There are 3 Laws that your rivers MUST follow if they exist on an Earth like planet. You won’t find a river on Earth that breaks any of the following laws follow the below laws and your maps will look realistic. Simples.
1, Just Like Us They Are Born
Every river has a source. They have to come from somewhere and every source usually starts high up in the mountains. Water then moves from a position of high elevation to one of low elevation – or in other words it begins to run down the mountain/hill.
So when you’re looking for a place to start a river look for the mountains on your map and move towards the sea.
2, Rivers Are Lazy
Water takes the path of least resistance and as rivers are big movements of water they follow the same rule. That means if you place water on a surface it will find the easiest way to get from a high altitude to a lower one.
You can see this effect if you watch your window when it’s raining outside. Droplets of water will hit your window and slide down, however they never slide in a perfectly straight line. There is always a little wiggle from left to right as the droplet finds the path of least resistance over the surface of the glass.
It’s because of this law that you’ll see bendy rivers such as the Amazon. As the water travels over surfaces with different levels of resistance it alters the course of the river.
3, They Want To Be One With The Sea
All rivers have the desire to join the sea. You’ll never see a river just randomly stop in the middle of a body of land. It is possible that it may go underground to reach the sea but these are rare circumstances. Most of the time a rive will move towards the sea or another river and so become bigger and stronger the closer to the sea it gets.
One thing that some new cartographers do is to end a river halfway in land or they might end it in a lake. This would never happen in real life as even lakes have exit rivers that lead down to the sea. Without the flow of water provided by an entrance and exit river the lake would stagnate and become poisonous to drink from.
Also it’s worth noting that a lake may have multiple entrance rivers but it will only have one exit river.
The Course Of A River
Here's a great little diagram that shows the course of a river and the different characteristics at each of the 3 parts of a rivers life.
If you follow the general shape and course of the river above in the diagram along with the 3 Natural Laws then your rivers will not only look realistic but they will add extra life to your maps.
How do you name the rivers in your world?