"Peter R." wrote:

Where is the northern turn-around point?

There's a long island in the river. The lower end lies about even with the
lower end of Central Park. La Guardia airspace goes to the surface close to the
upper end of this island. You have to turn before you get to the end of the
island. That point is downstream from Spyten Dyvil. The last time I went past
the island, I was in a boat, and it was many years ago, but, at that time it
was basically uninhabited.

I imagine that the north end is
capped due to La Guardia's airspace, correct?

Yep.

Oh, and what's the upper
altitude limit? 1,100 feet, too?

Yep.

George Patterson
Great discoveries are not announced with "Eureka!". What's usually said is
"Hummmmm... That's interesting...."