a river in N South America, rising in S Venezuela and flowing west, then north as part of the border between Colombia and Venezuela, then east to the Atlantic by a great delta: the third largest river system in South America, draining an area of 945 000 sq km (365 000 sq miles); reaches a width of 22 km (14 miles) during the rainy season. Length: about 2575 km (1600 miles)