Quaternary International 62 (1999) 69 }74 Sedimentological characteristics of small rivers with loessic headwaters in the Chaco, South America Oscar Orfeo CECOAL-CONICET, Casilla de Correo 291, 3400 Corrientes, Argentina Abstract Some sedimentological characteristics of #uvial environments located in the northeastern region of the Chaco}Pampa plain were analysed in order to determine the possible in#uence of the loessic headwaters of local basins. Concentrations of suspended sediments, as well as grain size of the river beds, were compared. Unexpectedly, it was found that the suspended load has an inverse relationship with the hydrometric levels, with the exception of highly saline streams (conductivity values between 4300 and 11,000 S cm). The mean concentration of suspended sediments was 63.1 and 206.3 mg l in #ood and low water periods, respectively. The grain size of bed sediments was relatively homogeneous along the whole hydrological cycle. Sandy}silty bottoms predominate in channels, followed by silty}sandy and clayey}silty bottom types. Sediment transport mainly occurs in accelerating #ow suspension, overcoming the critical erosion velocity. During #ood periods, some parameters exceeded by 70% the values recorded during the low water phase.  2000 Elsevier Science Ltd and INQUA. All rights reserved. 1. Introduction The Eastern Chaco, located in the northeastern region of the Chaco}Pampa plain, is formed by the distal areas of the alluvial fans of large rivers (Iriondo, 1993) The study region includes the distal areas of the Pilcomayo and Bermejo Rivers (Fig. 1). The #oods of these alloch- thonous rivers discharge large volumes of water into a plain landscape crossed by old #uvial belts, generating basically swamp environments. A series of local #uvial networks were developed in the Central and Eastern Chaco, all having similar character- istics. Collectors are small sinuous streams #owing in abandoned channels of large allochthonous rivers. Their upper basins (a few hundreds of square kilometers in each case) are covered by a loess mantle. Such areas are dominated by in"ltration, with little runo! and small slopes, resulting in a very small contribution of sediment to the channels. The middle and lower parts of the basins are developed on hard, impervious clay, paludal in origin. The region is a sunken block, covered by large permanent swamps and dense vegetation. Fluvial geomorphological elements are not clearly organized in hierarchical terms. Fertonani and Prendes (1983) de"ne this type of area as a `Non- Typical Hydrological Systema. Finally, the local rivers #ow into the regional collectors, the Parana H and Para- guay Rivers. The pattern and geographical location of these small rivers create particular characteristic sediment nature and dynamics, including silty beds and an inverse cor- relation between water discharge and concentration of suspended solids. These particular systems appeared in the Chaco plain during the middle Holocene (Iriondo, pers. comm.) The landscape is a!ected by alternating seasons of drought and periods of prolonged #ooding, which has resulted in the progressive coalescence of the water bodies and the saturation of the storage capacity of the basins. Climate, topography, structural conditions, and subsurface lithology de"ne the local drainage system (Patin  o and Orfeo, 1986) and shape the main landscape characteristics (Nei!, 1986). 2. Location of the studied basins The studied area comprises more than 30 local basins along 400 km of the Paraguay}Parana H collector (253 00}283 30 S; 57330}59330 W) (Fig. 2). The area covers the distal parts of the Pilcomayo and Bermejo alluvial fans and a sector of the Paraguay}Parana H belt (Iriondo, 1993). Considering the geomorphological setting of the region, the results obtained in the present investigation can be extrapolated to the entire Eastern Chaco, from 20 to 303 S. 1040-6182/00/$20.00  2000 Elsevier Science Ltd and INQUA. All rights reserved. PII: S 1 0 4 0 - 6 1 8 2 ( 9 9 ) 0 0 0 2 4 - 5