Oh yes, water.
Got to have it to float the boat. 
I like your start.
I have tried different things for rivers.
Still not happy with what I have.
Nice to see others ideas.
i built mine directly on the map/carpet.
i clear off the green texture with a heat gun or a grinder. I know it's heavy tools for modelling but very very efficient

The heat gun dissolve the green so you can clear off all the grass.
The grinder plane the grass but could dangerous for the support of the map/carpet because you can make holes in if you don't care.
The water is made with floor glue mixed with silicone (like Pattex one). This mix is essential if you don't want to see your glue crack when drying. The river is now flexible.
I put blue color in the final mix to get an uniform undercoat.
When dry, i dry brush blue paint, then white.
i make the brillant look of the river with glue-varnish. Severals pass are needed to give a water look !
and now , i can roll the map/carpet for transportation.

that's magic !
cheers
Eric