Still, it can be difficult to place control points at decreasing elevations, to ensure that the river flows down and that it does not rise above the ground. I implemented a plane which effectively shows what terrain is down hill from the previous point and if the current control point is above the previous point.
Adding this simple feature makes placing rivers much easier as the Flood Grid shows you the only possible regions which additional control points can be placed.
This video shows the interface for placing rivers with Flood Placement enabled. The grid moves with the selector (3d cursor) and appears orange if the elevation of the current position is higher than the previous point in the river. The end of this video shows the map loaded into a separate program which loads the features of the saved file.