Which river is the largest tributary of the Mississippi River by volume?

The Ohio River is the Mississippi River’s largest tributary by volume.

The Ohio begins at Pittsburgh, Pennsylvania, where the Allegheny and Monongahela rivers meet. It flows for about 1,579 kilometres—981 miles—along or near several state boundaries before joining the Mississippi at Cairo, Illinois.

By long-term average discharge, the Ohio carries more water at its mouth than the Mississippi does immediately upstream of their confluence. The Ohio’s average flow there is about 7,960 cubic metres per second, compared with roughly 5,897 cubic metres per second for the Mississippi upstream.

This makes the Ohio hydrologically dominant at the junction, even though the Mississippi retains its name downstream. The Missouri River is longer and has the larger drainage basin, so it is a frequent alternative in quizzes; however, the question asks for largest by volume, which points to the Ohio.