DOWNSTREAM HYDRAULIC GEOMETRY OF THE UPPER RIO CHAGRES, PANAMA

The Rio Chagres drains more than 400 km2 of mountainous topography in central Panama. Despite a steep, narrow valley geometry, abandoned meander bends indicate the sinuous river is actively incising. Bedrock is exposed discontinuously along the channel, but is mostly close to the surface and exerts control on channel geometry. Channels both follow and cut across geologic structures such as joints, bedding, and faults. 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 with small knickpoints. Coarse sediment is introduced via tributary channels and landslides, and grain-size distribution at a site correlates strongly with coarse sediment sources. Except for the headwater reaches, LWD is largely absent. This probably results from both very high decay rates for wood and high values of stream power along the channel. Where present, LWD does not apparently serve any geomorphic function such as sediment storage or channel stabilization, as is common along temperate rivers. Flow in the Upper Chagres has been gaged since 1930. The river has an exceptionally flashy hydrograph, with stage changes of 10 m recorded during a few hours in a channel 65 m wide. We surveyed 40 short reaches throughout the Upper Rio Chagres network, at sites with drainage areas ranging from 0.5 to 407 km2. The relation between discharge and drainage area was linear among these sites, both for measured low flows and inferred high flows. Downstream hydraulic geometry exponents for high flow are 0.37 for hydraulic radius, 0.5 for channel top-width, and 0.19 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 dataset. Grain size correlates poorly with both drainage area and reach gradient, and reach gradient correlates poorly with drainage area. Values of D84 are 13-102.5 cm, and high flow exceeds critical shear stress for D84 at about half the reaches. Both unit and total stream power peak just under halfway down the drainage basin.