We carried out a series of measurements to provide data for testing models of downstream fining in an actively braiding gravel bed river, in particular, the model developed in the previous paper. We sampled a series of nine cross sections for bulk grain size distributions over a downstream distance of 5.4 km on the North Fork Toutle River, near Mount St. Helens, Washington. In addition, we sampled two of the nine sections for detailed analysis of surface grain size distributions in the cross‐stream direction by Wolman counting within areas of distinct mean grain size (“patches”). Our theory predicts that the rate of downstream fining depends on the distribution of deposition and a relative mobility function that depends on the characteristics of the grain size patches. Application of the theory to the study area shows that most of the observed downstream fining can be explained using patchiness as the dominant mechanism.