Below is my workflow for terrain/water features.
I use the online utility Terrain2STL to create 3D terrain maps for carving.
From the map above, the generated STL in meshmixer looks like this (perfect for 3D carving with Aspire or Fusion360):
If you are doing 2.5D carving, take the generated STL and slice off the land terrain in Meshmixer using the Plane Cut tool, leaving the waterway behind. Then take a screenshot of the waterway in Meshmixer from the top, like so:
Crop the screenshot to the map only, then feed it into an image tracing application (Illustrator used below) to create vectors from the image. For something like this, I prefer Illustrator’s Image Trace tool using the Shades of Grey profile.
Then you need to tweak the generated vectors until they look good and can be cut with your bit.
This process takes time (not to mention the learning curve for the software) but produces very good results.