A three-dimensional ﬂow model that uses the RNG k-ε turbulence model and a non-equilibrium wall function was applied to the River Klarälven in the southwest part of Sweden. The objectives were to study the nature of the ﬂow in the river bifurcation and to investigate the short-term sediment transport patterns in the river. The effectiveness of three-dimensional ﬂow models depends upon: (1) how well the river geometry and it surface roughness are modelled; and (2) the choice of the closure model. Improvements were obtained by modelling the river in two parts: the entire river reach, and a selected part. Composite Manning coefﬁcients were used to account for roughness properties. The method requires a calibration process that ensures the water surface proﬁles match the ﬁeld data. The k-ε model under-predicted both the extent of ﬂow separation zones and the number of secondary ﬂow regions having a spiral motion, in comparison with the RNG k-ε model. The 3-D model could predict with good accuracy both the general and secondary ﬂow ﬁelds in the river. The results agreed well with the 3-D velocity measurements using an acoustic Doppler current proﬁler. A conceptual model was developed that accounts for the development of secondary ﬂows in a river bifurcation having two bends. The main ﬂow feature in the river cross-sections was the existence of multiple counter-rotating spiral motions. The number of spiral motions increased as the river bends were approached. The river bends also caused vorticity intensiﬁcation and increased the vertical velocities. The application of the 3-D ﬂow model was extended by solving the sediment continuity equation. The sediment transport patterns were related to the secondary ﬂow ﬁelds in the river. The sediment transport patterns at the river bifurcations are characterized by the growth of a sandbank. Copyright © 2004 John Wiley & Sons, Ltd.