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From AACWWiki

River boats provide a variety of capabilities for both sides.

Preventing river crossings

One capability of ships is their ability to prevent enemy ground forces from crossing rivers.

4 boats or more are needed, whatever their order is (offensive or defensive), to block the crossing of enemy ground forces in a water region. Look carefully for blocking all the water region (sometimes you can cross a river using two different regions to reach the same land region. This is a very valuable capability, since you can prevent some areas from infiltrations while you concentrate your troops in other strategic places.

However, a blockade can be negated by just one enemy ship entering the blockaded river region. A small blockade of one, two or three gunboats may not prevent a single enemy gunboat from remaining in a river region and ending a blockade. But 5 or 10 gunboats is very likely to immediately drive off or sink a single gunboat and maintain the river crossing blockade. Sometimes you need a large number of ships to guarantee blockade of a river region. The bottom line is if both sides have ships in the same river region, the river is not blockaded and both sides can cross the river.

Amphibious landings

A second major capability of ships is to land amphibious forces behind enemy lines. The Tennessee, Mississippi and other rivers offer excellent opportunities for attack behind rebel lines. However, once landed behind enemy lines, those forces must receive supply either along a river or by a land route. Forts or entrenched land forces with artillery can block river supply. Land forces with artillery need a certain level of entrenchment (5?) before they will block supply. River invasions will be very shortlived if the invasion force cannot receive supply. One temporary solution is to use fully supplied river transports as a supply source. River transports can also be converted into supply depots but they still need an unblocked supply line either by river or land to resupply once depleted. River transports are substantially cheaper for creating supply depots than using wagon supply trains.

Raid support

Finally, river ships are very good for dropping off cavalry for quick raids by either the US or the CSA. However if evacuation is necessary, getting troops out can be very tricky for raids or failed regular water invasions. It's not absolutely certain but it may be that if even one enemy ship is in or enters the same river region as troops loading onto ships, that enemy ship will halt the entire loading process. If enemy troops are in close pursuit of a raid or your evacuation is under attack and your troops cannot load onto ships if your opponent is able to get ships into the loading region, your force can be lost. It takes five days to load onto ships. Just beating off the one enemy ship, regardless of the size of your navy, halts the loading process. Not good if you are in a hurry to escape. An alternative escape is to use the non-ship river transport. River transport only takes one day to load, so you are on the river much quicker instead of being pounded by ground forces.

In general

The Union needs as strong a river force as they can create. Every ship can be put to good use, especially ironclads. Ironclads are the fighting ships that can survive bypassing forts and can defeat enemy navies. Gunboats are better used as blockade, escort or scouting ships. The Union should be aggressive in use of their river capabilities.

Thoughts for the CSA

The Rebels need a reasonable river force as well to disrupt river invasions/evacuations and also allow movement across rivers. In particular, they need ships if they intend to use land forces around Nashville or Bowling Green. The rebels should always bear in mind the use of entrenched artillery to block river supply as circumstances dictate.

References

• Original version based on a post by Jagger at the AGEod forums