LDoc said:
Trench warfare isn't exactly natural to the large confederate/us border. One of the reasons it was so developed ont he Western front is that the area they were fighting in wasn't very large or easy to manuvere in. Unfortunetly most of were the Civil War was fought is the exact opposite.
The overall frontier between the USA and CSA was very long, but I think that geography, terrain, and supply routes tended to limit the areas in which really large armies were most likely to be concentrated. In the east, there was a pretty large area cut through by the Appalachian mountains, relatively thinly populated and with only a few roads and fewer rail lines, where it was a lot more difficult to operate with a large army. West of the Mississippi, a very thinly scattered population and lack of many railroads seriously limited the number of troops that either side could effectively deploy or supply. Even along the Mississippi, there were large areas that had few people or roads and were dominated by swamps and bayous. All in all, this is going to mean that in a trench warfare type situation, the thick trench lines will probably tend to be concentrated in the more suitable terrain, such as eastern Virginia, the Shenandoah valley, western and central Tennessee, and/or parts of Mississippi, Alabama, and Georgia. The other areas would probably have much more thinly spread troops in a series of camps and outposts.
Even in OTL World War I there was a similar phenomenon. The trenches and defensive works in Alsace, the mountainous southern end of the western front, tended to be more lightly held than areas further north, and in some places they were more like a series of outposts than a continuous line. On the front between Italy and Austria-Hungary, the heaviest troop concentrations were in the east, where the land near the frontier was relatively flat, while the mountains were usually held by troops more thinly spread out in a series of outposts. On the eastern front, the line was generally too long to get the troop density that existed on the western front, but both sides tended to concentrate more troops in what they considered the most vital areas, so those parts of the front could develop a fairly thorough network of trenches.