SURFACE GRAIN-SIZE DISTRIBUTION OF GRAVEL BARS ON THE RIO CHAGRES, PANAMA

Sediment sampling of fluvial gravel bars was conducted along a longitudinal profile of the upper Rio Chagres, Panama, to investigate downstream trends in the grain size distribution. The Rio Chagres is located in eastern Panama and flows into Lago Alhajuela, a reservoir which provides water to the Panama Canal. The upper Chagres drainage area is approximately 400 km2, and the mean annual rainfall is 2840 mm per year. Point bars and alternate bars were sampled above and below confluences of 5 major tributaries with the Rio Chagres. A modified version of the Wolman pebble count was used, in which a measuring tape was stretched along the bar parallel to the flow of the river, and clasts were selected at 25 cm intervals. Clast size, lithology, and roundness were documented, and a total of 100 clasts were sampled at each site. Preliminary results show no strong evidence of downstream fining along the longitudinal profile of the river as a whole or between tributary junctions. The data instead indicate that the range of the grain size distribution, based on downstream changes in the size difference between D50 and D84, increases in the upstream portion of the profile, peaks near the middle, then decreases in the downstream portion of the profile.