Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)

First Advisor

Richard Kesel


This study was aimed at gaining a better understanding of the sedimentological and morphological characteristics of boulder-dominated, coarse gravel-bed rivers associated with alluvial fans in humid tropical climates and the transport processes of boulder-size clasts within these systems. The study focused on the Rio Toro Amarillo, a braided, coarse gravel-bed river in Costa Rica which drains an active volcano within the Atlantic Watershed of Costa Rica. The investigation involved mapping and measurement of the distribution of braid complexes and channels; gradient, grain-size distribution, lithofacies, and micromorphology of the channel-beds and bar surfaces; elevational differences between channel bed and bar surface; and flow direction in a abandoned channel reach of the river. The data demonstrated that coarse-gravel-bed rivers associated with alluvial fans in volcanically active areas are capable of transporting large volumes of sediments in a short period of time. High-sediment-concentration flows are suggested to be an active contributor to the transport dynamics of boulder-size clasts during non-catastrophic conditions. Inducement of such flows would result from changes in the morphology of the system, such as channel shifting, and possibly from destruction of bed forms. Consequently, transport dynamics in these rivers are dictated, in part, by site-specific conditions which can result in significant changes in the morphology of the river. A new micromorphological feature was observed in the Rio Toro Amarillo and was called "circular rib." A circular rib consists of an accumulation of large clasts in a half-circular pattern. Boulders and cobbles are lodged against this ridge of large boulders at the stoss-side of the deposit in an imbricated manner. Lee-side deposits generally consist of finer-grained sediments. Morphologically, circular ribs result in a sudden decrease in elevation towards the downstream side of the ridge; the elevational difference being at least the magnitude of the largest clasts forming the ridge.