USGS1976
The Lower Rio Bermejo in many ways is atypical of most rivers. In general it narrows as it flows downstream; its flow decreases downstream even though the average annual rainfall at its mouth is twice that at its head (1,200 mm vs. 600 mm) but its suspended-sediment discharge increases considerably downstream. The Lower Rio Bermejo begins where the river breaks out of the Andes Mountains. At first it flows over an alluvial fan in a wide braided channel with low banks that are overtopped during floods. Leaving the braided channel the river meanders for the remainder (80 percent) of its length in a narrow channel with high banks that do not seem to have been overtopped recently. The meanders are frequently cutoff, and along the river there are many oxbow lakes. There are also several abandoned stream channels. Old soil horizons, commonly seen in the banks, and terraces indicate the river has downcut. Tectonic movement in the basin probably has had a considerable effect on the river. The overall gradient of the Lower Rio Bermejo along the channel is 18 cm/km; the slope of the land surface in the basin parallel to the river is 32 cm/km. Water discharge and suspended-sediment concentrations have been measured at El Yacare and El Colorado, both of which are in the meandering part of the river. In 1970-71 the river at El Yacare, the upstream station, transported about half the suspended sediment transported at El Colorado and less than the Upper Rio Bermejo brought into the Lower Rio Bermejo. Therefore, between the head of the Lower Rio Bermejo and El Yacari (445 km) there was net deposition and between El yacare and El Colorado (575 km) there was net erosion. The river at El Yacare was narrower, deeper and swifter than at El Colorado. The Rio Pilcomayo is quite different from the Rio Bermejo even though they may appear similar on a map. The Rio Pilcomayo disappears near Laguna Escalante. The Rio Pilcomayo from Villa Montes, Bolivia to the Laguna is called the Rio Pilcomayo "Superior" and the river which appears about 160 km downstream is called the Rio Pilcomayo "Inferior." The Rio Pilcomayo "Superior" commonly overflows its tanks and changes its channel. It transports about 100,000,000 tonnes-1/ of sediment per year and the average-annual concentration of suspended sediment is more than 6 kg/m 3 . The Rio Pilcomayo "Inferior," which seems not to have flooded in several years, is meandering.
Rio Bermejo basin, Rio Pilcomayo basin↗