regions, they may be associated with floodplains or topographical depressions permanently or temporarily flooded, with different size configurations and hydrogeomorphological dynamics. 

Among the major river systems in South America, the São Francisco river basin, located entirely in Brazilian territory, has strategic importance in providing energy and water resources to the semi-arid northeast region, with about 30 million people. In it, there are small wetlands, considered in extension, but with great frequency, located in areas of headwaters and floodplains, mainly. 

In this basin, the Pantanal of the Pandeiros watershed synthesizes the main environmental impacts are existing in the São Francisco River basin. For example, high frequency of accelerated linear water erosion processes, known locally as voçoroca (gullies), deforestation since the 1970s, which continues for the production of charcoal, the presence of a small hydroelectric plant, the reduction of water flow since the 1990s and the intense dynamics of sediment transport. 

In the São Francisco river basin, there is an abundance of rocks and regolith formed by siliciclastic material, where their contribution to the dynamics of wetlands is still not well understood. The Pantanal of the Pandeiros wetland located in the floodplain of São Francisco River has a singular occurrence. Where it is at risk of lose to provide environmental services, through silting, the Pantanal of the Pandeiros River has complex hydrogeomorphological dynamics from the delimitation of a depressed area concerning its surroundings, marked by slope rupture, with a contribution to its flooding, the increase, and maintenance of the São Francisco river level during flood pulses, determining longitudinal dynamics in the wetland. 

Were mapped four different hydrogeomorphologic zones from a multiproxy approach: 

Isotopic tracers ²H and ¹⁸O, together with piezometers, demonstrate connections between the wetland and the floods of the Rivers São Francisco and the Pandeiros, strongly influenced by past and current geomorphological forms and processes. 

Raster images of digital elevation models and satellite, during flood pulses, plus ground-penetrating radar and analyses of geocoverages records ranging from environments predominantly sandy to clayey from upstream to downstream, respectively. Levels sandy material up to 3m deep, followed by a layer of pebbles and paleochannels. 

Sediments collected up to 150 cm deep date back to 3,246 years B.P., with a predominance of humid environment, via analysis of ¹⁴C and ¹³C. The average denudation rate in the last 1.5 million years, using ¹⁰Be, a high contribution of sediments were identified, which circulate in the basin, coming from specific regions, where erosive processes are predominant, such as gullies.