R. Seal et C. Paola, OBSERVATIONS OF DOWNSTREAM FINING ON THE NORTH FORK TOUTLE RIVER NEARMOUNT ST-HELENS, WASHINGTON, Water resources research, 31(5), 1995, pp. 1409-1419
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.