The Ohio River flows 981 miles from Pittsburgh, Pennsylvania, to Cairo, Illinois, and is the largest tributary by volume of the Mississippi River. While the river serves as a drinking water source for millions, it can and has frozen. However, a complete freeze-over is rare today, occurring far less frequently than in previous centuries. The river’s size and modern alterations make widespread, sustained freezing improbable under most contemporary winter conditions. The extent of the freeze is governed by complex physics and human intervention.
The Different Ways River Ice Forms
The formation of ice on a large, moving body of water like the Ohio River is a complex physical process. The river’s current and turbulence prevent the immediate formation of a smooth, continuous sheet of ice. Instead, the first stage of freezing occurs in supercooled, turbulent water, creating fine, needle-like ice crystals called frazil ice.
These crystals have a slush-like consistency and are carried downstream. As the river flows, frazil particles cluster together to form circular, flat plates known as pancake ice. These plates develop raised edges from repeated collisions and primarily form in the main channel where water velocity is highest.
A different type of ice, known as border ice or shore ice, forms along the riverbanks in slower-moving, shallower water. This stationary ice grows outward from the shore. A complete, stable sheet of ice across the entire river only forms when the border ice meets in the middle, or when large quantities of frazil and pancake ice freeze together. Water velocity is a primary factor; if the flow is too fast, the river will only produce frazil ice, which moves as a thick sludge but does not form a solid cover.
Major Historical Freezing Events
Before modern navigational controls, the Ohio River experienced frequent and severe ice events that halted commerce for weeks. In the 19th century, the river commonly froze solid enough for people to walk across, with a record closure lasting 47 days in the winter of 1855.
The winter of 1917–1918, known as the Great Freeze, was one of the most destructive freezing events on record. The river froze over its entire length, with ice thickness estimated up to 12 inches in some sections. This severe freeze had disastrous consequences for river traffic, crushing more than 30 steamboats with shifting ice flows.
The Ohio River last froze widely in the severe winters of 1976–1977 and 1977–1978. These periods brought prolonged below-freezing temperatures, including a recorded temperature of negative 25 degrees Fahrenheit in Louisville. The sustained cold allowed for the formation of a foot of ice in some areas, freezing the river from bank to bank.
Factors That Keep the Ohio River Open
The primary reason a complete freeze-over is rare is the construction of the Ohio River Navigation System. The river’s natural state was much shallower, ranging from three to 20 feet deep, allowing the water to cool quickly and freeze. The current system of locks and dams has transformed the river into a series of deep, slow-moving pools.
These dams significantly increase the minimum depth; for example, the water stage at Cincinnati rarely drops below 25 feet. This greater volume requires a much longer and more intense cold snap to cool the entire column to the freezing point. The deep, impounded pools slow the water’s velocity, which facilitates the formation of a surface ice sheet that insulates the water below.
Industrial Warming and Climate Change
Industrial and urban influences also contribute to keeping the river warmer. Power generation plants along the river use vast amounts of water for cooling and then discharge the heated water back into the river, a process known as thermal pollution. This discharge can cause a localized temperature increase, calculated to be as high as a 7.2-degree Fahrenheit rise in the main channel near some plants.
This localized warming, combined with the general warming trend from climate change, significantly reduces the likelihood of a widespread freeze. The river now requires an unusually long duration of sub-zero temperatures to overcome the thermal mass of the deep pools and the heat inputs from industry. This combination explains why modern ice events are generally limited to frazil ice accumulation or localized border ice along the banks.