@Damien: Glad you like Natural Earth!

ScaleRank is a curated list of rivers that are regionally important at each “zoom” of the map. When the map is zoomed in more, more rivers should show. These integer values allow you to quickly throttle the amount of content visible. They are partly based on river discharge, but moderated a good bit to make sure there is balanced coverage.

StrokeWeight is a curated appearance guide for how thick to make the river stroke in points. River mouths should be thicker than their headwaters. Bigger rivers (more discharge) should have thicker mouths than minor rivers.