The foundation of Flowing is a concept called a "flow field", which is a collection of vectors arranged in a grid. If you remember from multi, a vector has a magnitude and direction; here, I just used the direction.
I used a (pretty brute-force) algorithm to make sure the lines were spaced out enough and didn't touch each other. Here's what they would look like normally, with no constraints on how close they could be to each other:
If I didn't space them apart, I'd have a bunch of lines all overlapping and looking more like the watershed map of the US, which shows every river in the country:
I must give a shoutout to Max Drekker whose work on chromatic abberation inspired some of the black and white ones. It's like looking through really strong glasses or binoculars.