Issue separating points into lists based on proximity to one another

Hello all,

I am attaching a file I have been working on which is attempting to generate linear networks based on a sampling of the image provided. The idea is to use grasshopper to produce linework rather than tracing it (in this case dry riverbeds).

I've started by simply creating a dense grid of points and sampling them for brightness, and then dispatched to get here. The most logical next step would be to measure each point against every other point, and begin to find who is closest, but with the amount of points being produced, I'm hoping to find a better way.

Any ideas or help would be greatly appreciated!

Alex