Albert Barnes Commentary 2 Chronicles 32:30

"This same Hezekiah also stopped the upper spring of the waters of Gihon, and brought them straight down on the west side of the city of David. And Hezekiah prospered in all his works." — 2 Chronicles 32:30 (ASV)

See the note on 2 Chronicles 32:3. Either at that time or afterward, Hezekiah channeled the water of this spring through an underground tunnel down the Tyropoeon Valley to a pool or reservoir (as noted in a marginal reference).