Because of the fact that it was very Deep and heavily polluted.
Explanation:
Actual main channel depths along the river vary; with a least depth of 24' or less at a few locations at the channel edges to a maximum depth of 108' being found mid-channel at the southern approach to Hell Gate.
Due to heavy pollution, the East River is dangerous to people who fall in or attempt to swim in it, although as of mid-2007 the water was cleaner than it had been in decades.
