How do I lay river around the city?

I've used the Venetian building which popped up as a "theme"

It lets you build canals which you drag out from a starting point like the roads. The building is 3x4 + 1 water square sticking out so I was lucky I had enough room to move it around. If you drag a canal out it stays behind when you pick the building up & put it somewhere else. The canals are manipulated using the road building menu BUT they are not roads so buildings still need to be... next to a road.