The effects of rotation on turbulent channel flow are discussed, particularly the influence of the earth 's rotation is analyzed and some of previous literature is reviewed. It is shown that Coriolis-induced secondary currents should be of importance if they have a magnitude of 1% of the downstream velocity. A simplified analysis is used to estimate the relative magnitude of the horizontal secondary velocity at the cross-plane center point for fully-developed flow in a wide channel as a function of Rossby number. Numerical model simulations are used to investigate the influence of the side walls and the magnitude of the secondary velocities for developing flow. The results show that the secondary velocities amount to about 1% of the downstream velocity for rather slowly flowing deep channels