How Many People Would Have To Die?

[JuliaL]

Each human has, what, 6 liters or so of blood? Let's say this is 1 cubic foot (guessing based on the size of 2-liter and gallon bottles).

Better guess-

6 liter =  0.212 ft^3

I think you have to multiply the number of people by about 5.
Say, 200 million?

Edited for further absurdity:
http://everything2.com/title/Tigris

In contrast, at the city of Amara, the Tigris is only 600 feet wide and the depth varies anywhere between 6 ½ to 13 feet. It is also at Amara where the marshlands drain off a lot of the river and it narrows to a width of only 200 or so feet.

https://en.wikipedia.org/wiki/Tigris

Discharge

for Baghdad
 - average
1,014 m3/s (35,809 cu ft/s)

So, using the Tigris as the example of river width, say 450ft,  going from zero ft to 4 ft back to zero ft or an area of 4x300=1200 ft^2 and 200mi long for a volume of 3.6*10^10 liters representing 6 billion people.
Flowing at 20 mi/hr.
One heck of a wine press to keep this going.
At least there'd still be a billion people left.