Happy Valley Reservoir was created in 1897.
Happy Valley Shanghai was created in 2009.
Happy Valley Beijing was created in 2006.
Happy Valley AA was created in 1950.
Happy Valley-Goose Bay was created in 1941.
A Romance of Happy Valley was created on 1919-01-26.
Near Valley Village, California, the primary reservoir is the Pacoima Reservoir, located in the nearby San Fernando Valley. This reservoir is part of the Los Angeles Department of Water and Power's water supply system and is used for water storage and recreational activities. Additionally, the Tujunga Reservoir is also situated not too far from Valley Village.
If it is the name then it is proper and should be capitalized Happy Valley but if you are saying, I saw a happy valley as in the valley was happy then it is common.
Lake Chabot
Glenmore Reservoir was created in 1932.
Toorourrong Reservoir was created in 1885.
Waitakere Reservoir was created in 1910.
Hisehope Reservoir was created in 1906.