It's a 60 mile long tunnel and in order for water to flow through it, you need either pumps or a downhill gradient.
I'd guess the reason for the 800 ft is because the reservoir it'll draw from is near sea level.
I'd guess the reason for the 800 ft is because the reservoir it'll draw from is near sea level.
Maybe instead it needs to pass under the rivers [2: cross-section] surrounding New-York, which may be much deeper, especially when it comes closer to the bay passing Queens and Brooklyn [2: map]
1. https://en.wikipedia.org/wiki/Mississippi_River
2. https://gordonsurbanmorphology.wordpress.com/2014/10/26/wate...
(The Army Corps of Engineers has great detailed depth surveys for most of NY's waterways[1].)
Edit: There's also a higher-resolution render of the tunnel layout here[2].
[1]: https://www.nan.usace.army.mil/Missions/Navigation/Controlli...
[2]: https://extapps.dec.ny.gov/docs/water_pdf/nycsystem.pdf
I believe Tunnel #3 connects to the Catskill Aqueduct[1], which draws from the Schoharie and Ashokan reservoirs. Both are at least a few hundred feet above sea level (the Ashokan is about 600 feet above, since it was formed by flooding a valley in the Catskills).
But I have no idea why they dug it so deep, given that! Maybe to give themselves an (extremely) ample buffer for any future infrastructure in Manhattan.
One diagram I saw indicated 2 different layers of bedrock. I didn't find anything real clear, but it can be that the lower layer is a more suitable material for the tunnel.