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.
Prior to widespread development of agriculture and industry, nutrient and sediment discharge to a large part of the coast and shelf would have been negligible, so marine waters would have been relatively transparent, except during and shortly after relatively uncommon storms. Apparently, most coral reef areas of Puerto Rico were able to endure the episodic influx of sediment and nutrients, perhaps because during these episodes of high discharge (mainly tropical disturbances such as hurricanes), waves and currents are also strong, which inhibits deposition and promotes transport of the sediment to the shelf edge and shelf slope.
Beginning in the early 19th century, substantial land clearing and modification, first for agriculture and later for urban development, resulted in increased sediment yields, and increased sediment and nutrient influx to coral reef areas. This terrestrial sediment in runoff may be a source of pathogens that affect coral reefs. Large parts of Puerto Rico have been reforested since the mid-1940s, but sediment transported into the river valleys during the agricultural period is still being transported through the river systems. Although, nitrogen and phosphorous concentrations in river waters are well within regulatory limits, current concentrations are as much as 10 times the estimated pre-settlement levels. Fecal coliform and fecal streptococcus concentrations in many Puerto Rico rivers are near or above regulatory limits. Unlike sediment discharges, which are predominantly episodic and intense, river-borne nutrient and fecal discharge is a less-intense but chronic stressor to coral reefs found near the mouths of rivers. The constant anthropogenically increased sediment and nutrient discharge to the Puerto Rico shelf are believed to have contributed to widespread degradation of coral reefs in Puerto Rico. Negative effects of river-derived sediment and nutrient discharge on coral reefs are especially pronounced in nearshore areas of the north, southwest, and west coasts of the island. A comprehensive and systematic survey of Puerto Rico coral reefs would enable the establishment of a baseline with which to determine which reef areas are at risk, and what measures can be taken to protect and restore these valuable natural resources.
Abstract
Introduction
Purpose and Scope
Acknowledgments
Methods
Description of Study Area
Climate, Physiography, Geology, and Geomorphology of Puerto Rico
Puerto Rico Coast and Shelf
Water Quality of Puerto Rico Rivers and Coast
Reefs of Puerto Rico
Effects of Suspended Sediment and Sedimentation on Coral Reefs
Analysis of Water and Sediment Discharge
Surface-Water Discharge in Puerto Rico
River Sediment Discharge in Puerto Rico
River Water Quality in Puerto Rico
River Water and Sediment Discharge Associated with Hurricane Georges
River and Sediment Discharge to Puerto Rico’s Coast and Shelf
Effects of Human Activity on Puerto Rico Watersheds, Rivers Systems, and Reefs
Summary and Conclusions
Cited References
The citation for this report, in USGS format, is as follows:
Warne, A.G., Webb, R.M.T., and Larsen, M.C., 2005, Water, Sediment,
and Nutrient Discharge Characteristics of Rivers in Puerto Rico, and their
Potential Influence on Coral Reefs: U.S. Geological Survey Scientific Investigations
Report
2005-5206, 58 p.
This document also is available in pdf format: PDF, 6.9MB
Please visit http://pr.water.usgs.gov/ for more information about USGS activities in the Caribbean (Puerto Rico and the U.S. Virgin Islands).
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