An attempt is made to understand the channelling effect observed in the Rhine valley by the use of simple models. We consider two-dimensional flow of a homogeneous rotating viscous fluid over a valley. Two different approaches are described: (i) a linear theory of a horizontally averaged flow over a valley of finite depth; (ii) a numerical model. The linear theory gives wind profiles in the valley which compare favorably with the results of the nonlinear numerical model. There is qualitative agreement with observations. An analysis of the linear solution reveals that the channelling effect is a feature of Ekman flow where advection is important above the valley, but not in the valley.