Making a dug-in path and water area for a river is easy enough, but when you want a river to flow in the correct direction and at a certain speed, then it becomes more complicated. After doing some testing, I have figured out how to exactly make a river flow in the direction you want. The best way to illustrate this is with pictures. I will call each "Dot" in a water area you place using the water area tool a "Vertex" (Plural Verteces.)

Let's start with the key:



The Blue Circles are the 2 End points of the river and the green circles are the middle (If any) Verteces for adjusting speed or changing direction.

This example has the fundamental rules of a functioning river: Each End consists of 2 verteces, and the mid points are even on both sides.



Direction
Now it is time to learn how to adjuct direction. The direction your river flows in directly depends on the order that you place your verteces. Below is an example of order of placement and river flow direction.


[Each number represents its order]

This is the simplist type of river, 4 verteces. As you can see, you start with the bottom left, then bottom right, then top right, then top left, and the river flows upwards.
[Remeber that you can't move the camera in any way during the placement, and to end placement either move the camera or rightclick.]

You can also have the river change directions using midpoints like so:



In this picture you can see that it is a very similar placement order, with the exception of the right Midpoints placed before the top End and the left Midpoints placed after the top End, while the bottom End is always first.

Here you can see a slightly more complicated river:



I recommend that while placing rivers you count in your head or alloud the number of midpoints the river has, that way you can be sure to make it even.


Speed
The speed also has a general rule for it: The longer the space between Ends/Midpoints or Midpoints/Midpoints, the faster it looks. Here is a picture to illustrate what I mean.



The time it takes for Point A to get to Point B is EXACTLY the SAME as the time it takes to get from Point B to Point C. As your math teacher will confirm, if a car goes 1 mile in 1 minute it is going faster than a car going 10 feet in 1 minute.

This means that when making a river, you need to keep in mind that the river needs to stay at a steady speed by making the verteces evenly spaced. If you want to have a faster river, use less Midpoints. If you want it slower, use more.


Others
I don't have a picture for it, but you need to make sure the vertecies line up nicely from side to side or else the river will look distorted. If one Vertex is higher (Closer to another vertex or End) than the equal Vertex on the other side, it will look as if there is a horizontal line through your river because of the different (uneven) speeds.

A river will also look distorted if it has uneven midpoints, remember that you can add or subtract midpoints by clicking on the river verteces and clicking Delete to take away, or click on a line of the water polygon with the water polygon tool selected to add a vertex.

Forks or Splits:
A Fork or Split in a river requires another river water polygon area. Below is a simple example of how to make a river Fork.