Ha! I was looking at old maps today too. Only back to the 1800s though. That bit was labelled "Mill Lade" back then. Nearer the Cromarty Firth it was labelled "The Canal". Anyway, this is an important farming area. I'm sure these are still used for drainage but possibly also for irrigation. Various dictionary definitions for Canal and the OSM Wiki state "to carry useful water...". I'm inclined to leave it as "waterway=canal". I associate a canal with ships, boats, towpaths, etc. but apparently it can be much smaller. Mark this as resolved? What do you think?