![0002:3101: Hoover Dam canyon wall outlet works in operation, January 24, 1958 ["Shown is needle valve discharge occurring during the Gibson tests on generating Unit A9. This view was taken from the Arizona side of the central section roof at the base of the dam."
[USBR photo no. P45-301-10789-3A; R.C. Middleton, photographer]](https://d8e7jbdw4fu0e.cloudfront.net/4452/287bfce0-d406-11ed-9523-d9c58254ed2b-wxTv2BY.lg-nt@2x.jpg)
Hoover Dam canyon wall outlet works in operation, January 24, 1958 ["Shown is needle valve discharge occurring during the Gibson tests on generating Unit A9. This view was taken from the Arizona side of the central section roof at the base of the dam." [USBR photo no. P45-301-10789-3A; R.C. Middleton, photographer]