These images of Irvine Lake both come from Tom Pulley. The first is from a card postmarked 1948, and the second is undated.
According to the Irvine Ranch Water District: "Irvine Lake (formerly known as Santiago Reservoir) was formed by the Santiago Dam, built in 1931. It was constructed by The Irvine Company and [the] Serrano Irrigation District (today know as the Serrano Water District). The lake was opened to the public for fishing in 1941. ...The lake stores approximately 28,000 acre feet of water. Originally the lake served the agricultural and farming communities surrounding Irvine Lake. Today the Serrano Water District provides drinking water to Villa Park and some parts of Orange. Irvine Ranch Water District uses untreated water from Irvine Lake to supplement its irrigation water only. IRWD is a partial owner of the lake, but Serrano Water District runs Irvine Lake and its concessions."