A Landscape Built by Sand
The Taquari River Megafan is one of the largest active sediment deposits on the planet. Covering an area of roughly 50,000 square kilometers, it is a major feature of the northern Pantanal, the world's largest tropical wetland. The fan began forming during the Quaternary period as the Taquari River, flowing from the highlands of the Planalto, dumped its heavy sediment load upon reaching the flat Pantanal basin. This process created a massive, gently sloping cone of sand and silt that stretches 250 kilometers from its apex to its toe.
The core process driving the megafan's growth is called avulsion. The river carries so much sand that it continuously clogs its own channel. As the channel bed rises, it eventually becomes higher than the surrounding floodplain. At this point, often during a major flood, the river breaches its natural levees in an event locally known as an "arrombado," and carves a new, steeper path across the plain. This process has repeated itself for millennia, with the river periodically abandoning its old course and creating a new one, slowly building up the fan lobe by lobe. Satellite imagery reveals numerous paleochannels, the traces of the river's former paths, crisscrossing the landscape.
An Accelerated Crisis
While the megafan is a natural geological feature, human activity has drastically accelerated the process of sedimentation since the 1970s. The expansion of agriculture and cattle ranching in the highlands, or Planalto, has led to increased soil erosion. This surge in upstream erosion has overloaded the Taquari River with sediment, delivering about 72% of the total suspended sediment that enters the Pantanal Basin.
The consequences have been severe. The river channel is silting up faster than ever, leading to more frequent and larger "arrombados." One major avulsion event that began in the late 1980s has permanently altered the landscape, creating a persistently flooded area of about 5,000 square kilometers. This flooding displaces cattle ranchers and disrupts the ecological balance of the Pantanal, turning productive wetlands into sandy, barren areas. Decades of engineering efforts to dredge the channel or build artificial levees have been ineffective against the immense geological force of the river. The ongoing avulsion means the river is slowly shifting its mouth, a process that models predict might be complete by 2080, when it establishes a new permanent channel far to the north of its previous course.