This work was supported by a grant from CONCYTEC. We are very grateful to Holger Valqui and Oscar Moran for their critical comments, and to ClaudiaMachicado and Jose Choufor their comments and help in compiling and analysing part of the DNA sequences that support this work. Thanks also to Arman- do Bernui, Javier Espinoza, Jairzhinho Ramos, Luis Marky and Cristian Orrego, for their contribution to discussions, and especially to Mrs Ellen M. Pragen for her editorial work.

This work was supported in part by a grant from the Consejo Nacional de Ciencia y Tecnologla (CONCYTEC). We thank W. Behrens and C. Santa Cruz for helpful advice on HPLC analysis and T. cruzi culture, and H. Guerra and C. Guerra for critically reading the manuscript.

The authors gratefully acknowledge the financial assistance of CONCYTEC from Peru and DSF-IRD (JEAI) from France. We express our thanks to members of the Chayahuitas community who were willing to share with us their knowledge about medicinal plants. The authors are grateful to Candy Ruiz from LID-UPCH, for technical assistance in plants extractions.

Because increasing climatic variability and anthropic pressures have affected the sediment dynamics of large tropical rivers, long-term sediment concentration series have become crucial for understanding the related socioeconomic and environmental impacts. For operational and cost rationalization purposes, index concentrations are often sampled in the flow and used as a surrogate of the cross-sectional average concentration. However, in large rivers where suspended sands are responsible for vertical concentration gradients, this index method can induce large uncertainties in the matter fluxes. Assuming that physical laws describing the suspension of grains in turbulent flow are valid for large rivers, a simple formulation is derived to model the ratio (α) between the depth-averaged and index concentrations. The model is validated using an exceptional dataset (1330 water samples, 249 con...