On Hatching-tide: Shell-Shocked (04/11/2012) Topic you can read the following:

As any forestborn will know, turtles are not native to the Twelveswood. And so it was all the Wailer could do to cry “Turtles!” before staring, shell-shocked, as the little critters continued their leisurely parade, taking their leave of Gridania by way of Black Tree Brook.

On the other hand, in-game map displays Black Tea Brook for the name of main river at Gridania, as you can see right on this screenshot:



Grab your erasers!