We have had so much talk on this subject and so many maps fail this point that there is a River Police badge now for officers of the CGuild to catch miscreants in the process of making a map with duff rivers.
We have stated many times that water flows down hill and others, especially Waldronate, have given a list of points that must be true for water in a non magical and natural terrain and yet we still have those maps coming in.
So this tut is not so much about what we should be doing with our rivers but exactly how to marry the real world with the pen in your hand.
So first lets state a few really basic known properties about water in a non magical terrain.
1. Water flows downhill and it does so in the direction of steepest decent.
2. Rivers originally start with water which has fallen from rain.
3. Rivers ultimately end in the sea or in rare cases may evaporate into nothing.
We will add more shortly but lets dwell on these for a second.
Point 1: Rivers flow downhill.
If you don't know what is high ground and low ground about your terrain then you cant get your rivers traveling in the right direction. Trying to make terrain heights based on already specified river patterns is hard work. So determine your terrain height FIRST.
The statement also implies that two separate bits of water will always follow the same path. This is generally true. So when two rivers meet they BOTH then travel in the SAME direction. I.e. rivers do not spontaneously fork or split into two and go different ways around an obstacle. Only in a situation where one path cannot take the combined flow from the source does the water then split into different routes. So rivers always join up and do not split up.
Since rivers travel in the direction of greatest decent you cannot have rivers on the top of a hill or along any ridge. In fact when a rain drop falls it goes into exactly one 'Catchment Area' and these areas are usually separated by hills and ridges. A drop of rain just one side of a ridge falls into one catchment area and another just the other side of the ridge perhaps a few feet away go into a different catchment area. Those catchment areas collect the water into streams and rivers and form a 'Drainage basin' and each basin will eventually have exactly one river to the sea. So find the high points in your terrain and divide up your terrain into catchment areas based on them.
If your river enters a zone that in all directions means that it now has to go uphill then you are in a basin. Only in a basin will a lake possibly form. Lakes are a much scarcer geographical item than most fantasy maps have them. When we have a lake it can do one or more of the following :-
1. Fill up until it overflows out with another river.
2. Seep into the ground (i.e. overflow into the ground)
3. Completely evaporates. This is also very rare indeed.
If the lake overflows to another river then it is true that it flows out with exactly one river. It does not fork rivers out from a lake unless there is a temporary swell in rainfall, causing flooding. This is true because if there were two exits from a lake then the level would fall until there were just the lower one to exit the water. If you had two exits the same height then one would erode and the other would silt up. Which one is down to the chaoctic nature of the universe but in geological terms it would happen very quickly.
So you can have multiple rivers entering a lake but only one exit river. In effect a lake is a bit like a very fat bit of river.
A bit more complexity.
Rain or a river can hit an area of rock that has big cracks in it or is very permeable. In either case the water can go underground. From here it can gush out of another big crack or cave mouth. The cave mouth can be above or below the level of the surface of a lake. This is a spring.
When rain permeates through pourus ground then it will hit a water table. This is a layer of saturated ground and the water which makes it up travels towards a spring very slowly. The 'surface' of the water table might be horizontal underground or it might vary in height undulating. Where the surface ground terrain height drops away to a level below that of the water table thats where springs will occur and water will drain from the water table feeding the spring. Spring water which has permeated for a very long time in the water table is the stuff thats very pure and often safe to drink. So springs occur part way up a hill side and they are always at a very similar level - i.e. they follow the contour of the terrain. You have to dig a well to a depth to reach past the water table before the well fills with water.
So a bit of terrain comprising of a hill with a cliff on one side cant have wells on the hill. The rain falling on the hill goes into the ground or runs away. That going into the ground will fall out of a cave somewhere on, or at the bottom of the cliff.
Not many. The main one is that rivers erode terrain and pick up sediment. It can drop this sediment which then prevents the water from flowing in that direction so it much change. Thats called meandering. When the river has passed over a rapids in comparatively soft terrain it will be loaded with sediment. When it then flattens out and slows down it will dump off much of that sediment causing lots of meandering. In those situations the river can 'braid'. It splits and forks into many small rivers all flowing in approximately the same direction. In all but the most rare cases will that river rejoin and come back into one channel again tho.
joćo paulo has pointed out that the largest island where a river has forked around it is a mere 20 square kilometers.
This river is going through its very temporary meanderings to find one course through this region. One day soon one side will dry up. In any case 20 square km is very small.
When a river carries a lot of sediment it can dump it off right next to the sea. For the same reasons the river will braid but instead of rejoining, it hits the sea first. In this case it can form a delta where it spreads out into the sea. These deltas are usually small in comparison to the length of the river. The Nile delta is about 100 miles north-south but then the Nile is 4000 miles long. At the very least a river should have to pick up enough sediment so that its well loaded by the time the delta STARTS.