The Brigach and Breg rivers
The Green, Gunnisson, Dolores, San Juans, Virgin and Gila rivers among others.
the amazon river has 9 main tributaries called Marañón, Japurá, Rio Negro, Ucayali, Purus, Madeira, Tapajós, Xingu and Tocantins.
haha i don't know.... :)
If considered by contribution to Mekong, the main tributaries would be Se San river, Se Kong River in Cambodia (about 25%), Chi-Mun River in Thailand (About 8%) and Nam Ngum and Nam Cadiam in Laos...
not 100% sure, but think it's 29