A large amount of water suitable for most uses is available in the lower Rio Grande de Anasco Valley, the major source of which is the Rio Grande de Anasco which contributes about 95% of the surface water inflow to the lower valley. River flow at El Espino exceeds 100 cu ft/sec about 85% of the time and 200 cu ft/sec 50% of the time. Average daily flow for the driest months of the year (February, March, and April), is almost always <100 cu ft/sec. In contrast, the average daily flow for the wettest, months of the year (September, October, and November), is > 120 cu ft/sec. During the study period, flows of the Rio Canas averaged about 5 cu ft/sec. The lower valley is underlain by igneous rocks that have been eroded to depths of 350 ft or more below sea level. The valley is filled with 250 ft or more of limestone and clay, that in turn is overlain by as much as 100 ft of alluvium. The amount of groundwater available is unknown. There are large volumes of water in the saturated mostly fine-grained alluvium of Zone II, but as a whole the alluvium does not yield water readily to wells. Sand and gravel deposits associated with former river channels yield an estimated 100 to 150 gal/min to wells. The principal source of groundwater is the limestone of Zone III, that reportedly yields as much as 500 gal/min to wells. The quality of surface water especially that of Rio Grande de Anasco was very good. Specific conductance seldom exceeds 250 microsiemens/cm, even at low flows. Both salinity and sodium are low, falling in the Cl-S1 irrigation water classification. Water quality in the lower 5,000 ft or so of the river was affected by saltwater encroachment from the sea. The water quality of the other streams and canals in the lower valley was variable depending on susceptibility of saltwater encroachment, contamination from man-made sources, and concentration of minerals by evapotranspiration. Specific conductance however seldom exceeded 500 microsiemens/cm and the water usually falls in the C1-S2 classification. The quality of groundwater in the alluvial aquifer was about the same as that of the water of the Rio Grande de Anasco except where encroached by saltwater or contaminated. The water from the limestone was more mineralized than that of the alluvium (about 600 to 700 microsiemens/cm), and was somewhat similar to that of the smaller streams and canals in the valley.