Uruguay, a river of South America, which rises on the W. slope of the Sierra do Mar in the Brazilian province of Santa Catharina. During the first 75 m. of its course it flows N. W. through the plains of Las Vacas, where it unites with the Pelotas and is increased by numerous small tributary streams. It is subsequently joined by the Pepiri-Guassu, and turns S., forming the boundary between the Brazilian province of Sao Pedro and the Argentine provinces of Oorrientes and Entre-Rios, from all of which it receives many additions. At its junction with the Ibicuy, which rises in the Sierra Geral, it is 600 yards wide. In lat. 30° 14' S. it receives the Quaraim, an important tributary, 160 m. long, and navigable for 30 m., which marks the boundary between Brazil and Uruguay. The Uruguay is here 1,500 yards wide, and divided by a line of wooded islands. Thence to its mouth it is the boundary between the Argentine Republic and Uruguay. The western shore is high, steep, and wooded, the eastern very varied and picturesque.

The next important tributary is the Arapey from the east; here the bed becomes rocky and the current more rapid, until it forms the cataract of Salto Grande, which is about 250 m. above its junction with the Parana. This is frequently passed by steamers during high floods, and above it there is unimpeded navigation for vessels drawing 5 ft. for a distance of 300 m. Below it the river is rapidly augmented by numerous tributaries, until in the lower part of its course, for nearly 100 m., it becomes a lake, varying in width from 4 to 7 m. The right bank is here low, wooded, and often marshy; the left is formed by the slopes of hills 200 to 500 ft. high, intersected by numerous streams and interspersed with settlements. In lat. 34° it unites with the Parana to form the Plata. The Rio Negro is its largest affluent. The total length of the Uruguay is 1,020 m. Its waters are always clear and limpid. Its islands, much less numerous than in the Paranį, are mostly high and rocky. On the island of Higuerita is a populous town.

On its banks are the Uruguayan towns of Fray Bentos, Paysandu, and Salto, the Argentine towns of Concepcion and Porto Ruiz, the port of Gualeguaychu, and the Brazilian towns of Uruguayana and Itaqui. The main stream is navigable about 600 m. for flat-bottomed steamers and 200 m. more for boats. The annual freshets occur in September or October, sometimes with great rapidity. The average rise is 20 ft., but occasionally there is a difference of 40 ft. between extreme high and low water.