A three‐dimensional hydrodynamic model is applied to flows in open channels. The model incorporates a second‐moment turbulence‐closure model that has demonstrated considerable skill in simulating turbulent flows in laboratory experiments and in various geophysical and engineering boundary layers. The closure model consists of differential equations for turbulence energy and turbulence length scale. The remaining second‐moment equations are reduced to a set of algebraic equations in which tendency, advection, and diffusion terms are omitted. To account for the effect of the free surface on the bulk of the channel flow, a modification of the macroscale equation is introduced; the rest of the model equations and their attendant nondimensional constants remain unchanged. The model performance is assessed using laser‐Doppler anemometer measurements on the centerline of a large number of laboratory, smooth and rough, homogeneous and stratified, open‐channel flows with different values of the aspect ratio. Good agreement is found between the model and data in every case. Because the model is based upon a self‐consistent framework and is able to reproduce the many experiments provided here, the model can be used with confidence in environmental applications.