in neketulos and possibly and other bridge if you are levitating over it these can kill you happend to me in the middle of the bridge starting getten bitten and didnt make it in time to the other side.
They are level 5 or 6 and drop teeth. Give the teeth to the merchant next to the lake for several gold each, plus good xp. This is a great way to earn gold and xp at level 6 to 8. Plus you raise several nice factions.
the ones in rivervale are not level 10 tho.. i'm not sure but i didn't think they were large piranhas, i thought maybe just piranhas, and i think they are around level 5 or 6... been a long time so i could be wrong tho.. and there is a couple quests involving the rivervale piranha.