The effect of a surface cover (ice-cover) on transverse spreading of a tracer released at a constant and steady rate into a turbulent channel flow is investigated. Velocity, depth and transverse profiles of tracer concentration were obtained at four cross-sections downstream of the tracer source. It is shown that the transverse diffusion coefficient for ice-cover conditions, in a straight rectangular channel, is approximately 50% smaller than the corresponding coefficients for open water conditions. However, when these coefficients are normalised with the product of shear velocity,u*, and hydraulic radius,R, they do not differ significantly. An expression is derived for evaluating the average transverse diffusion coefficient over a channel reach by using concentration measurements at one station downstream of tracer source. This expression is found to give good results.