Creating realistic waterways

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Where do you even put rivers?! Where do they start? How wide/narrow should they be? How do tributaries work? Where do they end?! AAAHHHH!!!!!

Don't panic, I'm here to help!

First, I want to start off stating some basic facts about rivers:

• They generally start at higher elevations (mountains & hills) due to snow melt or accumulated precipitation, and flow to lower elevations. 
• They can run in any direction as long as it's from a higher elevation to a lower one. So they can flow north, south, east, west, northeast, northwest, southeast, southwest, etc.
• Rivers will flow until they either reach another body of water, or dry up/absorbed into the ground.
• Rivers can cut through stone wherever/whenever the flow is fastest/heaviest or be detoured wherever it is slower/more gentle. This means that a twisting/winding river is generally slower and a faster moving river tends to be less twisting. This is, of course, depending on how hard the stone is that the river is coming into contact with though over time it would eventually wear the stone down.
• Brackish water is where fresh & salt water mix. It can be clear but it tends to be muddied or greenish water.  Brackish doesn't mean foul or black water.
• Water speed & depth are not in direct correlation to the width of a river.  The width of a river is not an indication of the speed of the water nor the depth of it. A wide river can be moving just as slow/fast as a narrow river and both can be any depth.

Second, some points about rivers and society...

I often hear people talk about how "towns spring up along rivers" although this is true, it's not because it's necessarily a river. A better phrase is "towns spring up around resources".  Rivers are unpredictable at times and can change course, over flow, or dry up; so just because it's a river isn't a good basis to set up camp.  Towns spring up wherever there's a resource or means to etch out a living, even if that living is from trade.  They are just as likely to be found along trade routes as they are rivers.  Many towns in the US were established because they were along well-known animal migration trails and made hunting a lot easier.  And if a river has a stone or heavy clay bed, they might not be suitable for farming thus making it a less than ideal place to start a homestead much less a town.  You're better putting towns, villages, and cities were there is at least one other resource than just water. And the more resources an area has, the larger your town/city will be.

Some other things to keep in mind when plotting your river's course:

• Wider & deeper rivers will be more attractive to those looking to trade as it allows for ships to pass through easier.  
• Wider & deeper rivers are also great for logging towns as well, not only for the water mills but for transporting logs themselves.
• Narrow & shallow rivers are great for fishing as well as for detouring water flow to fields for crops.
• Creeks & streams provide cold fresh water that's easily detoured to homes & fields.
• Animals usually cross at shallow, slower flowing areas of rivers thus making them a good hunting spot but not a good place to set up a town.
• Towns & villages are best placed around straighter parts of a river because bends can overflow and wipe out structures.

Third, rivers can be a resource for more than food & water for humans. They can also provide:

• clay for building foundations
• moss for roofs of homes and bedding
• water for livestock
• water for forest animals (and also ease of hunting)
• source for cleaning & dying clothes
• reeds for thatched roofs, baskets, mats, etc.
• a natural deterrent for invaders if they are difficult to cross and/or have predatory animals living in or around them.

Now...let's talk about width of rivers on your maps

You're going to want to start your rivers usually in the mountains as smaller streams flowing into one bigger river that eventually leads to the ocean.  It's important to keep in mind that as you draw these rivers, you're specifically indicating that the land is going from a higher elevation in the mountains to a lower one at sea level or below.  This means that as you add things around your rivers, they will be at a higher elevation, even if it's slightly.  

The more twists you add, the more hard rock you're implying is around the bends.  Again this shapes what's around your rivers.  With bends in river, it's a good solid place to add bridges just before or after but keep in mind that rivers swell and shrink with the seasons so give some space for that.  Most people wouldn't build at the bend of a river but just before or after it so if the river overflows, it won't wash away their structures.

The width/depth of your rivers depends on how many smaller streams/rivers contribute to it and how much snow melt or precipitation is in the area.  If you're adding several smaller streams from snowy mountains leading to a "main" river, the river itself should be quite wide or deep.  You can make a river deep but narrow or shallow but wide.  You can even made it quite wide and deep but that usually happens the closer and closer it gets to the end/ocean.

HOWEVER, I want to always remind you that maps are meant to be more representational than exact so if you need a river to be more visible on your map, it's ok to draw it a bit wider to emphasize it.  You can always go much more "to scale" when you draw an area/battle map.

How many rivers should I add?

Did you know that there are countries without ANY rivers? Usually deserts or island countries, these places have alternative means of getting water to their people.  So if you don't have a lot of rivers, that's fine!  Some countries without river are the Bahamas, Kuwait, Marchall Islands, Monaco, Saudi Arabia, and more.

In the opposite vein, Russia has roughly 100,000 rivers which is more than any other country in the world! So ultimately it comes down to how many you want/need for your story.

River Length...

Again, they are usually from your mountains to your oceans.  If that seems too far, know that the Nile river is 4132 miles or 6650 km long. (Click here to see several real world examples of river lengths.)

A note on lakes, ponds, & other standing bodies of water...

You can have rivers run to non-oceanic bodies of water as long as that body has a reason it isn't overflowing into the nearby land.  It is not uncommon for lakes or ponds to overflow and cause temporary streams/rivers until they water height is lower than the surrounding land again.  This occurs with things like annual snow melt as well as sudden heavy precipitation (such as an annual monsoon in a valley between mountain ranges).

You can have standing bodies of water slowly drip into an underground water source or spring up from one.

You can even have them with no direct water source or outlet other than the usual precipitation and evaporation cycle in an area. Remember, water will pool at your lowest elevations so even if the water is from rain, it can easily pool into a lake.  BUT these types of bodies have a different eco-system all their own and will form things like algae, insect swarms, and other still-water attracting animals.