Thanks, all. I knew the foliage was going to be front and center in this image, so I spent quite a bit of time on that. Otherwise it's coming together pretty quickly. (For me, anyway.)
I wanted to share these two images to show how the terrain came together. I started with a 2-meter DEM downloaded from the
New York State Geographic Information Systems website ("before" image). It's pretty nice quality but of course it includes many modern details such as an expressway and other roadbeds. The exported .ter file was imported into a new TG scene and "terraformed" to remove those and then, using old survey maps as references, restore the location of the Mohawk River channel, insert the canal, and add procedural detail ("after").
It was all pretty simple because all the changes were on the flat valley floor. I have another project in mind that would involve editing some DEM slopes to restore surfaces that have been excavated away. Maybe there's a way to use a painted shader to paint the elevations back in? Doesn't sound easy.