Operator bias characterizing stream substrates using Wolman pebble counts with a standard measurement template

This study characterizes the variability associated with using a standard measurement template to perform Wolman pebble counts of coarse stream substrates. Four 10-count and two 12-count replicate experiments were conducted by five operators at three sample sites within gravel-bed streams using a gravel template. Results show significant differences between different operators sampling the same location, as observed in previous studies conducted without a standard sampling template, as well as significant variability within each operator's replicate samples. Variability was highest in clast estimates larger than D50 suggesting that these metrics cannot be consistently derived from Wolman pebble count samples. Variability was also highest in the low order study site suggesting that the sedimentary characteristics of low order streams increase the chance for operator bias. In light of our findings, we suggest that traditional 100-particle Wolman counts be utilized with more caution than is the current norm.