How to make a river channel?

140476351-GP-River-rLeftRight.skp (163.1 KB)
As a topic, How to make a river channel (two sides)?
I am using Sketchup to import some lines and curve to rhino, but I can’t generate face in Rhino.
Best Thanks! (See attached)

Join your segmented curves into single polycurves , rebuild the poly curves to get cleaner curves with similar point counts, then loft the rebuilt curves to generate your surfaces

check the help in rhino for details on each command in bold