How to get your rivers in the right place

[Realmwright]

All very good points. I remember only a little from jr high Earth Science, but I remember lots from college geography courses. The bottom line is that, as our illustrious post leader pointed out, water must flow downhill and/or gather in lowland areas. It is very good advice to figure your terrain features and mountains BEFORE placing rivers because I tend to think you only get a say in one or the other. Mountains tell you where rivers can be, and vice versa. As for the bit about water tables and springs, it makes sense, but it's also pretty deep (no pun intended). If most other mappers are like me they get a blank outline and then stare at it dumbfounded about what to do next. Great, I have a shape of the landmass......now what? Figuring out the surface terrain is enough of a task without trying to incorporate soil density.

[Caenwyr]

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.

Hi Redrobe! I'm still pretty new on this forum, but I'm working my way through as many topics as possible. I just came to this tutorial and I must say I really like it! As a geographer I can confirm every bit of information you give here. When it comes to the quoted post though, I was wondering if perhaps you could tell me the software you used for this simulation. I would be very interested indeed! Especially since I'm looking for a "natural" way to draw rivers in my map.

Many thanks in advance!

[Redrobes]

Hi Caenwyr, that software was my own and its unreleased. Its a total PITA to use and it has no GUI so it would be a problem releasing it. However, there is Wilbur which is written by another member here and the author of Fractal Terrains. Wilbur is free and downloadable and also calculates river path predictions based on similar criteria to my app simulation. So check this link out:

http://www.fracterra.com/software.html

[Midgardsormr]

I found an interesting factoid that probably only the most ardent of us will apply: The ratio between the length of a meandering river and the length of a line drawn from its source to its mouth approximates pi (3.14159…).

Pi -- from Wolfram MathWorld

[RobA]

I found an interesting factoid

Sounds more like a "fractoid"

-Rob (not very punny) A>

[Gumboot]

The alluvial plain near a lowland river tends to be very fertile because the soil around it is regularly exchanged. While the "Fertile Crescent" eventually wore out and suffered from desertification due to overfarming, the Nile's flood plain, particularly the delta area, remained fruitful through thousands of years of continuous farming. The primary difference was that the Nile floods regularly, which refreshes the nutrients in the soil. The Tigris and Euphrates flood plains have remained fertile, but not to the same extent because their flooding is less reliable. The rest of the land in that area was merely irrigated from the rivers and did not receive the benefit of the floods. As a result, the nutrients were depleted, salt levels rose, and the desert claimed what was once very good farmland.

More obviously, of course, vegetation tends to be thicker around rivers because that's where the water is! In the American Great Plains (where I grew up), the tall grasses will grow everywhere, but you only naturally find trees near the rivers because the rest of the land just doesn't have enough moisture to support them. Trees planted far from the rivers tend to be rather sickly, and they often fall down during windstorms. Typically on someone's car.

You're quite right, although trees don't grow where rivers are, as such. Rather both trees and rivers tend to end up in areas of high rainfall. Water in a river isn't much use to a tree, because trees don't grow in rivers. But rivers tend to flow where there's more rainfall. Trees get their water mostly from rainwater falling on the ground where they're standing. The water that isn't absorbed by trees ends up in rivers.

Generally speaking:

Higher rainfall = more trees + more rivers
Lower rainfall = less trees + less rivers

[Hai-Etlik]

You're quite right, although trees don't grow where rivers are, as such. Rather both trees and rivers tend to end up in areas of high rainfall. Water in a river isn't much use to a tree, because trees don't grow in rivers. But rivers tend to flow where there's more rainfall. Trees get their water mostly from rainwater falling on the ground where they're standing. The water that isn't absorbed by trees ends up in rivers.

Generally speaking:

Higher rainfall = more trees + more rivers
Lower rainfall = less trees + less rivers

Actually on the great plains it is the rivers directly allowing the trees to grow, they aggregate the water, and bring in water from rainier areas, which allows the trees to grow in the river valleys despite not having any more rain than the surrounding plains.

[mssandhu]

Good tut! I do have a question - I have been struggling to work how to get the same look to rivers that you find in any good atlas - they start thin, meander in very 'random' line pattern and join up to get wider until finally ending in a delta or an estuary. When I try, the rivers obviously come out looking overly smooth sided, and they do not start with near invisible thin streams that eventually coalesce to form a river. Any ideas please? (PS I have only just started using GIMP 2.8 - never realised until friends told me that there was anything better than Microsoft Paint!)

[Korash]

You might want to look at one of RobA's scripts here. I am not sure how it fits into 2.8 but might be worth a try. Let us know how it works out.

[Midgardsormr]

I had a thought the other day concerning the interface between rivers and seas, so I thought I'd bring it up here. We have discussed delta formation in fairly great depth, but there hasn't been a more general conversation about how rivers affect coastlines.

To start the discussion off, we know that as a river nears the sea and loses momentum that it can drop silt, which is what causes a delta to form. Usually, the delta will extend the coastline outward, as can be seen around the Mississippi Delta and the Nile. In contrast, other rivers may form an estuary, where the river erodes the coastline away, allowing the sea to move further and further inland. Because the estuary is filled with a mixture of seawater and river water, it usually has lower salinity than the ocean, but higher than the river. Many deltas also have this feature, and in both cases, the water level and level of salinity may be dependent on the tide. In some cases, a river ending in an estuary may actually run backward at high tide, as pressure from the narrowing channel temporarily forces the water to run uphill.

Are there any other interesting coastal formations caused by rivers? Have I got anything wrong there?