A peculiarity of the Orinoco river system is the Casiquiare canal, 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 Orinoco and Amazon.
Marañón and the Ucayali are the two rivers that join to make the amazon.
the Congo river
Two rivers that join the Mississippi River are the Missouri River and the Ohio River.
Tributaries are Marañón, Japurá, Rio Negro, Ucayali, Purus, Madeira, Tapajós, Xingu, Tocantins Rivers.
The confluence of the Alleghaney and Monogahela Rivers form the Ohio River at Point State Park in Pittsburgh. Thus the three rivers that "come together" at that point are the Ohio, Allegahaney and Monogahela.
Ohio, Monongahela and Allegheny
Where two rivers join, this is known as the confluence.
Both the Murrumbidgee and the Darling Rivers are significant rivers which join the Murray on its northern side.
The rivers Yonne, Marne, and Oise are the main rivers joining the Seine river along its course.
The Republican and Smoky Hill rivers join at Junction City, Kansas to form the Kansas River.
the Ohio,Missouri,Arkansas, and the red river
The Monongahala River (i'm not quite sure i spelled that right), and the Allegheny River