Data from 29 streamflow-gaging stations, including 9 stations with daily suspended-sediment concentration, and data from 24 water-quality stations were compiled and analyzed to investigate the potential effects of river sediment and nutrient discharges on the coral reefs of Puerto Rico. The largely mountainous watersheds of the 8,711-square-kilometer island of Puerto Rico are small, channel gradients are steep, stream valleys tend to be well-incised and narrow, and major storms tend to be intense but brief; hence flooding is rapid with peak discharges several orders of magnitude above base discharge, and flood waters recede quickly. Storm runoff transports a substantial part of fluvial suspended sediment from uplands to the coast, as indicated by sediment data from a set of nine streamflow-gaging stations representative of runoff from watersheds considered typical of conditions in Puerto Rico. For example, the highest recorded daily sediment discharge is 1 to 3.6 times the annual suspended-sediment discharge, and runoff from major storms induces sediment transport 1 to 32 times the median annual sediment load. Precipitation associated with Hurricane Georges in September 1998 is estimated to have averaged 300 millimeters across the island, which is equivalent to a volume of about 2.6 billion cubic meters. Analysis of runoff and sediment yield from Hurricane Georges indicates that more than 1.0 billion cubic meters of water and at least 2.4 million metric tonnes of sediment (and as much as 5 to 10 million metric tonnes), were discharged to the coast and shelf as a result of this major storm. Because of their relatively small size, dams and reservoirs of Puerto Rico have relatively little effect on total discharge of water and sediment to the coastal marine waters during major storms. The presence of reservoirs, however, may be detrimental to coral reefs for two reasons: (1) coarse sediments deposited in the reservoir can be replaced by finer sediments scoured, if available, from the river channels and flood plains below the dam; and (2) the loads of phosphorus and ammonia reaching the coastal waters may increase as organic matter decomposes in the anoxic bottom waters of the reservoir. Rainfall, water discharge, sediment discharge, and sediment yield vary across the island. Mean annual runoff for the island is estimated to be 910 millimeters, about 57 percent of mean annual precipitation (1,600 millimeters). Mean annual suspended-sediment discharge from Puerto Rico into surrounding coastal waters is estimated to range from 2.7 to 9.0 million metric tonnes. Hydrologic and sediment data associated with Hurricane Georges indicate that sediment yield is generally proportional to the depth of storm runoff. Discharge and sediment-concentration data indicate that during this storm, river water and sediment that discharged into the marine environment generally formed hypopycnal plumes (buoyant suspension layers). Generally, hyperpycnal (density) plumes can develop in areas with high discharges and sediment concentrations. Both hypopycnal and hyperpycnal plumes distribute suspended sediment over broad areas of the Puerto Rico shelf and shelf slope. Comparison of long-term suspended-sediment discharge and watershed characteristics for Puerto Rico with those of other river systems around the world indicates that Puerto Rico rivers are similar to temperate and tropical upland river systems.

Analysis of slope angle and rock type using a geographic information system indicates that about 66 percent of the San Juan metropolitan area (SJMA) has low to no susceptibility to earthquake-induced landslides. This is at least partly due to the fact that 45 percent of the SJMA is constructed on slopes of 3 degrees or less, which are too gentle for landslides to occur. The areas with the highest susceptibility to earthquake-induced landslides account for 6 percent of the surface area. Almost one-quarter (23 percent) of the SJMA is moderately susceptible to earthquake-induced landslides. These areas are mainly in the southern portions of the SJMA where housing development pressures are currently high because of land availability and the esthetics of greenery and hillside views. The combination of new development and moderate earthquakeinduced landslide susceptibility may indicate that the southern portions of the SJMA may be at greatest risk.

The Puerto Rico research site consists of the 113 square-kilometer Luquillo Experimental Forest (LEF), administered by the U.S. Forest Service, and the nearby Río Grande de Loíza drainage basin, an urbanized and agriculturally- developed watershed. This combined region serves as a terrestrial laboratory for the study of issues related to the global loss of tropical forest, and the associated changes in land-use practices. Findings from the WEBB research help scientists understand how vegetation, landscape, and people interact to affect the quantity and quality of water and the erosion of the landscape. The results of this work can be applied not only to Puerto Rico, but also to many other regions, where deforestation and rapid land-use change are issues.

On the morning of September 18, 1989, a category-four hurricane struck eastern Puerto Rico with a sustained wind speed in excess of 46 m/s. The 24-h rainfall accumulation from the hurricane ranged from 100 to 339 mm. Average rainfall intensities ranging from 34 to 39 mm/h were calculated for 4 and 6 h periods, respectively, at a rain gage equipped with satellite telemetry, and at an observer station. The hurricane rainfall triggered more than 400 landslides in the steeply sloping, highly dissected mountains of eastern Puerto Rico. Of these landslides, 285 were mapped from aerial photography which covered 6474 ha. Many of the mapped landslides were on northeast- and northwest-facing slopes at the eastern terminus of the mountains, nearest the hurricane path. The surface area of individual landslides ranged from 18 m2 to 4500 m2, with a median size of 148 m2. The 285 landslides disturbed 0.11% of the land surface in the area covered by aerial photographs. An approximate denudation rate of 164 mm/1000 y was calculated from the volume of material eroded by landsliding and the 10-y rainfall recurrence interval.