Imbalanced bedload transport in a river confluence: A field study

The confluence of the regulated Tolten River and its tributary, the unregulated Allipen (south of Chile), has proved dynamic in the last decade. Daily bedload measurements with a Helley-Smith sampler are obtained from a field campaign over 3 months in the high-flow season. The goals are to quantify total bedload, its distribution in space, and bedload texture, and to understand the balance in bedload between the tributary and main river. The bedload transport varies 200-fold, with a maximum of 5000 t/d (ton/day). The discharge varies fivefold, with a maximum of 900 m(3)/s. Two-thirds of the total bedload is transported through the deeper area of the cross section and gravel is predominant (64%). However, the spatial distributions of bedload differ substantially depending on whether the contribution of the tributary to the total discharge is dominant or the main river is dominant. The paper focuses on this "imbalanced" bedload distribution.