Cobb has noted correctly these are highlights. A river flowing toward the bottom edge of the screen will not necessarily have highlights that are moving. Highlights are formed by water deforming as it passes over rocks and such that are under the water (Those objects do not move) or by wind causing ripples.
If you can find some old Hanna-Barbera cartoons you can see economic and minimalist animation at its best. They'd create a series of three, maybe four, very simple frames that suggest movement and loop those. The frames might simply be a series of blue forms that appear to move toward the bottom of the frame. For more examples of this technique, search the web for "animation cycles" or "cyclic animation."