Not merely the tagline for a bad B-movie, it's now my cardinal rule when embarking on riverine maps.
I had found an embark spot with a confluence of rivers which carved up the map nicely and thought, "Hey...I could build a nice defensive fort using the rivers as a natural moat."
When I got there, I was wowed by the unit list...three pages of nothing but longnose gar, sturgeons, pikes and sea lampreys. "No biggie", I think. "At least there aren't any carp. Those should should be pretty docile. I'll just fish around the edges near the fort."
And so things went smoothly for the first year. And then suddenly, my Adept Miner cancels a task due to dangerous terrain. ??! Oh crap...he fell in the water. Wait, no...he's fighting a sea lamprey. Ok, he killed it. He's close to the bank, I can have him carve a ramp out! Hmm...it almost looks like those other fish are swarming him...Oh. My. God.
Chunks everywhere. A minute or two later, another lamprey pulls in my fisherman and rips his arm off. Then my carpenter gets pulled in by a longnose gar while he was just walking by with an armload of wood. He crawls for the shore, and then the gar knocks him back eight tiles. Underwater.
By this point, the rivers are pretty much red, and the survivors are seriously thinking about building a wagon to get the hell out of Coilrivers.