dolphins which are not smart at all. They are sturdy "boles" tarred black, driven deep into the bed of the canal near bank

Such dolphins are all along the Fraser River, too, for tying log booms to. The interesting thing is the origin of that use of the word. I read somewhere it had to do with Delphi, and the port there being the first port to make use of dolphins for tying up boats. (I'm sure it's Googleable. )