Thread: How to get your rivers in the right place

Ok so far this has been said before so I will show an example of what this means and hope that the example will help people get their rivers is better shape.

I want to make a terrain. To start with we need the terrain height so I will draw on some colored bands to show where the high areas will be.

If I mark on this map where the catchment areas will be and the direction of water within them then you will see that it will flow towards the middle where there will be some uncertainty about what it would do next.

In my example there are two exits off of the map where water could reach the sea. I am going to show later that water will chose just one of these. If you put down your rivers so that they followed the blue lines then that would be good. Here is a diagram with the blue lines perpendicular to the contour lines. Our rivers should be on these lines somewhere. What were expecting is that a lake with rivers should form in the thick blue section. If one side of the map had an inflow of water then it would run into the lake and out of the other side. If there was no inflow then the lake would drain to one side only because you can only have one exit. What happens to the river bottom right depends on whats past the bottom right side but we should get something here because all the water within that catchment area has to go somewhere.

So providing you come up with something that fits the bill then it would be alright.

I have run a simulation and got it to predict where the rivers would be based on some rainfall. I set it up so that either side of the map was an exit for water. The result is below. The middle part of the map started as a lake and then drained to one side. The middle of the map is an area of confusion so almost anything could have happened and it would have been reasonable.

What was fixed tho is that all the rivers follow the blue lines meaning that they are perpendicular to the contour lines and thus running downhill as fast as it can. Also no rivers crossed the pink lines of the catchment area borders because those lines determine which way water flows. Thats how we can predict where the rivers should be on a map without having to resort to a simulation.

I am attaching a movie in MPG format to show the simulation running. I hope this will show whats going on and why it was fairly easy to predict where the rivers would have been from the height terrain. Though a lake forms in the middle it is unsustainable and one side won out - in this case the right hand side.

Very interesting stuff - I had no idea about most of this, but it all follows and makes sense when one thinks about it.

I have a couple of questions:

- It looks to me as if the lake would drain to the right in your example anyway, because it looks like the terrain rises up to the yellows/oranges just off the the left of the map. So the fact that it does do that makes sense to me. I'm not sure what the river in the bottom-left quadrant is doing though, it seems to fill into the lake and then the lake retreats from it into the top half of your map and then drains to the right? So where is the river water going from the bottom left while it's doing that, is it just seeping into the ground? (also, what software are you using to do the simulation?)

- One thing I've found curious in the real world is northern canada - if you go look at it on google maps ( http://maps.google.com/ ) you'll see there that a huge swathe of the country around Hudson Bay, is covered with small lakes (you can see this in the map mode really clearly). Where do all those come from? It looks to me like the water table is above the land surface here, is that right? Or is something else going on? (e.g. permafrost in the ground is having an effect?). Actually looking at it, northern Ontario nearest Hudson Bay is actually lacking these small lakes (there are several rivers going to HB instead), and the same goes for southwest of HB, but to the east and west and northwest of HB we're back to lots of lakes. I suspect the geology has an effect here (is the precambrian shield that makes up a lot of canada impermeable, maybe?). That may be one thing to explain here too - the effect of permeable vs impermeable rock that the water is going over.

Thanks for doing this anyway, really interesting stuff!

Originally Posted by Redrobes

2. Rivers originally start with water which has fallen from rain.

A river can also be the result of melt water from mountain snowpacks. In fact that is where most of the water comes from where I live (central oregon). When you have a river like this you get high and low points. The highest points usually come during spring and early summer when the snowpack starts melting. Late summer and into fall it tapers off, getting to its lowest in the winter. Winter replenishes the snowpack and the cycle starts over again.

Good look at how rivers form though, very helpful (have some rep).

Being Canadian I have to question your view on lakes being more rare than they are usually presented in Fantasy. Honestly most maps I've seen tend to lack lakes in a number I would expect when placed in a northern region that would be subjected to glacier action.

Originally Posted by Talroth

Being Canadian I have to question your view on lakes being more rare than they are usually presented in Fantasy. Honestly most maps I've seen tend to lack lakes in a number I would expect when placed in a northern region that would be subjected to glacier action.

Yep, some areas are covered in lakes. If you've flown over southern Louisiana and SE Texas, you know what I mean. Here's an example from google maps.

EDG:
The terrain might raise up to the left but there was a single flat base level which went across the middle of the map to either side. It could have gone either way. It would have been reasonable to have a river going east to west or west to east.

During the sim if the water height is not very much then it does not show so if the water spreads out then it can vanish from the view. I set it not to permeate or evaporate and its running on my GeoTerSys app. There is in fact rain and water all over the map just not very much of it. The app is not ideal either. Its not without problems and differences with the real world. It is objective however, so it can come up with an answer which lacks my bias.

Talroth, Asharad:
Perhaps there are more lakes than I thought. It was a personal observation not really steeped in fact. Our world has some crazy places on it which defies all sensible mapping. Heres a good one...

http://maps.google.co.uk/maps?hl=en&...37708&t=h&z=11

It seems that there is a band across the world from Canada, Norway, Finland, and right across Russia in a small set of latitudes which do have a tremendous amount of lakes. Below that line they get a lot less numerous. But best ignore that statement - some places do have a lot of lakes. I dont know why this is or whether it has anything to do with water tables. I would doubt it. I wonder because of the latitudes that it might be because for some part of the year lots of water is locked up as snow and ice and then later it all thaws into lakes and then refreezes again. Maybe that climate precludes long rivers because theres no continuous steady water flow.

Nomadic:
Sure, should have said rain and snow. It was basically that water has to gain the energy to rise up in height before losing it all as it falls back to the sea.

I am not much of a geographer but have been wrestling with some of these things trying to model them in water flow to try and get the results looking natural. We could do with more expertise with geographers and geologists on this site.