In a mountain landscape rivers and lakes should not miss, as they give a characterustic touch in our scale model (if they are good placed). The use of the true water is not reccomanded, becouse it produces humidity problems which are not good with our rotable materials and our electronic system.
The easier solution is to use resins (es. woodland scenics). First make the bed of river, or of the stream or of the lake, by using thin brown dusty backdrop.
Then apply some small stone and some little broken branches. Put then the resin.
To simulate some little ripples on the water surface, you can blow (for example wih a hairdryer) when the resin hasn’t dried yet. At drying done you will be surprise for the result.