As described in the reports of the PROCUENCA-SAN JUAN project, the SJRB is located between 10º and 12º 30’ north latitude and between 83º 30’ and 86º 15’ west longitude and includes southeast Nicaragua and northeast Costa Rica (Map 1). Its boundaries are defined by the basin of Lake Nicaragua and its tributaries, the San Juan River and its tributaries, and the allied coastal zone. From the point at which the San Juan River leaves Lake Nicaragua to its mouth(s) at the Caribbean Sea, its length is roughly 200 km.

Most of the basin is below 500 meters above sea level and rises from the coastal Caribbean flood plains of Indio Maiz (in Nicaragua) and Tortuguero (in Costa Rica) to the 1500-3000 meter highlands of northern Costa Rica and to just over 1,600 meters above sea level in Nicaragua. Rainfall is greatly influenced by this topography and reaches between 4,000 mm to 6,000 mm over nine to eleven months in the more humid sections and between 1,000 mm and 2,000 mm in the drier zones around Lake Nicaragua,  which have a dry season of about seven months. Average annual temperatures range between 20°C and 28°C although, at the higher elevations, they can drop to less than 10°C. As can be expected, this variation in elevation, topography, and distance from the sea has created at least 14 different life zones that vary from very dry tropical forest to montane rainforest (Map 2).

Map 3 shows the political/administrative divisions of the SJRB. In Costa Rica, the political-administrative division is by provinces and cantons—seven of which are entirely within the SJRB (La Cruz, Upala, Guatuso, Los Chiles, San Carlos, Sarapiquí, and Pococí). A few others are only  partially within the basin and are omitted from the project. In Nicaragua, departments and municipalities make up the political/administrative units. One of these, the San Juan River Department, lies entirely within the basin, as do parts of five others (Masaya, Granada, Boaco, Chontales, and Rivas). Within the project area, these departments are further divided into 37 municipalities (Table 1). Most of the  information available for describing the basin comes from data captured at the municipal, canton, province, department, and national levels. Very little of the information required in this characterization is available from data gathered within physical or biographical boundaries.

An estimated 1,070,000 people live in the basin, 780,000 (70%) in Nicaragua and nearly 290,000 (25%) in Costa Rica. Over 40% of the population of the Nicaraguan portion lives in but four of the 37 municipalities and, within these, in just three cities (Masaya, Granada, and Juigalpa). Fifty-five percent of the population is rural and, although scattered unevenly throughout the basin (in part, because of the number and size of the basin’s parks and reserves), the population density in Nicaragua is 46/km². On the Costa Rican side, 85% of the population is rural and the population density, at 22/km², is less than half that of Nicaragua.