Katherine Rinne, "The secret life of Roman fountains", in Places 12:2 (1999), 76-81.
Roman fountains, at least until the twentieth century, were fed by a vast yet simple aqueduct system that exploited the natural law of gravity to distribute water. Water flowed freely within the aqueduct channel, but once it reached the city, it was constricted in underground pipes that created the necessary pressure for distribution. Unlike mechanical systems that force water into unnatural contortions, a gravity system nurtures, exploits and enhances water's natural abilities as it flows through its "watershed".
Three pre-industrial, gravity-driven aqueducts still operate in Rome today: the Acqua Vergine, Acqua Felice and Acqua Paola. Each is based on an ancient aqeuduct - using the same water source, following the same route into the city, and in some cases reusing portions of the original aqueduct channel. Each water system occupies its own "watershed", that is a bounded topographic area, outside of which the water cannot flow. This article demonstrates how the the design of the public fountains is determined in part by its relation to its water source.
Madonna dei Monti Fountain, Giacomo Della Porta (1589), Copyright, Katherine W. Rinne, 2001
Aquae Urbis Romae: the Waters of the City of Rome