Since Upper Annex Lake was a naturally existing lake at high elevation, engineers decided to drill a hole in the side of the lake, about 150 feet below the water surface, and use the natural lake for water storage rather than build a large dam on a stream. The Annex Creek Hydroelectric Project initially came online with water taken from behind the dam by a pipe and put into the penstock to generate power while the tunnel was being completed. The 1,481 foot-long tunnel and lake tap was completed on Feb. 14, 1916. It is believed that it was the first lake tap in North America, and the first ever to save the water rather than drain the lake.