Uruguayyo͝o´rəgwā, gwī, Span. o͞oro͞ogwi´, o͞oro͞owī´, river, c.1,000 mi (1,610 km) long, rising in S Brazil and flowing in an arc W, SW, and S to the Río de la Plata, an estuary; it forms part of the Brazil-Argentina border and most of the Argentina-Uruguay line. Its chief headstream is the Pelotas of S Brazil; the Río Negro of S Brazil and Uruguay is an important tributary. The lower river is navigable for oceangoing vessels for 130 mi (209 km) upstream. Its upper course is broken by waterfalls. Salto Grande Falls is the site of a joint hydroelectric project of Argentina and Uruguay. Argentinian environmental concerns over Uruguay's construction of a pulp mill on the river led to a prolonged dispute that finally led in 2010 to an agreement to create a joint environmental monitoring program for the river. Concordia, Argentina, and Salto and Paysandú, Uruguay, are on the river.

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