The Ucayali River is one of the most dynamic large rivers in the world, with high rates of channel migration regularly producing cutoffs. In the lower portion of the Ucayali River, before its confluence to the Marañon River where the Amazon River is born, the increase in water and sediment discharge triggers bends with secondary channels (transitional stage from purely meandering to anabranching), which influence the planform migration rates and patterns of the sinuous channels. Based on remote sensing analysis, a comparison of planform dynamics of bends with and without secondary channels is presented. For the case of a bend with secondary channels (Jenaro Herrera, JH), detailed field measurements for bed morphology, hydrodynamics, bed and suspended load are performed for low-, transitional- and high-flow conditions (August, February and May, respectively). Additionally, a two-dimensional depth average hydraulic model is utilized to correlate observed migrating patterns with the hydrodynamics. Results indicate that the secondary channels have disrupted typical planform migration rates of the main meandering channel. However, at high amplitudes, these secondary channels reduce their capacity to capture flow and start a narrowing process, which in turn increases migration rates of the main channels (meandering reactivation process), suggesting that an imminent cutoff along the JH bend is underway by pure lateral migration or by the collapse into the existing paleochannels.

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Multiple factors control the flow distribution in a river bifurcation. However, one factor that requires further analysis is the effect of upstream reservoirs. Such is the case of the bifurcation of the Mezcalapa River into the Samaria and Carrizal Rivers located in the lowlands of the Grijalva River basin, Mexico. Four dams were commissioned in the upper basin, the first in 1967 and the last in 1987. Since the late 1960s the distribution of the flow at the bifurcation has varied considerably. The flow captured by the Carrizal River between the late 1960s and mid 1980s gradually decreased from 40% to 10%, and later, between the mid 1980s and early 2000s, it increased up to 75%. During the same period, the Samaria River had a reversed pattern. Here, using a two-dimensional hydro-morphodynamical model, the impact of an upstream reservoir (that causes homogenization of the flow hydrograph and sediment retention) on bed morphology and flow distribution in a river bifurcation is analyzed. The results indicate that the sediment retention is the factor of largest impact on the flow distribution. The upstream channel incises, and the process propagates to the diffluent with the larger energy gradient developing positive feedback were the erosion increases the hydraulic capacity to capture more flow, resulting in more erosion. The flow homogenization has also an impact. The suppression of peak flows reduces the sediment load capacity in the diffluent with lower energy gradient, generating sedimentation at its entrance and diminishing its hydraulic capacity.

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