How and where are aqueducts built?

Aqueducts are built in areas where you have a bunch of motivated end users (like a town or group of farmers) at a low elevation in need of a more reliable source of water located somewhere fairly nearby at a higher elevation. The aqueduct builders construct a series of canals, elevated channels, and tunnels as required to get the water from the source to the end users. Some good examples:
a) Roman engineers built aqueducts throughout Italy and France from mountain water sources to serve city dwellers
b) Water-needy Southern California cities and farms are served by an aqueduct that brings them water from sources in Northern California
c) New York City is supplied by an aqueduct and tunnel system from sources upstate.
d) Inca farmers in coastal valleys built irrigation aqueducts from sources higher up in the Rockies
e) Native American cultures in Phoenix area built irrigation canal systems that diverted water from sources at higher elevations to irrigate their crops.