How can a hurricane reverse the flow of massive rivers?


Using Ida reversing the Mississippi as an example, sounds pretty insane to me.

In: Earth Science

It is pretty insane. The Mississippi is a massive amount of flowing water to reverse. It’s an incredible feat.

But you can achieve the same feat on a smaller scale yourself. Fill a cup 3/4 full of water. Tilt it towards your mouth. Notice where the water line is. Now blow on the water. Watch the water line receed. You are the hurricane.

Hurricanes bring a “storm surge” with them when they make landfall. The power of the sustained winds at the leading edge of the storm literally pushes seawater up onto the coastline, sometimes many feet deep.

The swampy outlet of a river may only have a few feet of grade over many miles, so a twenty foot storm surge will make the ocean higher than the river and it will flow backwards up the river for a good distance.

It happens almost twice a day, every day. To be specific, high tide occurs roughly every 12 hours and 25 minutes. During high tide, tidal rivers flow backwards as the oceanic water level overcomes the river’s flow and the estuary is forced further back into the riverbed.

Now, the hurricane is tidal forces on steroids and can push the flow of the river much further back into the riverbed.

The water in the storm weighs less… more specifically, there is less air holding the water down. Big storms have very low barometric pressure. On weather maps these “low pressure systems” are marked with a big letter “L”.

Low pressure makes it both easier for clouds to turn to rain and for the ocean to get taller than it normally would be. Since the ocean is taller it flows back into the Mississippi because it is relatively “down hill”

All rivers (well… at least the majority of them) have their flow reversed when the tide is high, which happens roughly twice a day.

Hurricanes have a very low pressure area which effectively sucks up seawater, causing a higher and longer than normal high tide, causing the Mississipi to flow backwards longer than normal and on a larger scale in terms of volume of flow.

This might be off topic, but I was once fly fishing in windy wyoming, and the float I was using was pushed up stream by the powerful 20-40 mph afternoon wind.

It appeared that the surface of the stream was flowing uphill

The end of the Mississippi as it reaches the ocean has a very gradual slope. The river’s elevation only drops a few feet over the last several miles.

One of the hurricane’s effects is “storm surge”, it raises the ocean’s surface level at the shore by 10,15, 20 feet. This is caused by the wind physically pushing water onto the shore and by the low air pressure of the storm allowing the ocean to rise higher.

So now you’ve got the ocean 20 feet higher at the shore than it usually is. That puts the ocean’s water level higher than the river’s water level for miles back inland. So the ocean water freely flows back up the river channel until the river channel is as high as the storm-surge water level down at the coast.

Wind can push water around.

Really strong winds can push a LOT of water around, see:

A hurricane can be blowing/moving such that the wind is pushing ocean water “backwards” up the river temporarily. Think of it more like the fresh water coming down the river being stopped and spilling over the banks where it meets the oncoming ocean water being pushed up the river.