The mouth of the Missouri River is just north of St. Louis, MO and slightly south of Alton, IL. It actually converges with the Mississippi river there at the confluence of the Missouri and Mississippi Rivers. The Mississippi River then flows to the Gulf of Mexico (it forms the Louisiana/Mississippi boarder.) The mouth of the Colorado River (and the Colorado River Diversion) is also in the Gulf of Mexico near Matagorda, TX. The Yukon River lies mostly in Alaska with a portion in Canada’s Yukon Territory. The Yukon River flows into the Bering Sea with it’s mouth at the Yukon-Kuskokwim Delta.

Mouth of the mississippi is in Louisiana into the gulf of mexico

Mouth of the Colorado is near Arizona in Mexico into the pacific ocean

Mouth of the yukon is in Alaska intoi the pacific ocean