Mississippi: How Big a ship would fit?

I'm working on a TL, but I'm having difficulty with the issue of ships sailing on Mississippi. Quite simply, I know very little about how river boats worked, so I'm wondering how big a ship would have to get before it could no longer navigate up Mississippi river?
 
i live on the very river you speak of :)

I wouldn't say very big ships, like the massive cargo ships that travel the oceans today.

I have seen some rather large boats with a shallow draft, mainly used for tourism and such (you cant go to high now, as there are dams on the river further up
 
It's not a matter of size per se, it is a matter of design. The Mississippi is plenty wide enough to accommodate most ocean going vessels. The problem is the depth. I'm sure that if an ATL nation felt the need to (like if it were frequently fighting wars over control of Mississippi shipping) they could probably find a way design massive warships with very little below the water so that they could navigate the river. I'm not exactly sure how big, since I'm not an engineer, but it sounds doable.
 
They might be able to be built big but how much armour could they hold- if a ship doesn't have a very deep draft wouldn't putting a lot of armour in her make it very top heavy and unstable? Although now I think about it they did have special ironclads on the Mississippi during the American Civil War so it is doable as long as you keep within certain limitations.
 
With no waves there isn't much need for deep draught so wide, shallow draught boats can be quite big. But they are stuck on the river system and are screwed in the open sea.
 
According to Wikipedia, from St. Paul to Baton Rogue, the Corps of Engineers maintains a depth of at least 9 feet (since the 1930s). That said, it's much deeper in most places. For example, the average depth is about 120 feet. In St. Louis, the average is about 30. Even as far north as Minneapolis, it's 16 feet. It gets somewhat shallower as you get closer to New Orleans. Throughout its course, you get sand bars and other obstacles that shallow the river.

I know that doesn't help much if you're interested in the 1800s but my guess is that you'd be safe for much of the river with a draught of about 10 feet. Before the 1850s though, you'll not be able to get anything north of Minneapolis.
 
Before the 1850s though, you'll not be able to get anything north of Minneapolis.

I believe that's 1950's and the '60s, when the St. Anthony Falls locks were built to clear the falls. But, you still can't get past the northern suburbs (a dam around Coon Rapids, I believe).

you cant go to high now, as there are dams on the river further up

The dams of the Mississippi (starting with St. Anthony Falls) have locks allowing for commercial travel. Freight barges still run all the way between Minneapolis and New Orleans.

EDIT: I'm not sure when they started building the dams and locks, before that there would have been some sections of the rivers there (in the places where the dams were built, or just upstream of them in their reservoirs) that might have some rapids, but they were still navigable by riverboats. This system ends at around Cairo though. Dams built below that are pretty much just for flood control, AFAIK.
 
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I believe that's 1950's and the '60s, when the St. Anthony Falls locks were built to clear the falls. But, you still can't get past the northern suburbs (a dam around Coon Rapids, I believe).
The first lock and dam system at St. Anthony's Falls was built in the early 1850s according to a source I read (I can find it again if you'd like). The lock channel was 16 feet deep and the dam wasn't so much a dam as just something to concentrate the power of the falls into something more usable for hydro-power.
 
According to Wikipedia, from St. Paul to Baton Rogue, the Corps of Engineers maintains a depth of at least 9 feet (since the 1930s). That said, it's much deeper in most places. For example, the average depth is about 120 feet. In St. Louis, the average is about 30. Even as far north as Minneapolis, it's 16 feet. It gets somewhat shallower as you get closer to New Orleans. Throughout its course, you get sand bars and other obstacles that shallow the river.

.

Uhm.

Not being an expert by any means, but it might be best to clear up something a little. They run tankers up to Baton Rouge, and have for a long time. How it is kept clear, I do not know, but is probably a mostly natural process. It would be better to say gets shallower as you get closer to Baton Rouge.

Here is a quote: "Between Baton Rouge and the Gulf of Mexico is where deep-draft ships operate; further upstream the Mississippi shallows and only barge traffic is allowed."

http://www.maritimesun.com/news/river-floods-may-trigger-halt-of-ship-traffic/

A guess is when the river starts meandering heavily to the north, and changing its loops as it did so many times, it tends to make shoals, preventing useful shipping consistent depths for deeper drafts. Also the gradient may pick up as there are a lot of bluffs to the north.
 
The first lock and dam system at St. Anthony's Falls was built in the early 1850s according to a source I read (I can find it again if you'd like). The lock channel was 16 feet deep and the dam wasn't so much a dam as just something to concentrate the power of the falls into something more usable for hydro-power.

Okay, I knew they had a dam there to power the mills that long ago, but I didn't know they had a lock. (I kind of assumed they didn't, because I never really could see it in the pictures).
 
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