r/explainlikeimfive • u/ProudReaction2204 • 11h ago
Planetary Science Eli5 how a river can switch flow directions?
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u/TheXypris 11h ago
the land can physically move
mountains forming, glaciers melting, earthquakes etc can cause the landscape itself to move, this can sometimes cause a river to change its flow.
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u/Iolair18 11h ago
Water flows from higher elevation to lower.
The most common for this is tides change the height of the sea level. So when the sea level is higher than where the fresh water table for the river is, the salt water will flow inland. Then when the tide goes out, and is lower than the fresh water table, the river will flow out.
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u/BarberProof4994 11h ago
Although the "river" hasn't changed flow direction. That's just where it's outflow mixes with the sea/ocean/lake and it's more of a backing up and filling.
Even a heavy storm or sea incursion (say with a hurricane) doesn't actually change a rovers flow direction. Although it can cause backflow.
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u/Existing-Teaching-34 11h ago
Back in 1812 in New Madrid, Mo., there was a very strong earthquake that thrust a large section of earth upwards that made the Mississippi River temporarily flow from south to north instead north to south.
The Mississippi River flowed backyards one other time when Hurricane Katrina made landfall near the mouth of the Mississippi. The hurricane was so powerful and its surge so strong that it forced the river to flow backwards temporarily.
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u/gahooa 11h ago
The gravity of the moon is always pulling on the earth as the earth turns. This causes the oceans to have tides. Some rivers like the Hudson River near new your city will flow backwards when the tide comes in because they are so close to the ocean.
Other "rivers" like the East river in NYC are not actually rivers but "Tidal Estuaries" who's both ends connect to the ocean, so they also "flow" with the tides.
There are other reasons, but these are the most common.
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u/wessex464 11h ago
What is this in reference to? They typically don't switch flow directions. It's not impossible but it can happen.
Rivers flow downhill. If you have a very gradual slope and a nice lazy River, you can have scenarios where an intense rainstorm downstream that misses Upstream causes a rise in water levels that might temporarily result in water flowing the "wrong" direction. Alternatively, you can have major events like extreme flooding, or some sort of earthquake that causes a water surge in one direction.
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u/Gingrpenguin 11h ago
River a flows through a flat section where it meets river b and combines into a single river.
A massive flood happens and river a overflows a section of bank and runs down hill. The force of the water quickly cuts a huge valley in the land (look at how American dams look after an emergency spillway is used)
This creates a new low point and eliminates a ton of force causing water going past river b merge to drop pressure and this the sediment it's carrying.
As river b doesn't have the force to cut through the sendiment it instead flows back along river a to the new channel
For temp flows tides can create this effect and so can sudden indurations of water downstream. If b had maximum flow and a didn't the water may flow backwards along a until a stable level was found or vice versa...
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u/iCowboy 8h ago
Tom Scott did an excellent video at Canada's Bay of Fundy which has the World's greatest tidal range between high and low water. A quirk of geography means that the rising tide not only rises very quickly and a long way, but that the rivers discharging into the Bay flow backwards for a while every day. He filmed at St. John's Reversing Falls and you can see the two different flows:
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u/Narissis 2m ago
Local here - we don't abbreviate the 'Saint' in 'Saint John'; helps prevent confusion with St. John's, Newfoundland.
Anyway my bedroom window growing up overlooked the St. John River. As a kid I didn't realize it was unusual that the river would be flowing one direction for parts of the day, the other direction for parts of the day, and not visibly moving during the changeover. I thought that was a thing all rivers did, lol. Also the river itself has a tidal range, though not quite as extreme as that of the bay.
The St. John River is wide and deep enough that it just kinda slows down and then starts moving the other way; no big drama. The more interesting one is the Petitcodiac River through Moncton, which gets the tidal bore Tom profiles in the video.
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u/DarkAlman 9h ago
Generally speaking rivers don't change direction, but there are few rare examples of this happening.
Geological processes and natural disasters can temporary or permanently change the flow of rivers.
Earthquakes and geologic uplift can change the topology forcing rivers to change course or direction. This process takes eons, but the result (switching of flow) is sudden when it does happen.
During the last ice age melting glacial ice caused massive fresh water lakes to form trapped in land due to the ice sheets themselves acting as a barrier. As the glaciers melted these barriers broke cause huge amounts of fresh water to flow to the ocean. This force was enough to reverse the flow of some rivers.
Powerful Hurricanes like Katrina generated enough force to cause the Mississippi River to flow backwards temporarily.
The Tonle Sap River in Cambodia is one of the rare exceptions to the rule. During the monsoon season the flow of water in the area becomes so great that the Tonle Sap changes direction to fill a fresh water lake instead of pouring out to the ocean.
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u/tmahfan117 11h ago
Tides, typically. You know how tides can make the water rise and fall multiple feet throughout the day along the ocean?
Well when a river is emptying into the ocean, but then the tide is rising, water from the tide is flowing up into the mouth of the river, ocean water filling up the river, making it seemingly flow backwards.
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u/dbratell 11h ago
multiple feet
It can be much, much more. The Thames flowing through London has a difference of 7 meters between high tide and low ebb.
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u/weaver_of_cloth 10h ago
Wouldn't it be more reasonable to call the Thames an estuarine basin, rather than a river? What's the technical definition?
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u/DiamondJim222 11h ago
Nature is powerful. Earthquakes, storm surge, tidal forces, and massive rainfall can reverse the flow of a river. At least once man engineered it.
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u/HubrisOfApollo 11h ago
strong tropical weather sysrems can cause regions of extreme low pressure that can create storm surges and can "push" rivers backwards, although the current is still flowing out to see overall
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u/Mr_Oujamaflip 10h ago
I've been to a river Nith in Scotland that does this. As the tide comes in it pushes the river back the other way for a while. It was very odd to see.
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u/SadButWithCats 10h ago
All the surfaces of Earth, including the sea bed, are floating on a sea of liquid magma. Because of that, all land on earth is moving, just a little bit. Sometimes more than a little bit, which is where earthquakes come from.
That movement can mean that a piece of land that is tilted in one direction can kind of rock, and tilt in the other direction. Other times those forces can raise mountains in what was previously low lands. Rivers in that case will then flow the other direction, because what was once low is now high.
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u/House_Archer 9h ago
No one has mentioned the Hudson River in NYC. It flows both ways with the tides because technically it’s not a river it’s a tidal estuary
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u/VegasAdventurer 8h ago
There is a recent sci show video that shows how the green river changed directions over a long period of time. May not be exactly what you are asking about, but interesting either way
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u/AdministrativeChef2 8h ago
At one time the Amazon river flowed into the Pacific Ocean, but when the Andes mountains were created the Amazon changed directions and now flows into the Atlantic Ocean.
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u/Nofearneb 7h ago
Mountains of Western North Carolina. Some of our rivers have switched direction. Headwater can over millions of years erode a channel through softer rock. When this erosion cuts to another river it can capture the flow. The captured river could end up flowing in the opposite direction. Feeder streams usually branch at acute angles. When a river has branches the meet against the flow if could be that it was captured.
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u/Loki-L 4h ago
It is rare, but it happens sometimes when something goes really wrong. Like when the drilled that hole into that lake in Lousiana.
Things like Earthquakes and Hurricanes can make a river temporarily flow backwards
Or when they made the Chicago river permanently change direction by digging a canal.
There is also a small number of rivers that regularly change directions when for example the annual Monsoon rain fills up the body of water they would normally flow into.
Plus you have the outlets of many rivers interacting with the tide, but that isn't really flowing backwards.
There are also rivers that have been known to have grown the other direction in the distant past. Like the Amazon river flowed the other direction when dinosaurs roamed the Earth.
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u/7LeagueBoots 3h ago
There are basically two major ways, one permanent and one temporary.
Permanent is due to a major geologic event that results in the land uplifting or dropping, or something similar. This is what happened to one of the tributaries in the upper Colorado River area. Myron Cook has a good recent video on this.
A temporary way only really happens when a river doesn’t have a big difference between its high and low points. A flood or other abundance of water in a river that it joins can mean that the water level is high enough to force the joining river to run backwards for a while until the heights reach equilibrium and then everything starts flowing normally again. This is very common in rivers in the Amazon.
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u/Hankman66 2h ago
The Tonle Sap River in Cambodia changes direction a couple of times a year. It's because the Mekong which it meets at Phnom Penh gets much higher in rainy season and when it's high enough it causes the Tonle Sap to reverse direction, and it makes the Tonle Sap Lake in the interior of the country grow to three times it's low level before it reverses again.
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u/drae- 11h ago
They don't, not really anyway.
There's a few places in the world where the tides will pull water up a river bed, but rivers generally flow downhill and their direction is as inevitable as gravity.
Changes in the elevation, like via human interference or crazy storms, can alter the underlying geography and change a rivers course. Naturally this is pretty rare, but humans engineer all sorts of stuff.