Rivers Are Jerks

[JasonUncensored]

Making sure rivers run logically can be a challenge.
They run INTO large bodies of water and they flow from high ground to low.
Other than that, I don't know all of the rules.
If I recall correctly, most rivers also flow towards the equator, correct?

[waldronate]

There's no real to/away from any particular point on the world except away from the high toward the low (unless your world is rotating fast enough that water and air wouldn't stick to it, but that's unlikely). Bodies of water fill up low spots so rivers will fow towards them, but not at the cost of going up and over a hill to get there.

Ocean and air currents, now, those have particular flow rules that relate to poles and directions of rotation. They will influence the amount of water but not the direction of flow.

[Midgardsormr]

The biggest other rule that is often broken by inexperienced fantasy cartographers is that rivers join together as they flow downstream, but almost never split, unless they rejoin soon after.

[rshall89]

If you wanted them to split again you could always use a lake to divert the river in 2 different ways, lake would probably be big or in a elevated area.

Rivers go from up to down elevation wise, using the quickest path. The equator doesn't influence the rivers direction. Rivers can cut path through mountains if its the only way out, Klamath, Columbia and Pit, all rivers that cut through the Cascade Range.

[Nomadic]

If you wanted them to split again you could always use a lake to divert the river in 2 different ways, lake would probably be big or in a elevated area.

The same rules that govern rivers govern lakes, so no this falls under the same rule. Rivers almost (almost though not always) never have two outlets from any point. Lakes likewise almost never have two outlets from any point. One outlet will eventually win out the erosion battle and the other will dry up.

[Sigurd]

Rivers may end in a lake but if they flow from a lake they take the most efficient path down. Any number of rivers in but normally only one route out. Lakes simply empty to a level that they are only drained by one exit.

Islands may happen in rivers especially where seasonal high water erodes a broader river bed. Even in these cases however the river will eventually choose the simplest most efficient way to go down.

Erosion will gradually straighten rivers quite a bit. The big exception to this is often a delta flood plain. In a flood plain really slow water is greatly influenced by the soil strata and may very irregularly. Increased flow speeds erosion and always leads to straighter rivers.

A very slow and lazy river can gain some of the features of a lake.


sigurd

[Steel General]

Rivers are.... filled with water

[Talroth]

The same rules that govern rivers govern lakes, so no this falls under the same rule. Rivers almost (almost though not always) never have two outlets from any point. Lakes likewise almost never have two outlets from any point. One outlet will eventually win out the erosion battle and the other will dry up.

True, but if you have a lake that is sitting on a shelf of exceptionally dense and level bedrock, then from the view of a purely mathematical model you can get a lake that then feeds into two different watersheds for a few thousand years. (And this is the 'almost' aspects to rivers never have two outlets.) I don't know of any river on earth that is split by a watershed in this manner, we usually only get islands a few miles long before the river is diverted back into a single course. However if a falls like Niagara was pushed up against a hill high enough and made of strong enough stone, it could conceivably spill water to either side of a small mountain range. However in Niagara's case it is spilling over the hard sheet of stone and eating away at the far softer stone below it, generating a nice canyon system. (This is far, far more likely to be what happens than splitting into two rivers. Just if your heart is set on a river that splits, this is the only way to do it for more than a few years.)


If you are struggling with drawing your rivers so they work well, then try drawing them backwards, draw your rivers uphill. Pick a point where you think a river might meet the ocean, and then start snaking lines out from that point, branch them off now and then with the goal of collecting water from all areas you can. if you run into a hill top, stop. When you can't go any farther with the river system then go back to the coast, move up or down a ways, and start another river. If it looks like it is about to run into a hill top or existing river, then again stop.

After that, look around for areas that are over hills and haven't had lines drawn to them to drain them. Then you have to think about how you'll deal with them. Will they become a large lake that fills up a valley and then flows over the lowest point between two hills and then connect to one of the existing river systems? Or will it sit there and collect all the water into a pool at the bottom and have it evaporate? Maybe it will collect in a large pool in the bottom of the valley and then drain out through the ground.

[Redrobes]

For more, see:
http://www.cartographersguild.com/showthread.php?t=3822

[RobA]

One other good ref is the Wikipedia entry on continental divides:

http://en.wikipedia.org/wiki/Continental_Divide

Note the exception to "all rivers flow to the sea" rule in the basins of N.America.

-Rob A>