The Catskill Aqueduct is one of three aqueduct systems that supply water to NYC. I would guess that "blow-off" refers to a valve that allows water to escape the pipeline if the pressure in the system exceeds a certain threshold.
This is the counterpoint to my walk across the US. Instead of seeing a million places for just a minute each, I'm going to spend a million minutes exploring just one place. By the time I finish walking every block of every street in all five boroughs, I'll have traveled more than 8,000 miles on foot — all within a single city. Details!
Your donations allow me to keep walking full-time. If you think what I'm doing is valuable and you'd like to offer some support, I would be very grateful. On the other hand, if you think I'm a worthless bum, feel free to email me and tell me to get a job, bozo. Both are excellent options!