http://wiki.openstreetmap.org/wiki/Waterway
http://wiki.openstreetmap.org/wiki/Tag:waterway%3Dcanal
http://wiki.openstreetmap.org/wiki/Tag:waterway%3Dditch
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HB: use ditch not canal+boat=no to avoid overlap with the canal_cl layer.
Be careful to test that hydrostations' tailraces are not called ditches though!