OBSERVATIONS OF DOWNSTREAM FINING ON THE NORTH FORK TOUTLE RIVER NEARMOUNT ST-HELENS, WASHINGTON

We carried out a series of measurements to provide data for testing mo dels of downstream fining in an actively braiding gravel bed river, in particular, the model developed in the previous paper. We sampled a s eries of nine cross sections for bulk grain size distributions over a downstream distance of 5.4 km on the North Fork Toutle River, near Mou nt St. Helens, Washington. In addition, we sampled two of the nine sec tions for detailed analysis of surface grain size distributions in the cross-stream direction by Wolman counting within areas of distinct me an grain size (''patches''). Our theory predicts that the rate of down stream fining depends on the distribution of deposition and a relative mobility function that depends on the characteristics of the grain si ze patches. Application of the theory to the study area shows that mos t of the observed downstream fining can be explained using patchiness as the dominant mechanism.