Three rivers discharge onto the narrow insular shelf of Mayaguez and Anasco bays on the west coast of Puerto Rico. The bays are bounded offshore by a discontinuous line of reefs. The resulting pattern of carbonate and terrigenous sediments reflects both the source materials and the physical processes distributing sediments. The carbonate sediments are found only near and on elevated reef structures. Terrigenous sediments are widely distributed and are encroaching over the coral reef areas. A change in sediment distribution is occurring that will eventually result in loss of the reef environment except on the shelf edge. This change is the result of increased sediment load in the rivers.--Modified journal abstract.