Downstream Hydraulic Geometry along a Tropical Mountain River

Abstract

The upper Río Chagres basin drains 414 km² of mountainous topography in central Panama. Hillslopes exceed 45° in over 90% of the basin and are highly dissected by a predominantly bedrock channel network. A variety of igneous bedrock lithologies are exposed discontinuously along the channels, which both follow and cut across geologic structures. Local changes in channel gradient and geometry occur where the channel crosses a more resistant bedrock unit, but the longitudinal profile as a whole is concave. Coarse sediment is introduced via tributary channels and landslides. The basin has high values of discharge per unit area. Channel geometry, bed grain-size, velocity, and discharge were surveyed for 40 short (50–200 m-long) channel reaches throughout the basin to examine how downstream hydraulic geometry relations and other channel characteristics (i) compare to those from mountain drainages in temperate regions, and (ii) reflect site-specific variables versus variables such as drainage area and discharge that change progressively downstream. Downstream hydraulic geometry exponents are 0.36 for hydraulic radius, 0.43 for channel top-width, and 0.24 for mean velocity. These values are similar to the average values for rivers worldwide, despite the inclusion of step-pool, pool-riffle, and bedrock gorge reaches in the data set. The presence of strong downstream hydraulic geometry trends indicates that hydraulic driving forces sufficiently exceed resisting forces to override specific lithologic influences on channel width and depth when averaged across sub-basin to basin scales. Poor correlations between grain size and drainage area, discharge, or reach gradient and between reach gradient and drainage area or discharge suggest that landslides and bedrock lithology locally control grain size and reach gradient.