In 2019, the U.S. Geological Survey, in cooperation with the Upper Colorado River Wild and Scenic Stakeholder Group, studied the magnitude and recurrence interval of streamflow needed to initiate bed movement of gravel-sized and finer sediment in a segment of the Colorado River in Colorado to better understand sediment movement and its relation to flow regimes of the river. Two stationary hydrophone systems were installed on April 23, 2019 at the above Catamount Bridge stationary hydrophone site (above Catamount Bridge site), and two were installed on April 24, 2019 at the Radium stationary hydrophone site (Radium site). Pebble counts of coarse surface particles (greater than 2 millimeters [mm; 0.00656 foot]) were performed in the wadeable areas adjacent to the stationary hydrophones to provide general estimates of the size of the material that could be mobilized and recorded near the hydrophones. The particle size of the intermediate axis (B-axis) was measured with a gravelometer, and the largest bin size the particle did not pass through (the retaining sieve size) was recorded. Particles smaller than 2 mm (0.00656 foot) were recorded as less than 2 mm (0.00656 foot). Those particle size measurements are presented in this child item.