The Causes of River Reversal
Rivers, vital lifelines of our planet, have been known to reverse their flow, both permanently and temporarily. While permanent reversals are often caused by geological forces, temporary reversals can be triggered by natural events or human interventions.
Permanent Reversals
Permanent Reversals are irreversible changes in a river’s course, often resulting from tectonic shifts or large-scale geological processes. For instance, the Amazon River, once flowing into the Pacific Ocean, was redirected towards the Atlantic Ocean due to the uplift of the Andes Mountains.
Temporary Reversals
Temporary Reversals, on the other hand, are short-lived changes in a river’s direction, typically caused by factors such as seasonal variations, extreme weather events, or human activities. Tidal rivers, for example, experience daily reversals due to the gravitational pull of the moon and sun.

Example of both Natural and Artificial Permanent Reversals
Permanent Natural Reversals
- Amazon River: Originally flowed into the Pacific Ocean but was redirected towards the Atlantic Ocean due to the uplift of the Andes Mountains during the Cretaceous period.
- Wisconsin River: Once flowed into the Great Lakes Basin but was rerouted towards the Mississippi River due to glacial activity during the Pleistocene Epoch.
Permanent Artificial Reversals
- Chicago River: Reversed direction in 1900 to divert sewage away from Lake Michigan, a drinking water source, by constructing the Chicago Sanitary and Ship Canal.
Example of Daily, Annual and Intermittent Temporary Reversals
Temporary Daily Reversals
- Tidal Rivers: Rivers near the coast often experience daily flow reversals due to tidal forces. Examples include the Hudson River, Saint John River, Salmon River, Yarra River, Thames River, Senegal River, Gambia River, Shatt al-Arab River, Salween River and Mekong River.
Temporary Annual Reversals
- Krupa River: Flows into the Neretva River during high water levels, reversing its direction.
- Qiantang River: Experiences a tidal bore that can cause temporary reversals.
- Tonlé Sap River: Flows into the Mekong River during the monsoon season.
- Amazon River: Experiences a tidal bore that can cause temporary reversals.
Temporary Intermittent Reversals
- Mississippi River: Storm surges caused by hurricanes.
- Chicago River: Intensive rainfall.
- Pearl River: Storm surges caused by severe typhoons.
The Tidal Bore: A Natural Phenomenon
Imagine a massive, moving wall of water surging up a river. That’s essentially a tidal bore. It’s like a natural tsunami, but smaller and happening regularly. Think of it as the ocean trying to flood a river, and the river putting up a fight.
The tide comes in from the sea, and as it reaches the river’s mouth, it gets funneled into a narrower channel. This creates a powerful surge that can travel miles up the river, sometimes even reaching inland cities. It’s like when you try to squeeze water out of a balloon, but instead of popping, it forces its way out in a powerful jet.

A River's Double Reversal: The Case of the Chicago River
As you may have observed, the Chicago River has experienced two distinct flow reversals. Initially, it was permanently reversed in the early 1900s to divert sewage away from Lake Michigan. However, in recent years, the river’s flow has been temporarily reversed during periods of heavy rainfall to prevent flooding. While the permanent reversal was a major engineering feat to improve water quality, the river’s natural tendency remains to flow towards Lake Michigan. Although it would be challenging and costly to revert the river’s flow back to its original direction, understanding its natural inclination is important for managing its future.
Keep them clean!
As we’ve seen, rivers can change their course, both temporarily and permanently. These shifts are a testament to the dynamic nature of our planet and the intricate balance of forces that shape our waterways. Reflecting on these events reminds us of the importance of protecting our rivers and understanding the factors that influence their flow.
F.A.Q.
River reversals can be caused by both natural and human factors. Natural causes include tectonic shifts, glacial activity, and extreme weather events. Human interventions, such as dam construction and canal digging, can also lead to reversals.
Permanent reversals are irreversible changes in a river’s course, often resulting from geological forces. Temporary reversals, on the other hand, are short-lived changes caused by factors like tides, extreme weather, or human activities.
Yes, rivers can naturally reverse their flow due to geological changes, such as the uplift of mountains or the movement of tectonic plates.
The Amazon River, once flowing into the Pacific Ocean, is considered the longest river that has reversed its flow due to geological changes.
River reversals can have significant environmental consequences, including changes in ecosystems, disruption of wildlife habitats, and impacts on water quality.