Does a Monster Flood Have to Fuel a Monster Dead Zone Too?

[n2doc]

By Julia Whitty| Fri May. 13, 2011 2:24 PM PDT



Sediment-laden water pours into the Gulf of Mexico from the Mississippi and Atchafalaya Rivers. Credit: Norman Kuring, Ocean Color Team.


As you can see from the image above, a lot more than water comes down a river. The Mississippi and its distributary, the Atchafalaya River, carry an average of 230 million tons of soil into the Gulf of Mexico every year. On flood years, the total runs a lot higher.

That's going to cause downstream problems this year... all the way downstream to the ocean. But before we go there, let's take a quick look at how the system works.





Credit: USGS.


The graphic above shows sediment concentration and sediment discharge for major US rivers. The giant half-circle in the Gulf represents the Mississippi/Atchafalaya discharge. Why are the two rivers bundled together? Because they're really the same river: the parentheses bracketing a 100-mile-wide delta.





Mississippi delta switching during the past 4,600 years. Credit: Wikimedia Commons.

Before 1927's epic flood, the Mississippi was free to wander the delta, whiplashing between courses whenever a huge flood year like this one pushed it over its banks—a process known as delta switching. In the above image you can see some of its historical courses and when it ran them.

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http://motherjones.com/blue-marble/2011/05/does-monster-flood-have-fuel-monster-dead-zone-too

[jpak]

1. The answer is probably yes

The size of the Dead Zone is correlated with spring/summer discharge volume of the Mississippi River - and especially when flooding occurs in the Corn Belt after fertilizers have been applied (this elevates dissolved inorganic nitrogen and phosphorus concentrations).

The sheer volume of water entering the Gulf will produce a large surface lens of freshwater that will enhance stratification as well.

The only saving grace may be uptake/immobilization of Mississippi-borne nutrients in the Atchafalaya Basin (which retains much of its historical floodplains/bayous/marshes). That process, however, will depend on the transit time of water in the Atchafalaya system - too fast and little uptake/immobilization will occur - very slow = lots.

Expect a big summer Dead Zone that will only dissapate after hurricane passage.

[Sinistrous]

2. The shrinkage of the delta may abate for a while,

but I don't see that as a problem. It isn't the soil itself that causes the "dead zone", but the phosphorous and other nutrients it carries.

[RaleighNCDUer]

3. Look at the silver lining -

all that sediment will bury the millions of gallons of oil that remains on the ocean floor from last year's spill.

[FBaggins]

4. Not close enough to the spill to have that much of an impact... but yes, there are some positives.

Remember that much of what makes the area fertile is the centuries of annual flooding that deposited alluvial soils/silt/etc on the area. Mankind's desire to control the river for our own purposes ended much of that.