Rivers seem about as dependable as anything in nature. Water runs downhill, toward the sea, day after day. So the idea of a river suddenly reversing course sounds like something out of a tall tale. But it really happens.
Here's the short answer: a river can flow backward when something pushes water upstream harder than gravity pulls it down. A powerful earthquake can do it for a few minutes. A strong incoming tide can do it for a while each day in certain rivers. A flood of monsoon water can reverse a river for months. And in one famous American city, engineers did it on purpose.
The Night the Mississippi Turned Around
In December 1811, the first of a series of huge earthquakes struck near the Mississippi River town of New Madrid, in what is now Missouri. Two more big quakes followed in early 1812. They remain the most powerful earthquakes in recorded history to hit the United States east of the Rocky Mountains.
A naturalist named John Bradbury was on the river that first night. He later described a noise like distant thunder, falling trees, terrified animals, and the roaring of the Mississippi, whose current, he wrote, ran in reverse for a few minutes.
The quake in February 1812 was even stranger. Ground lifted along a fault and created temporary waterfalls on the Mississippi, sent waves rolling upstream, and blocked streams to form what is now Reelfoot Lake in Tennessee.
Tides That Push Upstream
In a few places around the world, the incoming tide is so strong that it forms a wave that charges up a river against the current. This is called a tidal bore. It usually happens where there's a big difference between high and low tide, and where a broad bay funnels into a shallow, narrowing river.
The world's largest bore is on the Qiantang River in China, where the wave can reach about 30 feet high and travel up to 25 miles per hour. Bores can be dangerous, but some also attract daring river surfers.
The River That Reverses Every Year
In Cambodia, the Tonle Sap River changes direction with the seasons. During the dry season, it drains the Tonle Sap Lake into the mighty Mekong. When the monsoon rains arrive, the swollen Mekong pushes water back up the river and into the lake, whose water level rises by about 33 feet.
When the rains end and the Mekong drops, the flow flips again, and the lake helps feed the Mekong. This yearly reversal works as a natural safety valve, helping prevent flooding farther downstream.
Chicago Turned Its River Around
In its natural state, the Chicago River flowed into Lake Michigan. As the city grew, so did its sewage, and an extreme weather event in 1885 raised fears for Chicago's water supply. In 1887, Illinois lawmakers decided to reverse the river. By 1900, a large new canal and a series of locks had turned the main stem around, so it flowed away from the lake and toward the Mississippi watershed instead.
In 1999, the American Society of Civil Engineers named the system a Civil Engineering Monument of the Millennium. Today, the main stem of the river flows west, away from Lake Michigan, the exact opposite of its original direction.
So whether it's an earthquake, the tide, a monsoon, or a crew of determined engineers, rivers can be talked into turning around. They just need a very good reason.
