That consideration is among factors making me want to fish for those Asian carp using a roundhaul net that circles and then closes immediately, keeping most everything in the net alive ----one reason being so that game-fish species could be sorted from Asian carp, and returned to the waters the game-fish came from -- if removed from the water, at all.
My apologies for back-tracking to fisheries considerations, but there's a bit more along those lines...
Gillnets are species selective only in mesh size, and in just where and when they are set --including what parts of the water column the entangling meshes of gill, and/or trammel nets are placed in. Once a fish is caught in a gillnet, it doesn't necessarily die right away, but after some time does usually die, with this varying depending upon the type of fish, and just how gilled, or 'wedged' into the mesh, and how long the fish is in the net, etc. The fish that get away, or else those thus caught but chosen to be put back in the water -- can still suffer some amount of injury, which can lead to death, or lesser dysfunction later on. Sorry again to go on about commercial fishing aspects, when you were talking about eradication through blockage and chemical-like treatments...
As for the electric fence contemplations; I think I see better what you were intending regarding an overall plan, but the "fences" that are there presently being used did cost a pretty penny, which is something to consider The power consuptions of those things are rather high, and there's always risk that things could go haywire (leading to serious injury and death of human beings) given enough time such systems were in place.
The additional note you provided there at the end would bear more investigation;
How to make "a dam" that is usually "open"? I'm having trouble envisioning such a creature unless there be some side-sluice (or multiples of those) which could be closed off. One question, touched upon diagonally in your added "(note...)" -- what kinds of river flow does the Illinois require? That does vary, and quite a bit, too, depending upon weather conditions and need for sufficient drainage at any one time.
I must admit I don't know enough about the physics of the river systems to comment any further than idle speculation...
Eh, basically the dam would have a big gate in the middle, with the river flowing through it, as well as river traffic. The gate is only closed when a fish kill is needed.
This is not a tall dam — I’d envision maximum lake level at only the normal “flood level” of the river presently, in an average spring. It is also not there to physically block the carp: It’s purpose is to make killing all the fish in the river for the next 50 miles or so upstream practical, without severely affecting aquatic life downstream from that point. Flood control might be a secondary consideration.
Of course, the practicality of the dam also depends on what the value is of the downstream fishery, if there is still one. I’d add that rather than building the dam, it might be cheaper to dump in Potassium Permanganate at the point of the proposed dam, to deactivate the Rotenone as it goes downstream. This especially if “kills” are rarely needed.
Cost of the overall plan should be pretty modest compared to the ongoing value of the Great Lakes fisheries (including the sport fisheries.)
“Flow” is a good question and would need research.
BTE, no need to apologize for the “fisheries” thoughts. ‘Tis interesting. :-)