Looking at pictures is dangerous when you have the terrain bug

[Pattus Magnus]

My original modular hills were made of 1’ insulation foam had approximately a 2/3 rise over run slope. For the river bank terrain tiles, I was considering using 0.5’ insulation foam (a bit less common here, but still available), and using a slope of about 1/3. You’re right about the challenges involved in getting the edges to match, though. That’s one of the reasons the project never got beyond the planning stage!

[snitcythedog]

Another option that would be a bit easier to build and work with is to use that eight foot of foam that you said was screaming to be used as a big table spanning hills.  You can sculpt the sides to look like the sides of your riverbank and lay the whole sheet onto your normal gaming surface.  If you do a sheet for each table edge you have a river bed.  Storage might be an issue but it would remove the issue of matching edges and exact measurements. 

[Pattus Magnus]

I just came across this on the Bloodbeard’s Garage blog (lots of other cool stuff on there in several fantasy and sci-fi genres, too, if you’re looking for inspiration).

 https://bloodbeard.blogspot.com/2019/09/tutorial-jungle-island-felt-mat.html?m=0

Imagine doing something similar, but using colours to make a river bed like the photo in the original post… I think it would work using felt for the islands/ surrounding plains, like in the blog post, or you could use foam sheet or some similar material that doesn’t warp much if you want some vertical elevation. There’s quite a bit of flexibility for game layouts, too.