The attenuation of progressive, dispersive, oscillatory wave systems by dissipation within the viscous boundary layer in a long laboratory channel was studied both experimentally and theoretically, for the conditions of uniform water depth and over uniform impermeable slopes. In general, the observed dissipation exceeded that predicted by linear, small-amplitude theory for the boundary layer on the sides and bottom of the channel. On the hypothesis that the additional dissipation might be due to the presence of a similar boundary layer at the free water surface, the theory is extended to include the surface effect. The experimental results essentially confirm the theory—even in the markedly nonlinear region near shore.