Hi Elise. To achieve this in ArcGIS for a river centreline, I've used the Create Thiessen Polygons tool. If you convert the polygons to lines, then split the lines into small segments (the smaller the segment, the smoother the centre-line), then create points from the line segments - creating Thiessen polgons from these points will create a centre-line in the middle for any irregular polygon - albeit with a bit of superfluous geometry.
I had to manually edit to leave only the centre-line, but there might be a way to do this without manually editing. Which you'll need with over 2800 polys 🙂