Hydrology in the SJRB

The drainage network (Map 6) shows that the rivers of the northern sector of the basin are short in length, generally oriented north to south, and eventually find their way to Lake Nicaragua. Most of the rivers in the southern sector of the basin originate in Costa Rica, in the Cordillera de Guanacaste to the west and at elevations of up to 3,000+ meters in the Cordillera de Tilarán to the east. The high levels of precipitation along the northern flank of the Tilarán range contribute approximately 85% of the San Juan River’s total volume.

Table 2: Monthly and annual precipitation rates in the San Juan River Basin.

Estimated flows of the river are 475 m³/s at San Carlos, at the outlet from Lake Nicaragua. They increase to 833 m³/s just before the confluence with the Sarapiquí River and 1,308 m³/s at its outlet to the Caribbean. Twenty-six percent of the river's total flow at its mouth originates from Lake Nicaragua; 6.5% from inflow between San Carlos and El Castillo; and 67.5% between El Castillo and Sarapiquí.

Groundwater in the SJRB is abundant and of high quality, except on the Caribbean coastal plain, where it is salty. Aquifers with high-quality water have been found between 45 and 105 meters at the southern and western boundaries of the basin, and at 8 to 40 meters on the interfluvial plains on both sides of the Rio San Carlos, where they supply potable water to the populations of this zone.

Riverine sediments originate in the upper and middle parts of the basin’s watersheds and are the result of strong rains, the fragility of many volcanic soils, deforestation, road construction, and agricultural and livestock development. Lake Nicaragua is the depository for sediments carried by its tributaries. It also receives laminar runoff and subsurface drainage that carry dissolved, or suspended pesticides and fertilizers. Little is known of the volume of sediment arriving at Lake Nicaragua. At the Terrón station on the San Carlos River in Costa Rica, the contribution of sediment was calculated at 817 tons/Km²/year; at Peñas Blancas, 700 tons/km²/year; at Punta Viejo and Veracruz on the Sarapiquí River, it was calculated at 216 tons/km²/year. At the Guatuso station on the Frío River, a contribution of 298-tons/km² year was calculated. In Nicaragua, agricultural activity and deforestation have caused erosion problems and, as a result, floods and the diversion of watercourses. Periodic samples of suspended sediments have not been taken at key stations; however sedimentation can be verified in that some riverbeds have been filled in with the consequent problems of overflow, flooding, and the formation of new sand and mud bars.