Dam-trapping efficiency can be estimated by using the hydraulic residence time. On the regional scale, the global impact of several dams can be assessed by taking into account the spatial organization of dams along the river network. Therefore, in this study, a method is proposed to estimate the global-trapping efficiency, TEw, for any watershed containing more than one dam. This method is applied to the Adour, Dordogne, and Garonne River watersheds (southwestern France). The spatial organization of dams and its impact on TEw and on sediment yields measured at 37 TSM sampling stations over 2 years are discussed. Positive correlation between drainage areas and river sediment loads corrected from dam regulation using TEw, as well as comparisons between TEw-corrected sediment yields and sediment yields measured upstream from dams, point out the interest of the method in order to reconstitute the natural sediment yields. ¿ 2001 American Geophysical Union