Fifty sand samples were collected from west coast beaches and analyzed for grain size, composition, and textural features. The percent composition of the sands were expressed as percentages of carbonate shell debris, quartz and feldspar grains, igneous rock fragments, dark mineral grains and magnetite grains. Except for very few fresh water snails from the rivers and lithoclasts from limestone shorelines (less than 1%), the carbonate fraction is biogenic shell material from offshore reefs and shoal carbonate environments. The quartz and feldspars are from inland sedimentary rocks and alluvial deposits, and the igneous rock fragments (basalts) and dark minerals are from the basalts and serpentines of the central mountain range of the island.
Analyses of beach sand composition, river input sources, erosion of the coastline, and net shore drift indicators have been used to analyze the sand budgets for each beach. Longshore transport is from north to south. The beaches north of Mayagüez are mixtures of carbonate skeletal material, quartz and feldspar, and rock grains and dark minerals. Mayagüez beach is uniquely dominated by igneous rock fragments, and the southern beaches are mainly carbonate sands.
The west coast of Puerto Rico is dominated by the effect of the termination of structural mountain ridges at the shore which are separated by broad alluvial valleys. The ridges form a rocky coast, and the shoreline bordering the alluvial valleys is occupied by sand beaches The topography is influenced by the valleys of the Culebrinas, Añasco, and Guanajibo rivers (fig. 1) . Between the river valleys, the terrain is hilly to mountainous with steep ridges and deep gullies. The drainage pattern of the Culebrinas and Añasco rivers is well defined and runs from higher inland mountains to the coast. The Guanajibo river wanders over a broad alluvial plain and evaporation: is almost equal to water input for most of the year. From Aguadilla to Mayagüez, the insular shelf is very narrow, averaging less than 1 km width. South of Mayagüez, the shelf is relatively broad and reefs and shoals with depths of less than 10 meters extend 20 km offshore. The same pattern of ridge with rocky shoreline and valley with sand beach is present, but there are also areas of mangrove coastline and fringing reef coast because of the protection offered by the shoal continental shelf. Rainfall in the central mountains is greater than 200 cm/yr and 125 to 200 cm/yr for the north coast. Roberts, 1942 The south coast has less than 80 cm/yr of rainfall. Glynn, 1973 Most of this rain occurs during September, October, and November with hurricane events producing as much as half of the annual rainfall.
The general pattern of sand sources and losses for the west coast are:
The beaches of the west coast are discontinuous and form separate beach systems with little or no communication. Sand composition and net shore drift indicators have been used to determine the separate systems. Current movement and longshore drift on the west coast is from north to south.
Crashboat and Punta Borinquen beaches are isolated pocket beaches bounded by rocky shoreline. The narrow and steep shelf together with large areas of rocky shoreline limit exchange of material between these beaches. Crashboat beach is at the mouth of a dry wash. The sands are derived from the wash during flood rainfall and from onshore transport of carbonate sands. The shelf break is less than 100 meters offshore, so very little offshore transport is required before sands are lost over the shelf edge.
From Aguadilla to Punta Guanajibo, the terrigenous input is dominated by the direct sediment load from the rivers and erosion of the alluvial valley shorelines. Each of the rivers contributes a dominant set of minerals even though considerable overlap occurs (table 1). The composition of the beach sands from Aguadilla to Punta Algarrobo is similar, but the rocky point at Punta Jiguero divides this expanse of shoreline into two beach systems.
Table 1. Minerals available from rivers. Data for sand sized sediments.
# most abundant mineral
* significant concentration compared to beaches
| mineral | Culebrines | Añasco | Guanajibo |
| quartz | # |
* |
* |
| feldspar | # |
# |
* |
| epidote | * |
# |
|
| serpentine | # |
||
| opaques | * |
# |
# |
| magnetite | * |
# |
# |
The Aguadilla beach system, from Aguadilla to Punta Jiguero, is a fairly wide and relatively continuous beach. There is a narrow coastal plain of river alluvium and alluvial fan deposits from the hills. The Culebrinas River supplies quartz and feldspar to the beaches and samples from the river. Kattman, 1972 have more opaques and magnetite than found in the beach samples, so that the river contribution is significant for these minerals. The beach sands are characterized by high concentrations of quartz, magnetic minerals, altered feldspars, and carbonate shell material. There is severe erosion from the Culebrinas River to Punta Jiguero
The Corcega-Añasco beach system is separated from the Aguadilla beach system by the rocky headland at Punta Jiguero. Although wave refraction may carry some sand from Aguadilla beaches into the Corcega- Añasco beaches, most of the southern longshore transport from the Aguadilla beach is lost over the narrow shelf at Punta Jiguero. The composition of the Corcega- Añasco beaches is very similar to the Aguadilla beaches - approximately equal parts carbonate shell material, quartz and feldspar, and igneous rock material, opaques, and dark minerals. Although Corcega and Añasco beaches are separated by a rocky headland at Punta Cadena, there is no composition change in the sands, and there is evidence that reefs off Corcega beach are supplying biogenic shell material to Añasco beach. These beaches are probably part of a single beach system that extends from Rincon to Punta Algorrobo. Much of the sand supplied to this beach system comes from erosion of the alluvial shoreline and from offshore sources. The Añasco River is an episodic source of sands.
The grain size distribution of two of the beaches is bimodal (fig. 3) . Both Corcega and Rincon beaches have a small coarse mode and a major mode at the mean size of other beaches. The median size of most of the beaches falls between 0.8 and 1.6 phi, Añasco and Rincon beaches have smaller median size sands (fig. 4) . The smaller size of these beaches is related to wave energies. The composition is not that different from Mani and Corcega beaches.
The most abundant minerals in the Añasco River are feldspar, opaques, and magnetite. The normal output of sand from the Añasco River is very low. Because of the small tide range in Puerto Rico, the salt wedge in the rivers is not flushed out daily (fig. 5) . It may persist for months so that bed load transport is deposited in the river. During periods of higher rainfall (rainy season flood events and hurricane rainfall), the discharge level is raised and the salt wedge is forced out of the river. Measurements of the salt wedge, and measurements of suspended sediment load have not been made in detail, but scattered data from Kipple, et al., 1968 Rickher, et al., 1970 and Grove 1977 show a non-linear relationship between suspended sediment concentrations and river discharge (fig. 6) . At about 14 cubic meters per second flow, the suspended sediment load jumps drastically. At this time, the salt wedge is moved out of the river and bottom sediments are entrained and moved from the river either to the beach directly, or into offshore sand bodies that supply the beaches. The discharge of the Añasco River usually exceeds this value during September through November (fig. 7) . The beach system is terminated at Punta Algorrobo where a sea wall and fill area has been collecting sediments for more than 15 years.
Mayagüez beach is bounded immediately offshore by a terrigenous mud sediment facies which is very low in carbonate content. This mud facies and the lack of carbonate grains in the beach sands suggests that no onshore sediment transport is occurring. There is severe erosion along the southern half of Mayagüez beach. So much rip-rap has been added to long stretches of the beach for protection that the coastline is virtually a man-made rocky shoreline.
The Joyuda beach system extends from Punta Guanajibo to Joyuda. Most of the south longshore transport from Mayagüez beach is either piled up at the mouth of the Guanajibo River where it leaves the beach system via truck, or is moved offshore at Punta Guanajibo. Some of the bypassed sand may be returned to the Joyuda beach system. South of Joyuda, mangrove shorelines serve to separate the beaches. The sands are dominantly quartz, igneous rock fragments and dark minerals, and carbonate shell material. The source of terrigenous material is coastal erosion. There are no rivers south of the Guanajibo River drainage.
Ostiones beach is unique and isolated. It is more than 90 percent Halimeda flakes derived from a shoal area directly west of the beach. The carbonate beaches north and south have different carbonate grain constituents. This clearly indicates that there is little exchange with the other beaches.
Mela beach is isolated from Ostiones by the channel leading to Puerto Real. Erosion contributes some terrigenous sand, but most of the beach is carbonate shell material from fringing reefs and shoal carbonate deposits. The sands are transported south into the reef-sand bar system across the mouth of Boqueron Bay.
The Boqueron-Combate beach system is a mix of carbonate grains from offshore and quartz grains from an inland alluvial deposit west and south of Boqueron. The transport system carries sand from the bay, past a non-beach shoreline out of the bay, and along the Combate shoreline. The sands are transported south past Cabo Rojo where they join carbonate sands that have been carried westward along the south coast. This has resulted in a large accumulation of sand on the southwest insular shelf described by Grove and Trumbull. 1978