Stausee Oberilzmühle, Reservoir in Salzweg, Germany.

Stausee Oberilzmühle is an artificial reservoir situated along the Ilz River near Salzweg. The water basin is surrounded by thick forest and features sandy areas for swimming and direct water access.

In 1939 the city of Passau bought Oberilzmühle to build a hydroelectric plant; from May 1942 the project passed to Arno Fischer's research institute, and prisoners from the Passau I subcamp of Mauthausen carried out the construction until the war's final days. After 1945 the Passau municipal works completed it in conventional technique between 1951 and 1955, and it has operated since 1953. In 1965 and 1966 the dam wall was raised, lifting capacity to a maximum of 500 kilowatts.

The reservoir is used for recreation: a public bathing beach lies on the west shore, while an unofficial nudist area occupies the east shore. Bathing bans in the Ilz recur because of foul sludge pollution, and in August algal blooms can sharply worsen the reservoir's water quality.

The site can be reached from central Salzweg and has parking spaces nearby. The beach areas are accessible and walkable in most seasons without special equipment.

The 2007 fish ladder on the western side lets fish bypass the 19 m high, 110 m long gravity dam. The reservoir is about 5 km long, only 40 to 50 m wide in its upper stretch and up to 200 m wide lower down, with a maximum depth of 14 m.