By what criterion is it decided which rivers are shown at which zoom level?

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1

Is it width? And if it is width, what width is shown at what zoom level? (The question is about the map at https://www.openstreetmap.org.)

2

Hello and welcome to the OpenStreetMap Community!

On the OSM default (Carto) style, rivers aren’t shown by real-world width but by tags, zoom level, and on-screen size:

  • Line rivers (waterway=river): shown from z8 upward, line width increases with zoom.
  • Riverbank/water polygons: drawn at all zooms, but labels appear only if the polygon covers enough screen pixels (way_pixels). That’s why wide rivers get names earlier.
  • Streams/ditches/drains: hidden until z14–15 to avoid clutter.
  • Labels: rivers/canals from z13, streams/drains from z15.

So there’s no fixed “X meters wide = shown at Y zoom” rule - it’s purely tag- and zoom-based, with polygon size thresholds for labels.
See also: openstreetmap-carto/style/water.mss at b7e0691161ba00671f9cedd0f957dd496c911590 · gravitystorm/openstreetmap-carto · GitHub

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