How does Mapnik draw rivers?

Here we see the river danube, drawn as a blue area:


The river danube however is just a way, a line going in the middle of the blue area. How does Mapnik know that the whole area of the danub shall be drawn blue?

thank you

Hi snupo,

if you have a look at any position of the river you see, that there is also the tag “waterway = riverbank”.

Thats why its rendered as area.

Welcome to the forum!

bingo, learned a lot, thank you so much, danke!