river and outlet of Lake Nicaragua, issuing from the lake's southeastern end at San Carlos and flowing along the Nicaragua–Costa Rica border into the Caribbean Sea at San Juan del Norte. It receives the San Carlos and Sarapiquí rivers during its 124-mi (199-km) southeasterly course through tropical forests, and near its mouth it forms three arms, the Juanillo Menor to the north, the Río Colorado to the south, and the San Juan proper. Navigation is impeded by rapids at Toro, El Castillo, and Machuca and is limited to boats with a shallow draft. During the migrations to California from the eastern United States between 1850 and 1870, travellers transferred at San Juan del Norte from Atlantic steamers to small boats, which went up the river and across Lake Nicaragua; they then travelled overland to the Pacific port of San Juan del Sur. Along with the lake it has been considered as a possible canal route between the Caribbean and the Pacific. The San Juan River has been the source of several boundary disputes between Nicaragua and Costa Rica regarding Costa Rica's use of the river. The conflict dates back to the Cañas-Jerez Treaty of 1858 signed by both countries. The treaty determined that the San Juan River belonged to Nicaragua, but Costa Rica was allowed commercial access and obtained the right to “free and perpetual” navigation of the river. Throughout the 19th and 20th centuries and into the 21st century, the treaty was reinterpreted and altercations were common (especially during talks concerning the river as a possible canal route); these were often resolved though arbitration.

river in the southwestern United States, rising in the San Juan Mountains of southern Colorado, on the west side of the Continental Divide. It then flows southwest into New Mexico, past Farmington, northwest into Utah, and west to the Colorado River near Rainbow Bridge National Monument in southeastern Utah. The river is 360 mi (580 km) long and is not navigable. Its chief tributaries are the Animas, Los Pinos, Piedra, La Plata, and Mancos rivers, all in northwestern New Mexico. In that area, where the river valley widens, there is some irrigated farming. The section of the river where the boundaries of New Mexico, Utah, Arizona, and Colorado meet and where the San Juan enters the Colorado Plateau, into which it has carved numerous S-shaped canyons more than 1,000 ft (300 m) deep, is known as the Goosenecks. From there the river flows in a relatively straight canyon to the Colorado. The Navajo Dam on the San Juan in northwestern New Mexico is part of the Upper Colorado River Storage Project.