Water Mud

The Geometry > WaterMud brush allows you to paint dirty or muddy areas of River objects.

The color and properties of muddy water are set in the River object. See Adding Rivers and Water Objects for details.

As opposed to the regular mud, the water mud simply affects the color of the river.

I.e., according to the painted areas, the game will perform blending of the two river colors that are set for the river in its properties: Clean Type (for clear water) and Muddy Type (for muddy water).