We establish the average wavelength dependence for rough-surface reflectance S_rs(λ) in Lake Superior and determine its magnitude at any individual site by extrapolating the total reflectance or raw remote sensing reflectance R_rsT(λ) measured at the surface to the asymptotic limit of zero scattering where S_rs(λ) ≅R_rsT(λ). The results show that S_rs(λ) differs from the flat-surface Fresnel reflectance of sky radiance used in standard determination of the remote sensing reflectance R_rs(λ) attributed to the scattering of light by particles and molecules in bulk water. In waters containing colored dissolved organic matter (CDOM), R_rs(λ) can be very low and radiometric measurements at ground level can often lead to negative estimates of R_rs(λ) if we assume simple mirror-like reflectance of the sky radiance as the basis for correcting radiometric data for surface reflectance. We examine the differences between Fresnel reflectance and S_rs(λ) and estimate the ratio of the concentrations of CDOM versus suspended particles that could produce negative values of R_rs(λ) if assume Fresnel reflectance in calculation of R_rs(λ).