In the past I would have cover the whole piece (minus the space ship) with a coating of PVA glue and sand to get a ground texture but that always had a fake look. Nature rarely has a uniform texture so this piece has about 60-70% sand coverage. Most bank erosion is the pink foam ground down with a stone bit and painted and silt deposits were made with clay. After painting gravel deposits were added (used a commercial product from Gamesworkshop that has a nice look and doesn’t require paint). The river banks are 2 layers of 3/4 inch pinkfoam which lent itself to doing overhanging banks.
The ship was designed and carved from scraps of 2" pink foam after browsing a book on bugs; I wanted an organic, possibly living ship. After painting the whole piece it just didn’t have enough impact so that lead to Trying to do "tendril" and suddenly I hit on the shredded "sail" which certainly convey the idea that nature has had her way with this structure. (The sails can be either the remains of a solar sail or a temporary shelter made by the survivors- but that is a mystery for the player characters to determine.)