APPLICATION OF THE PEBBLE COUNT - NOTES ON PURPOSE, METHOD, AND VARIANTS

The pebble count procedure (Wolman, 1954) is the measurement of 100 ra ndomly selected stones from a homogeneous population on a river bed or bar, which yields reproducible size distribution curves for surficial deposits of gravel and cobbles. The pebble count is widely used in ge omorphology (and increasingly in river engineering) to characterize su rficial grain size distributions in lieu of bulk samples, for which ad equate sample sizes become enormous for gravels. Variants on the origi nal method have been proposed, one of which, the so-called 'zig-zag' m ethod (Bevenger and King, 1995), involves sampling along a diagonal li ne and drawing data points from many different geomorphic units. The m ethod is not reproducible, probably because it incorporates stones fro m many different populations, and because an inadequate number of grai ns is sampled from any given population. Sampling of coarse bed materi al should be geomorphically stratified based on the natural sorting of grain sizes into distinct channel features. If a composite grain size is desired, the areas of the bed occupied by different populations ca n be mapped, pebble counts conducted on each, and a weighted average d istribution computed.