Rivers are channels of water that effectively drain water inland out to sea. They flow downhill with gravity, meaning that most of the time they have a predictable direction. However, some extreme and rare scenarios can cause them to flow backwards.
A video on Twitter went viral in 2022 showing what it claimed to be the Murray River in South Australia flowing backwards. While some questioned when exactly the video was taken, the Murray River is known for occasionally flowing backwards.
Rivers can flow backwards when their normal direction of flow is disrupted by a competing water source. In the case of the Murray River this is the nearby Goulburn River, which at high flows effectively represents a barrier, sending the water back up the Murray.
Extreme weather events can also send rivers running backwards. A 2012 article from the US Geological Survey (USGS) details the conditions that forced the Mississippi River to flow backwards for nearly 24 hours during Hurricane Isaac. Its levels were raised to 3 meters (10 feet) above its normal height as water surged upstream.
"This reversal of flow of the mighty Mississippi is but one measure of the extreme force of Isaac," said USGS Director Marcia McNutt in the article. "While such events are ephemeral, they are yet another reminder of why we need to respect hurricane warnings."
Hurricanes are capable of having such an influence on rivers as waves caused by powerful winds turn on rivers that may in normal weather have a modest speed of flow. Hurricane Katrina actually topped Isaac in making the Mississippi flow backwards at a height 4 meters (13 feet) above normal, but hurricanes can also make levels fluctuate as their spiral motion can drain rivers unusually fast.
Sometimes rivers can change direction with deadly consequences. In a piece for The Conversation, professor of geography Vamsi Ganti described how “channel-jumping” events known as avulsions have sometimes caused “some of the deadliest floods in human history.”
“Avulsions on China’s Yellow River killed over 6 million people in the late 19th and early 20th centuries,” he wrote. “Similar events have been linked to the decline of Mesopotamian civilization along the Tigris and Euphrates rivers in what is now Iraq, Kuwait, Turkey and Syria.”
Avulsions differ from the river-reversal of the Mississippi in that rather than keeping a river on-course but flowing backwards, they see a river suddenly up and carve out a new path. This can take them into human settlements, hence coming at such a potentially deadly cost.
River reversals can also take place over time. The Amazon once flowed in the opposite direction, from east to west. Considering reversing the direction of the Earth's largest river is no trivial thing, geologists pondered the cause for a long time, but 2014 research demonstrated that nothing more than erosion was needed to explain the enormous shift.
So yes, rivers can run backwards. Sometimes rapidly, sometimes over tens of millions of years, sometimes only in the short-term and with no ill effects, and others at the cost of millions of lives.
On the topic of dramatic water systems, did you know that some lakes can explode?