Why The Ohio River Carries More Water Than The Mississippi Where They Meet

At Cairo, Illinois, the Ohio River meets the Mississippi, and the Ohio’s name ends there. From there south, all of the water is called the Mississippi. Yet on an average day, more of it comes from the Ohio. The smaller river carries the larger share, and it is not a close call.

The reason is rain, and where it falls. The Ohio drains some of the wettest land in the country, including the entire Tennessee River basin. The Mississippi’s drainage area is far larger, but much of it is dry plains that add plenty of area to the map and little water to the river. A storm over the Appalachian valleys can end up passing this small Illinois city on its way toward Memphis. Where that rain lands, and what holds it back along the way, decides which river carries more water to the meeting point.

The view from Fort Defiance State Park in Cairo, Illinois, where the Ohio meets the Mississippi. Editorial credit: W_lemay, CC BY-SA 2.0, via Wikimedia Commons By the time the Mississippi reaches Cairo, it already carries the Missouri River’s water. That gives it a drainage area stretching far across the Great Plains toward the Rocky Mountains. The Ohio approaches through a much smaller watershed to the east. Yet the Army Corps of Engineers describes the Ohio as supplying more than 60 percent of the combined flow at their meeting point.

The comparison concerns discharge, the volume of water passing a point each second. A river’s length cannot establish that amount, and its width alone cannot either. Water depth and speed also matter. At Cairo, the Ohio’s contribution is large enough to more than double the Mississippi’s average flow.

The Ohio River in West Virginia, in the wetter Appalachian part of its watershed. Much of the land draining into the Ohio lies in the humid eastern United States. Its tributaries collect rainfall in Appalachian uplands and across the Ohio Valley, carrying that water toward the same outlet at Cairo. Farther west, large parts of the Missouri watershed receive less precipitation. Those acres add considerable area to the Mississippi’s drainage map while supplying comparatively little water.

Rainfall also has several possible destinations. Some evaporates, and plants return more moisture to the air through their leaves. Water can remain underground before eventually entering a stream. Over the long term, drier parts of the Great Plains generally supply less river water per square mile than wetter eastern watersheds. The Ohio and Tennessee watersheds produce comparatively abundant runoff for their size, largely because of their higher rainfall.

Paducah, Kentucky, on the Ohio River, where the Tennessee River enters from the south. Paducah, Kentucky, sits at a junction that helps explain the Ohio’s size farther downstream. Here the Tennessee River enters the Ohio after a long bend through the Southeast. Beginning near Knoxville, it runs through eastern Tennessee and northern Alabama before turning north through western Tennessee into Kentucky.

The Tennessee is the Ohio’s largest tributary. Its mouth at Paducah is downstream of Louisville, so the Ohio passing Louisville has yet to receive this major addition. By Kentucky Reservoir, the Tennessee system drains about 40,900 square miles, adding a substantial southern watershed to the Ohio’s supply.

Kentucky Dam stands 22 miles upstream of the Tennessee’s mouth. The reservoir stretches 184 miles across Kentucky and Tennessee. Its flood storage helps reduce flood crests along the lower Ohio and Mississippi. Releases there alter when water reaches the junction at Paducah.

Kentucky Dam on the Tennessee River. Its reservoir holds back floodwater that would otherwise reach the Ohio sooner. The Ohio’s larger contribution describes a long-term average. The share arriving through either river changes with weather across their separate watersheds. Rain falling near Pittsburgh follows a different route and schedule than snowmelt entering the Missouri. Reservoirs can hold back part of a flood and release it later, further changing when that water reaches Cairo.

In spring 2011, heavy rain helped drive both rivers to dangerous levels around the city. On May 2, the Army Corps of Engineers opened the Birds Point-New Madrid Floodway by breaching a levee on the Missouri side. Water spread over farmland to relieve pressure on the flood protection around Cairo. By the time a satellite photographed the area on May 3, the Ohio at Cairo had fallen roughly a foot below its record crest.

At Fort Defiance State Park on Cairo Point, the Ohio and Mississippi bound opposite sides of Illinois’ southernmost tip. Their waters can remain visibly different after the banks separating them end. An astronaut photograph taken in January 2006 showed brown Ohio water alongside a greener Mississippi, reversing their more usual appearance. Sediment helped make the meeting visible; color itself could not establish which river carried more water. In that image, the boundary between the two remained visible three to four miles downstream. Barges were visible on the Ohio in the same photograph, using the river just above its junction with the Mississippi. At Fort Defiance, an observation building overlooks the point where the two rivers meet.