The flow through the ¿resund is mainly driven by the sea level difference between the Baltic Sea and the Kattegat, suggesting a functional relationship between the sea level difference and the flow rate. In existing hydraulic models, the sea level difference is assumed proportional to the square of the flow rate. While the underlying theory requires an almost constant coefficient of proportionality, observations show that the coefficients is dependent on the flow rate. This effect may be accounted for by adding to the relationship a term that is linear in the flow rate. Theoretical considerations suggest that the linear term may be due to rotational effects in a way similar to flow resistance in the geostrophic control model. Adding nonlinear frictional effects to that model, a simple expression results that combines the nonlinear effects of friction and acceleration and linear effects of rotation. This expression quite accurately describes the observed flow resistance in the ¿resund. ¿ American Geophysical Union 1995.