Ulmtalsperre, Reservoir in the Westerwald, Greifenstein, Germany

The Ulmtalsperre is a reservoir in the Lahn-Dill district of Hesse, impounded by an earth-fill dam on the Ulmbach stream. Its water surface is 570 metres long and 90 to 180 metres wide, covering 6.7 hectares, and the dam holds 740,000 cubic metres, or up to 820,000 cubic metres depending on the water level.

The reservoir was created by the Ulmbachverband, the water association that still operates it, to protect the Lahn from flooding. Its barrier is an earth-fill dam. In 2006 the reservoir was drained completely for a modernisation, and in 2012 the dam itself was repaired. Each winter the water is drawn down so the dam can take the Westerwald snowmelt in spring.

The name derives from the Ulmbach stream and the Ulmtal valley; the dam is also called Ulmtalsperre or Hochwasserrückhaltebecken Ulmbach. The reservoir is a local recreation area run by the Ulmbachverband, with a camping site, a bathing beach with a Liegewiese watched by the DLRG at weekends, a kiosk and restaurant, a children's playground and a bird sanctuary.

The shore is shallow only in the bathing area; elsewhere it is steep and overgrown, so bathers stay in the supervised section, where the DLRG watches on weekends. The water is drained in winter so the dam can take the Westerwald's spring meltwater. Parking spaces and a children's playground are on site.

The area is home to many waterfowl and other wildlife, making it a popular spot for birdwatchers. The land around the water remains mostly wild with tall grasses and trees growing along the edges.