The Orange-Fish Tunnel
The 50-mile-long Orange-Fish Tunnel was built in the late 1960s / early 1970s to carry water from the Orange River at the Hendrik Verwoerd / Gariep Dam (Inlet Tower at 30°41'26.00"S 25°45'46.78"E) to the Great Fish River / Sundays River catchment for irrigation.
The gradient is 1:2000, and there is not a lot of energy left when it gets to Teebus (Outlet at 31°25'22.53"S 25°38'13.53"E). There *is* a small HEP station there, but the tunnel was built because the dams in the Great Fish River valley and the Sundays River valley were all silting up, leaving no storage for irrigating the enormous areas of productive farmland.
The tunnel is the longest continuous enclosed aqueduct in the southern hemisphere and the third-longest water supply tunnel in the world, see http://en.wikipedia.org/wiki/Orange%E2%80%93Fish_River_Tunnel
You will find various documents and photographs in the Library - click on that Tab on the left of the screen.