Thread: Huge River Paradox

Originally Posted by Ceru

Several dark lines flow from both sides of the mountain and instead of coming together to form a valley or something, they flow north, creating a huge river up there and ending at the west coast. I know and read that all rivers flow downhill hill, but for this map in particular, the smaller rivers are flowing north instead of south, which makes no sense (since gravity pulls everything down, I'm assuming most rivers would flow south because 'south' is 'down') so, I just can't imagain it going up north.

The pull of gravity on a planet is the sum of all its particles causing a pull on another object, including those in the northern hemisphere. The direction of the pull of gravity is towards the center of the planet. All the stuff on the planet is concentrated around an abstract point towards which they are all pulled, and because a planet is round, that point is at the center of the globe. North and south are defined by the rotation of the planet, not its gravity. Or the orbit of the planet around its star or the rotation of its star etc. north and south are arbitrary.

Basically, to gravity, it does not matter where on the globe you are, rivers always flow downhill, towards the center of the Earth.

Here's a map of the Earth I made using GIMP. It's still a bit rough, but I think it includes all the major features.

Originally Posted by Wikipedia

The Nile [...] is a major north-flowing river in Africa, generally regarded as the longest river in the world.

- Wikipedia article on The Nile. Some major rivers actually flow from south to north; it's not that uncommon.

and isomage, I love that 'map'!!

-asp

Originally Posted by Naeddyr

The pull of gravity on a planet is the sum of all its particles causing a pull on another object, including those in the northern hemisphere. The direction of the pull of gravity is towards the center of the planet. All the stuff on the planet is concentrated around an abstract point towards which they are all pulled, and because a planet is round, that point is at the center of the globe. North and south are defined by the rotation of the planet, not its gravity. Or the orbit of the planet around its star or the rotation of its star etc. north and south are arbitrary.

Basically, to gravity, it does not matter where on the globe you are, rivers always flow downhill, towards the center of the Earth.

Okay, thanks, got it. That makes everything easier to comprehend, but with the way the waters are flowing, it's not flowing to the center of the world. It's going north and then west. Is that possible?

And thank you altasilvapuer and Isomage for the 'duh' moment. But, I could've sworn the Nile was the only river in the world that flowed from south to north (it was either from a fact sheet or from a teacher -those or I'm wrong again).

Completely forget about rivers and compass directions. The two things are not related. Instead, think about rivers and mountains. Rivers always flow away from the mountain peaks. If they run from one range of mountains toward another, they'll curve away from the second range, much like the rivers in the Midwest United States do. They form downhill from the Rocky Mountains and flow eastward until the land starts to rise headed for the Appalachians, then they curl south toward the Caribbean Sea.

Rivers that start in the northern part of the Black Hills, though, such as North Dakota's Red River, will flow northward into Canada, eventually reaching the Hudson Bay.

On Earth, rivers that flow northward are relatively rare, but they do exist. Most are relatively short, but aside from the 4000 mile long Nile, there are three in Russia over 2000 miles (the Lena, Ob, and Yenissey).

Possibly the most well known river other than the Nile that flows to the north is the Niagara River, though it is only 40 miles long.

Thanks, I'll keep that in mind when I edit my map again.

Originally Posted by Ceru

Okay, thanks, got it. That makes everything easier to comprehend, but with the way the waters are flowing, it's not flowing to the center of the world. It's going north and then west. Is that possible?

I just want to make sure: you do realise that when I refer to the center of the planet, I mean the point underground half between Cartoon America and Cartoon China, right? Not a place on the surface, but a point at the middle of the globe.

The reason that you did not get the expected river flow is that there is an outlet to the sea that is lower than the ridge between the bottom and top areas of the map. The attached file shows the the too-high area in red and the too-low area in yellow. If you lower the ridge area and/or raise the low area then your river will flow as desired.

Another thing you can try is to paint in the desired river channel in Wilbur using a small brush at a fairly low level. Wilbur uses an algorithm that always follows the path of steepest descent and so will always generate plausible river channels by following the terrain. If you give it a desired channel that's lower than the surrounding terrain and connects to your sink (the lowest points or edges of your map) then the river network will follow that path.

A point of confusion that might need clearing: north, south, east and west correspond to top, bottom, right and left on the images. Up on the map corresponds to out of the screen (lighter pixel values) while down corresponds to into the screen (darker pixel values). I mention this only because it's a fairly common difficulty that people have had when I've explained it before.