# How far out into the Gulf does the Mississippi fresh water go? **URL:** \ **Category:** Factual Questions\ **Created:** [September 29, 2011, 10:35pm UTC](https://boards.straightdope.com/t/how-far-out-into-the-gulf-does-the-mississippi-fresh-water-go/598078 "2011-09-29T22:35:45Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Mangosteen](https://avatars.discourse-cdn.com/v4/letter/m/6a8cbe/32.png) [@Mangosteen](https://boards.straightdope.com/u/Mangosteen)\ **Post date:** [September 29, 2011, 10:35pm UTC](https://boards.straightdope.com/t/how-far-out-into-the-gulf-does-the-mississippi-fresh-water-go/598078/1 "2011-09-29T22:35:45Z")
How far out in the Gulf of Mexico could one drink the water that flowed from the Mississippi? Could this water be economically “captured” in large 100,000 gallon bags and floated over to Galveston or Corpus Christi? (And trucked or sent by rail into the dry west Texas desert). I’ve think I recall hearing about this being done elsewhere. ---
**Author:** ![treis](https://avatars.discourse-cdn.com/v4/letter/t/bc79bd/32.png) [@treis](https://boards.straightdope.com/u/treis)\ **Post date:** [September 29, 2011, 11:09pm UTC](https://boards.straightdope.com/t/how-far-out-into-the-gulf-does-the-mississippi-fresh-water-go/598078/2 "2011-09-29T23:09:09Z")
The Mississippi doesn’t have a real mouth on the Gulf of Mexico. It branches off into a delta and the water becomes brackish. I don’t think any water with a drinkable salinity makes it to the Gulf of Mexico. ---
**Author:** ![FatBaldGuy](https://avatars.discourse-cdn.com/v4/letter/f/ecd19e/32.png) [@FatBaldGuy](https://boards.straightdope.com/u/FatBaldGuy)\ **Post date:** [September 30, 2011, 12:04am UTC](https://boards.straightdope.com/t/how-far-out-into-the-gulf-does-the-mississippi-fresh-water-go/598078/3 "2011-09-30T00:04:47Z")
Even without the salinity, I wouldn’t want to drink anything from the Mississippi. ---
**Author:** ![mac\_bolan00](https://avatars.discourse-cdn.com/v4/letter/m/b3f665/32.png) [@mac\_bolan00](https://boards.straightdope.com/u/mac_bolan00)\ **Post date:** [September 30, 2011, 1:08am UTC](https://boards.straightdope.com/t/how-far-out-into-the-gulf-does-the-mississippi-fresh-water-go/598078/4 "2011-09-30T01:08:26Z")
believe it or not, the fresh water discharge from a river as big as the M is denser than sea water and could travel out to sea for several hundred miles. that’s because it has a lot of dissolved chemicals and suspended particulates so it’s heavier and “creeps” along the sea bottom on the continental shelf and eventually “spills” into the ocean floor. those supended particulates may eventually form thick sedimentary deposits on the ocean floor called “turbidites.” ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 30, 2011, 2:35am UTC](https://boards.straightdope.com/t/how-far-out-into-the-gulf-does-the-mississippi-fresh-water-go/598078/5 "2011-09-30T02:35:25Z")
> [@](#): > > believe it or not… I’ll choose “not” on that one. Yeah, it has dissolved chemicals, but nearly as much so as seawater, and merely suspended particles shouldn’t drag the fresh water with them. ---
**Author:** ![busterpickle](https://avatars.discourse-cdn.com/v4/letter/b/258eb7/32.png) [@busterpickle](https://boards.straightdope.com/u/busterpickle)\ **Post date:** [September 30, 2011, 5:35am UTC](https://boards.straightdope.com/t/how-far-out-into-the-gulf-does-the-mississippi-fresh-water-go/598078/6 "2011-09-30T05:35:39Z")
I assume the OP i saying an actual water column extending from the Mississippi into the Gulf of Mexico. I would have to say, that with a river delta, which diffuses the speed of the current, that said water column would exist but not as large if no delta. I was on a ship that was waiting at the mouth of the Mississippi and the water was quite turbulent as the river and gulf water mixed. So, depending on how you call it, maybe a few miles at most? ---
**Author:** ![Bearflag70](https://avatars.discourse-cdn.com/v4/letter/b/8e7dd6/32.png) [@Bearflag70](https://boards.straightdope.com/u/Bearflag70)\ **Post date:** [September 30, 2011, 5:50am UTC](https://boards.straightdope.com/t/how-far-out-into-the-gulf-does-the-mississippi-fresh-water-go/598078/7 "2011-09-30T05:50:16Z")
I Googled Mississippi River estuary and found this little tidbit. [http://na.oceana.org/en/explore/places/mississippi-estuary](http://na.oceana.org/en/explore/places/mississippi-estuary) ---
**Author:** ![mac\_bolan00](https://avatars.discourse-cdn.com/v4/letter/m/b3f665/32.png) [@mac\_bolan00](https://boards.straightdope.com/u/mac_bolan00)\ **Post date:** [September 30, 2011, 6:09am UTC](https://boards.straightdope.com/t/how-far-out-into-the-gulf-does-the-mississippi-fresh-water-go/598078/8 "2011-09-30T06:09:08Z")
> [@Chronos](#): > > I’ll choose “not” on that one. Yeah, it has dissolved chemicals, but nearly as much so as seawater, and merely suspended particles shouldn’t drag the fresh water with them. it’s called a density flow, caused by having a lot of particles in turbulent water. it creates a new liquid that is denser than sea water and can creep out all the way to the ocean floor. but your point in more “how long does it stay fresh water?” no geological work i know answers that. my own guess is “not for long.” ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [September 30, 2011, 6:21am UTC](https://boards.straightdope.com/t/how-far-out-into-the-gulf-does-the-mississippi-fresh-water-go/598078/9 "2011-09-30T06:21:51Z")
Following the[link](http://bigthink.com/ideas/21260)from [this post](http://boards.straightdope.com/sdmb/showpost.php?p=14308475&postcount=14)by Northern Piper, you can find this: > [@](#): > > The Mississippi’s effluent of fresh water is so massive (7.000 to 20.000 m³/sec, or 200.000 to 700.000 ft³/sec) that a plume of fresh water is detectable from outer space, even as it rounds Florida and up to the coast of Georgia. ---
**Author:** ![Tastes\_of\_Chocolate](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tastes_of_chocolate/32/3232_2.png) [@Tastes\_of\_Chocolate](https://boards.straightdope.com/u/Tastes_of_Chocolate)\ **Post date:** [September 30, 2011, 10:13pm UTC](https://boards.straightdope.com/t/how-far-out-into-the-gulf-does-the-mississippi-fresh-water-go/598078/10 "2011-09-30T22:13:22Z")
> [@FatBaldGuy](#): > > Even without the salinity, I wouldn’t want to drink anything from the Mississippi. Why ever not? Most of the cities located along the Mississippi use the river for at least part of their water supply, all the way down to [New Orleans](http://www.swbno.org/history_water_purification.asp). ---
**Author:** ![Great\_Antibob](https://avatars.discourse-cdn.com/v4/letter/g/e47c2d/32.png) [@Great\_Antibob](https://boards.straightdope.com/u/Great_Antibob)\ **Post date:** [September 30, 2011, 10:41pm UTC](https://boards.straightdope.com/t/how-far-out-into-the-gulf-does-the-mississippi-fresh-water-go/598078/11 "2011-09-30T22:41:45Z")
> [@mac\_bolan00](#): > > believe it or not, the fresh water discharge from a river as big as the M is denser than sea water and could travel out to sea for several hundred miles. that’s because it has a lot of dissolved chemicals and suspended particulates so it’s heavier and “creeps” along the sea bottom on the continental shelf and eventually “spills” into the ocean floor. those supended particulates may eventually form thick sedimentary deposits on the ocean floor called “turbidites.” Geophysicist here: I can confirm turbidites exist, but I’m not sure where the dissolved chemicals are supposed to come in. Most turbiditic formations in the GoM are formed from regular sediments (those suspended particulates), which are a natural part of the river system. Several good hydrocarbon deposits are actually trapped under turbiditic formations. The rest of your “new type of fluid” stuff is kind of odd. It doesn’t really have to be some bizarre new form of water. The sheer volume of fresh water and the density contrast will be sufficient to carry non-saline water flows tens of miles out to sea. Not all necessarily at the surface, mind. Just for confirmation, the annual Mississippi River “dead zone” is a result of all the nitrogen, phosphorus, and other stuff getting washed out to sea. That stuff is certainly carried quite a ways and does mix with salt water. ---
**Author:** ![penultima\_thule](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/penultima_thule/32/3833_2.png) [@penultima\_thule](https://boards.straightdope.com/u/penultima_thule)\ **Post date:** [October 1, 2011, 12:29am UTC](https://boards.straightdope.com/t/how-far-out-into-the-gulf-does-the-mississippi-fresh-water-go/598078/12 "2011-10-01T00:29:54Z")
> [@Mangosteen](#): > > How far out in the Gulf of Mexico could one drink the water that flowed from the Mississippi? > > Could this water be economically “captured” in large 100,000 gallon bags and floated over to Galveston or Corpus Christi? (And trucked or sent by rail into the dry west Texas desert). > > I’ve think I recall hearing about this being done elsewhere. First rule of water economics, don’t fight gravity. According to the City of Corpus Christi Water Department website > **[Home | City of Corpus Christi](https://www.corpuschristitx.gov/)** > > The official website for City of Corpus Christi. > [@](#): > > Approximately 28 billion gallons of water are treated each year. Its rated capacity of 167 million gallons per day is well above the peak summer demand of as much as 110 million gallons per day. So each 100,000 gallon bags will contribute 0.1% of daily demand. And 1000,000 gallons weighs 1 million pounds so it’s going to be prohibitively expensive just to lift it from sea level, before you start on the reclamation and transport. If the required flows can’t be captured and diverted from upstream, then it isn’t going to be economic for domestic purposes, probably by a factor of 100. For irrigation purposes you are going to need water volumes in the thousands of acre feet (an acre foot would hold about four of those bladders) ---
**Author:** ![rbroome](https://avatars.discourse-cdn.com/v4/letter/r/838e76/32.png) [@rbroome](https://boards.straightdope.com/u/rbroome)\ **Post date:** [October 1, 2011, 12:03pm UTC](https://boards.straightdope.com/t/how-far-out-into-the-gulf-does-the-mississippi-fresh-water-go/598078/13 "2011-10-01T12:03:46Z")
> [@mac\_bolan00](#): > > believe it or not, the fresh water discharge from a river as big as the M is denser than sea water and could travel out to sea for several hundred miles. that’s because it has a lot of dissolved chemicals and suspended particulates so it’s heavier and “creeps” along the sea bottom on the continental shelf and eventually “spills” into the ocean floor. those supended particulates may eventually form thick sedimentary deposits on the ocean floor called “turbidites.” I disagree. It is easy to demonstrate this isn’t true: simply observe the salt-water wedge that extends well up the Miss. River channel. It is, as one might suspect, on the bottom of the river. I suspect that you are thinking of the large fraction of the sediment load in the river that is transported along the bottom and on out into the Gulf. These sediments are flowing down the river, and are certainly heavier than either the fresh water in the river or the salt water in the Gulf. Those sediments act as you describe. ---
**Author:** ![mac\_bolan00](https://avatars.discourse-cdn.com/v4/letter/m/b3f665/32.png) [@mac\_bolan00](https://boards.straightdope.com/u/mac_bolan00)\ **Post date:** [October 3, 2011, 12:07am UTC](https://boards.straightdope.com/t/how-far-out-into-the-gulf-does-the-mississippi-fresh-water-go/598078/14 "2011-10-03T00:07:22Z")
> [@rbroome](#): > > I disagree. It is easy to demonstrate this isn’t true: simply observe the salt-water wedge that extends well up the Miss. River channel. It is, as one might suspect, on the bottom of the river. > I suspect that you are thinking of the large fraction of the sediment load in the river that is transported along the bottom and on out into the Gulf. These sediments are flowing down the river, and are certainly heavier than either the fresh water in the river or the salt water in the Gulf. Those sediments act as you describe. the sediment transport you are describing is “traction flow” which is simply the mechanical transport of sediments. density flow is different. it does create a new kind of liquid that is heavier than sea water. ---
**Author:** ![wevets](https://avatars.discourse-cdn.com/v4/letter/w/e9a140/32.png) [@wevets](https://boards.straightdope.com/u/wevets)\ **Post date:** [October 3, 2011, 1:23am UTC](https://boards.straightdope.com/t/how-far-out-into-the-gulf-does-the-mississippi-fresh-water-go/598078/15 "2011-10-03T01:23:23Z")
I think you may have that backwards, **mac\_bolan00**. From **Bearflag70** ’s link upthread: > [@http://na.oceana.org/en/explore/places/mississippi-estuary](#): > > The main channel is a classic example of a salt-wedge estuary—its surface waters contain little salt, but they flow over a wedge of salt water, which extends deep down for several miles up the estuary. This is not uncommon - most estuaries have denser salt water at the bottom and less dense freshwater at the surface: > [@http://www.des.ucdavis.edu/faculty/Richerson/ESP30/Estuaries.pdf](#): > > Partially mixed estuaries are nutrient traps. Nutrients enter with the river and salt water. Sinking particles from the fresher surface layer drop into the saltier subsurface flow that carries them towards them back of the estuary. The deep salty current then mixes into the upper layer laden with mineralized plant nutrients. and: > [@http://www.msc.ucla.edu/oceanglobe/pdf/densitysalinity/densityintro.pdf](#): > > Because of differences in density between fresh and salt water, salt water will move upstream in the estuary along the bottom, while fresh water will flow downstream along the surface. Some mixing occurs at the interface where fresh and salt water meet. This is called brackish water. Local conditions of temperature and water circulation may also increase or decrease salinity I’m not seeing evidence that the Mississippi has some unusual slurry of freshwater and sediment you’re talking about. ---
**Author:** ![mac\_bolan00](https://avatars.discourse-cdn.com/v4/letter/m/b3f665/32.png) [@mac\_bolan00](https://boards.straightdope.com/u/mac_bolan00)\ **Post date:** [October 3, 2011, 2:12am UTC](https://boards.straightdope.com/t/how-far-out-into-the-gulf-does-the-mississippi-fresh-water-go/598078/16 "2011-10-03T02:12:21Z")
ok, ok, ok. turbidite flows occurr at the off-shore region of the mississipi and while that water is often brackish, it does not represent a direct discharge from the river mouth (which is what i was implying before.) but it effectively explains the principal of density undeflow wherein sediment-laden water being of heavier density than clear sea water. the fresh water dicharge at the channels would certainly flow above heavier salt water. [http://pkukmweb.ukm.my/~kamal/sedimentologi/Geol306\_lec09%20-%20aliran%20graviti%20&%20laut%20dalam.pdf](http://pkukmweb.ukm.my/~kamal/sedimentologi/Geol306_lec09%20-%20aliran%20graviti%20&%20laut%20dalam.pdf) a better example of what i was babbling about would be the amazon estuary which dilutes sea water out to 100 miles and within that distance could already form turbidite flows that will travel even farther. ---
**Author:** ![wevets](https://avatars.discourse-cdn.com/v4/letter/w/e9a140/32.png) [@wevets](https://boards.straightdope.com/u/wevets)\ **Post date:** [October 3, 2011, 2:07pm UTC](https://boards.straightdope.com/t/how-far-out-into-the-gulf-does-the-mississippi-fresh-water-go/598078/17 "2011-10-03T14:07:25Z")
To get back to the OP, the brackish water mix resulting from freshwater outflow out of the Mississippi is detectable hundreds of kilometers from the Mississippi Delta. You might find the third image down on this page ([Welcome to USF College of Marine Science](http://www.marine.usf.edu/microbiology/regulation-mississippi.shtml)) helpful. The yellow & red colors representing salinities equal to or below 34‰ can be seen extending out to somewhere near 550 km from the Mississippi’s river mouth. However, even 33‰ is a lot more saline than freshwater. If you wanted to use a less restrictive cut-off for ‘brackish water’ vs. ‘seawater,’ you might decide to only use salinities less than 32‰, in which case you’d decide the brackish water extended out only to about 150 km. It all depends on how you want to define ‘brackish.’ ---
**Author:** ![wevets](https://avatars.discourse-cdn.com/v4/letter/w/e9a140/32.png) [@wevets](https://boards.straightdope.com/u/wevets)\ **Post date:** [October 3, 2011, 2:09pm UTC](https://boards.straightdope.com/t/how-far-out-into-the-gulf-does-the-mississippi-fresh-water-go/598078/18 "2011-10-03T14:09:34Z")
And if you scroll down to Figure 2 on this page: [Microbial ammonium cycling in the Mississippi River plume during the drought spring of 2000 | Journal of Plankton Research | Oxford Academic](http://plankt.oxfordjournals.org/content/26/11/1265.full) there’s a different map and salinity gradient measurement that may also be useful. ---
**Author:** ![treis](https://avatars.discourse-cdn.com/v4/letter/t/bc79bd/32.png) [@treis](https://boards.straightdope.com/u/treis)\ **Post date:** [October 4, 2011, 1:14am UTC](https://boards.straightdope.com/t/how-far-out-into-the-gulf-does-the-mississippi-fresh-water-go/598078/19 "2011-10-04T01:14:24Z")
> [@wevets](#): > > To get back to the OP, the brackish water mix resulting from freshwater outflow out of the Mississippi is detectable hundreds of kilometers from the Mississippi Delta. The OP isn’t talking about brackish water. S/he’s talking about drinkable water, and there is none of that in the Gulf. ---
**Author:** ![wevets](https://avatars.discourse-cdn.com/v4/letter/w/e9a140/32.png) [@wevets](https://boards.straightdope.com/u/wevets)\ **Post date:** [October 4, 2011, 7:31pm UTC](https://boards.straightdope.com/t/how-far-out-into-the-gulf-does-the-mississippi-fresh-water-go/598078/20 "2011-10-04T19:31:33Z")
Thanks for catching that, **treis** , I was distracted by the hijack… The OP may be referring to NASA’s MODIS instrument detecting a dark plume of Mississippi River water which ran through the Gulf of Mexico and into the Atlantic. That event is described in a paper here: [http://www.aoml.noaa.gov/phod/docs/Mississippi\_River\_water.pdf](http://www.aoml.noaa.gov/phod/docs/Mississippi_River_water.pdf) But if you read the details of that paper, actual on-site sampling (the MODIS instrument can detect color changes in water but does not directly measure salinity) when the Mississippi River plume reached the Florida Straits it was around 35.5‰ and when it reached the South Atlantic Bight off the coast of Georgia it was around 34.9‰, neither of which are drinkable salinities by any stretch. They estimated that in general the salinity of the plume was about 0.8‰ less than the salinity of the surrounding water. Detectable, but you wouldn’t want to bottle it and sell it in grocery stores. Maybe more useful to the OP is that the Army Corps of Engineers has done studies into the salt water intrusion at the bottom of the Mississippi and has concluded that: > [@U.S. Army Corps of Engineers](#): > > Movement of the salt wedge has been correlated to the volume of riverflow using historical low-flow events. Discharges of 300,000 cfs or greater will prevent saltwater intrusion into the entrance of Southwest Pass. As flows drop to around 250,000 cfs, salt intrusion will reach the Head of Passes, which is the point of major trifurcation along the Lower Mississippi River (Figure 1). Distances are commonly measured in miles above the Head of Passes (AHP). Figure 2 relates minimum river discharge and salt wedge migration. Since I can’t post Figure 2, if you want to see it you’ll have to follow the link below; but the upshot of it all is that in a good rainy year, the saltwater wedge is outside the Head of Passes and the Mississippi is freshwater running out into the Gulf. Somewhere (I don’t know where and it will depend on how good a rain year it was) the freshwater will mix with salt in the Gulf. In a bad year, where the flow of the river is as low as 100,000 cubic ft. per second, the salt water wedge will be on the bottom between Kenner and New Orleans, about 110 miles upriver from the Head of Passes, and about 90 miles upstream from Head of Passes (somewhere between Concession and Algiers Point) the surface water will become too saline to drink. The publication may be found as a PDF at the bottom of the page here: [http://chl.erdc.usace.army.mil/chl.aspx?p=s&a=Publications!385](http://chl.erdc.usace.army.mil/chl.aspx?p=s&a=Publications!385) [Next page](https://boards.straightdope.com/t/how-far-out-into-the-gulf-does-the-mississippi-fresh-water-go/598078.md?page=2)