GEOLOGY OF THE PROVINCES OF BARAHONA AND AZUA. 189 



DRAINAGE. 

 RIVERS. 



The Provinces of Barahona and Azua are rather plentifully supplied 

 with streams, several of which are large. Many of the smaller ones, 

 especially the "arroyos," are not through-flowing streams at all seasons. 

 Some flow only after rainfall; others are perennial in their upper courses 

 only. 



Rio Yaque del Sur is the largest river in the region and, with the excep- 

 tion of Rio Guayajayuco, much the longest, its length being roughly 120 

 kilometers. Near its mouth it is about 50 meters wide and 3 or 4 meters 

 deep and flows between banks of soft alluvium at a velocity of approxi- 

 mately 4 kilometers an hour. It maintains the same general character 

 throughout the area covered by its delta deposits, which extend from the 

 vicinity of Alpargatal to the Bay of Neiba. 



At Palo Alto, in the delta area, the Barahona Company has maintained 

 a self-recording stream gage for over a year. The gaging indicates that 

 the low-water period in the river is from January to April. The minimum 

 recorded flow is 18 cubic meters per second. 



Above the delta the river flows in several braided channels, 35 to 75 

 meters wide, over a gravel-strewn bed that ranges in width from several 

 hundred meters to a kilometer or more. The current here is much swifter 

 than it is farther downstream, but the channel is at many points not too 

 deep to be easily fordable except in times of flood. 



Above Los Bancos the bed of the stream is more gorge-like. The water 

 flows swiftly through channels strewn with coarse boulders or over bed- 

 rock. Occasional floods are reported to raise the stream level 6 meters in 

 the gorges. 



The Yaque has numerous tributaries, most of which are small streams 

 that are dry during the greater part of the year. The longest tributaries 

 are Rio San Juan (80 kilometers), Rio de las Cuevas (55 kilometers), and 

 Rio del Medio (55 kilometers). Rio San Juan is itself a stream about 35 

 meters wide at its mouth. Its longer tributaries are Rio Vallejuelo (30 

 kilometers), Rio Maguana (25 kilometers) and Rio Mijo (55 kilometers). 

 Rio Guayajayuco, which drains the region adjacent to the Haitian 

 frontier, is a large stream 230 kilometers long. About 180 kilometers of 

 its course lie wholly in the Republic of Haiti and most of the remainder 

 forms a natural boundary between the Republic of Haiti and the Dominican 

 Republic. The headwaters of the stream are in Monte Cristi Province, 

 Dominican Republic. 



The Great Valley of San Juan is drained principally by the westward 

 flowing Rio Macasia, 80 kilometers in length, a branch of Rio Guaya- 

 jayuco. The longest tributaries of Rio Macasia are Rio Cana (80 kilome- 

 ters), and Rio Ceiba (40 kilometers). The east end of this large structural 

 valley is drained by Rio San Juan and its tributaries. 



