When you were 'sent up the river', you were sent to Newgate Prison. They took you from the courthouse down on Wall Street, put you in a boat and sailed you up the Hudson River to the Christopher Street docks. One block north stood the prison.
Built in 1797, Newgate was the first State Penitentiary in New York. It was considered a model of reform at the time, rehabilitating inmates by teaching them useful trades, and even allowing them to bathe in an indoor pool.
But the Manhattan Island developed much faster than anyone thought possible - as it would continue to do for another century - and within 20 years, the farmland that once surrounded Newgate had been completely developed.
It was no longer possible to march prisoners over to Washington Square to hang them from the Hanging Elm, nor could they be buried there, since the square had become a parade ground.