How to get your rivers in the right place

[Redrobes]

You would probably start with a salt marsh with light vegetation as the river feeding it dried up. Then the ground near to where the river came in would dry up leaving soil which would initially be salty but eventually would change and then more vegetation would grow on it. That vegeation would either stay pretty scrubby or turn into trees depending on how much water flow or rainfall you could get to the soil. But in essence it would turn into a normal bit of coast line alththough it would probably have an odd undulating shape to it until over time it would flatten. Either that or the land would erode back, or deposit sediments from the sea pushing the sea out depending on the sea currents. In which case the ground left behind would be a low land plain or level.

In the case in my local region of Glastonbury, Somerset, and Avalon which is part of the Somerset Levels where this has happened in actual fact the sea has come in and out several times in the last few 10,000 years and the situation is quite complex due to ice ages and thawing and changing sea levels. Around 2-3 thousand years ago it was at a stage where it was a brackish water logged plain but humans then got involved and cut channels into it to dry it up for farming. Now, large areas of it still floods in winter where farmers expect it, and abandon those fields to the water, and then almost all of it is dry in summer where its generally grass quickly shoots up and cattle put out in the summer. Somerset is called that for being the Summer Isles as the ancient people came in during the somer months only and the early name of Glastonbury in old Welsh (which predates the Latin from the Roman invasion) means the Isle of Glass because it is said that in winter the lands flood and then freeze over so its and island amongst the frozen glassy plains. So lots could go on in a place like that and the exact circumstances would drive what happens. It wouldn't be difficult to make a claim for quite a variety of outcomes for it. Geologically, 10,000 years is not a lot and things will settle down and the land will eventually go one way or the other.

[Razgriz05]

Thanks, that's a great explanation, especially with the bit about Somerset, that's something I was absolutely unaware of. I think I'll go for a scrubby vegetation in the dried estuary of the river, like something you'd see in Southern Spain, as it'll be a good fit for that sort of climate.

[Zeyami]

Very in-depth guide. Thank you for the help with plotting rivers!

[MoLAoS]

So I'm working on a fantasy strategy game where I wanted to do a lot of river based content.

The maps are procedurally generated. Below I've copied some text from a post about this issue:

Axioms is a fantasy game and as such the default map generator isn’t configured to generate a world quite like our own. Instead at the high end it generates a world roughly 10x the side of ours and it is roughly 70% land. Half the water area will be oceans, so 15%. Given a size increase of 10x for the overall surface area that means 1.5x as much square mileage.

12% of the world will consist of inner seas like the Baltic and gulfs like the one in Mexico. The Mediterranean Sea is roughly 0.5% of the surface of the world. That is roughly a third of the non-ocean salt water in the world. So those areas will cover roughly 8x as much surface area as they do here.

That leaves 3% of the surface area of the world for medium to large lakes, inland seas, and navigable rivers. This is 3x the percentage of our world that is fresh water. Which means it is 30x as much surface area covered in enclosed fresh water compared to our own.

The main purpose of this change is to reduce low quality space which limits the scale and complexity of civilizations. More great rivers, ideally in complex connected networks, more lakes, and more inner seas allows the world to open up proportionally.

There is far less wasted deep ocean. In a lot of cases the reduction in ocean market share just means sticking a big continent in the middle or having a vast and varied archipelago. For my purposes a bunch of empty ocean is low value. There is also a lot more valuable land proportionally which means more variety in states.

My question is specifically about navigable rivers. About how deep/wide generally would these rivers need to be? Is there any sort of river/heightmap algorithm that is commonly used that I can modify for map generation? Since the game uses a "province" system and not a hex or square system I don't need to get super fancy. If as noted in the post I want pretty expansive river networks compared to Earth is there anything special I can do with the heightmap to encourage that in a natural looking way? Of course as it is a fantasy game I can pretty easily do some "gods did it" stuff to fiddle heightmaps around.

[srm038]

My question is specifically about navigable rivers. About how deep/wide generally would these rivers need to be? Is there any sort of river/heightmap algorithm that is commonly used that I can modify for map generation? Since the game uses a "province" system and not a hex or square system I don't need to get super fancy. If as noted in the post I want pretty expansive river networks compared to Earth is there anything special I can do with the heightmap to encourage that in a natural looking way? Of course as it is a fantasy game I can pretty easily do some "gods did it" stuff to fiddle heightmaps around.

I worked on this a while ago as a WIP attempt: https://forhinhexes.blogspot.com/201...own-river.html

Essentially (and especially in a procgen) you can correlate the drainage area and local slope with the width, depth, and velocity of your river. That can then give you an idea of how navigable they are by considering the types of river vehicles available (flat-bottom barges vs ships etc).

[tartopom]

That was a helpful tutorial. Never thought I'd have to think about how water flows from mountains before making rivers.

But it's completely logic. I'm going to need my to revise my geography and geology in order to have natural placements.