I had fun adding a few more details to it :)

About what I discovered, to select the river bed in one click you can use the tool Loop Selection instead of selecting meshes one by one. And instead of the stitching tool to merge several mesh in one point, you can select them all and then right click > Collapse.

A part for these details the course was really nice and easy to follow. The rendering settings are simple but they work really well!

Thank you!