Here’s a short tutorial on making “wild river” terrain. The impetus for this was playing the Fords of Isen scenario, but the techniques obviously could be used for river terrain for any game.
Modeling
Making Alien/Mutant Vegetation From Straws
The seminal site Terragenesis has an interesting bit on making alien vegetation from drinking straws.
Modeling Tree Bases
The Joy and Forgetfullness Blog has a useful post on modeling good looking tree bases.
Making A Guard Checkpoint
Here’s a tutorial on making a nice guard checkpoint. It’d be useful for a variety of 20th century settings.
Modeling Sandbags
What’s a modern battlefield without sandbags? Here’s a nice tutorial on making some.