Adding a river to a drawing is easy with a couple of techniques. The first technique is to determine the horizon line. The horizon line establishes the viewer view such as a bird's-eye view, worm's-eye view or eye-level. The second technique is creating organic lines and not straight lines. Straight lines will give the impression of a road instead of a river. Once the river is created, add details to complete the drawing. (See Reference 1)
Determine the horizon of the drawing. Is it a bird's-eye view, (reference 2 shows a bird's-eye view) eye-level view or worm's-eye view? Draw the horizontal line to separate land from sky. A horizon line at the top of the will give a perspective of looking down at the river from the sky. This is a bird's-eye view. Looking up at the river from the bottom of the page is a worm's-eye view. The middle area is an eye-level view. (See Reference 1)
Start with the wide end of the river, and sketch an organic line to the top of the horizon line. Sketch another organic line parallel to the first line. Have the lines taper so the width is smallest at the top of the river and widest at the bottom of the river.
Add detail to the river scene such as trees and rocks.