Reservoir of Riga Hydropower station (Riga Hydroelectric Power Plant). The Riga Hydroelectric Power Plant was put into operation in 1974. In order to build Riga HES, a dam was constructed across the Daugava River through the middle of Doles Sala, half of which has since been flooded to make room for Riga Reservoir. Along with Doles Sala, there have been several other smaller islands drowned in order to fill the reservoir. The dam was built in the late 1970s.