FAA regs. for minimum altitudes over built-up areas

Gary Drescher wrote:
apparently, flying over the river isn't construed as flying over a congested
area.

Is there an FAA definition or example of what exactly is a "congested
area"? I had also heard of the 1000/2000 rule, so I was pretty
surprised to learn that the East River could be flown at under 1100
feet.

It seems that general aviationists are against against any kind of
restriction, and even Bloomberg used the analogy that just because we
have car accidents doesn't mean we shut down all the streets. But if
you have a fatal car accident at an uncontrolled intersection, it would
be pretty reasonable to make the intersection safer, maybe by adding a
stop sign or signal lights. The East River corridor is pretty crazy, a
narrow strip between 3 major airports. It doesn't seem like a bad idea
to shut it down.