At its mouth it forms a wide delta that branches off into hundreds of rivers and waterways that flow through 41000 Km2 of swampy forests. In the rainy season it can swell to a breadth of 22 kilometres and a depth of 100 meters. Most of the important Venezuelan rivers are tributaries of the Orinoco, the largest being the Caroni. The river is navigable for most of its length, and dredging enables ocean ships to go as far as Ciudad Bolivar, 435 km upstream.
A peculiarity of the Orinoco river system is the Casiquiare, which starts as an arm of the Orinoco, and finds its way to the Rio Negro, a tributary of the Amazon, thus forming a 'natural canal' between the Orinoco and the Amazon.