Some simple momentum advection effects are considered in a current aligned with the y axis on whichthere is superimposed a "cross" flow in the x-z plane. The cross flow coupled with horizontal shear in thecurrent tends to generate differences along the vertical in the longshore velocity, while vertical mixingtends to even out such differences. As in the scalar diffusion problem considered by Taylor, a balanceis possible between the two tendencies. The equilibrium velocity distribution may support considerablelateral momentum flux, which, in the case of zero rotation, is directed down the velocity gradient, allowingthe definition of an effective horizontal viscosity. When rotational effects are significant, both the senseand the magnitude of the momentum flux come to depend in a complex way on the total vorticity f + S,where f is Coriolis parameter and S the current shear. Some illustrative examples are calculated for cross flow produced by frictional effects in a boundary current. These show that... Abstract Some simple momentum advection effects are considered in a current aligned with the y axis on whichthere is superimposed a "cross" flow in the x-z plane. The cross flow coupled with horizontal shear in thecurrent tends to generate differences along the vertical in the longshore velocity, while vertical mixingtends to even out such differences. As in the scalar diffusion problem considered by Taylor, a balanceis possible between the two tendencies. The equilibrium velocity distribution may support considerablelateral momentum flux, which, in the case of zero rotation, is directed down the velocity gradient, allowingthe definition of an effective horizontal viscosity. When rotational effects are significant, both the senseand the magnitude of the momentum flux come to depend in a complex way on the total vorticity f + S,where f is Coriolis parameter and S the current shear. Some illustrative examples are calculated for cross flow produced by frictional effects in a boundary current. These show that...