Perfstausee, Flood retention basin in Marburg-Biedenkopf, Germany

Perfstausee is a flood retention basin located between Breidenbach and Breidenstein that covers 18 hectares during flood conditions and normally holds a water surface of 13 hectares. The facility includes dam structures and control systems that manage water flow.

Construction began in 1984 following severe February floods, and the facility opened on September 18, 1993. The project emerged from flooding events that demonstrated the need for a retention system.

The lake's name comes from the Perf, the Lahn tributary that feeds it; the reservoir is also called Hochwasserrückhaltebecken Breidenstein/Perf. The upstream Vorsperre lies in the Im Wehr nature reserve, designated in 1990 and covering 14.17 hectares, where a birdwatching stand looks over the water. On 27 August 2023 a lake festival with regional clubs marked the reservoir's 30th anniversary.

A roughly three-kilometre circular path loops the lake, crossing two metal bridges that replaced wooden ones in early summer 2024; one spans the Elsbach. Non-motorised boats are allowed, with a vending machine for stand-up paddleboards and kayaks and pedal boats from the Wassersportclub Breidenstein in summer. The boat jetty below the Seeblick restaurant was rebuilt in 2024 with three levels for changing water levels.

A 2015 European Environment Agency rating of "poor" closed the lake to swimming; research by Koblenz University of Applied Sciences from 2017 to 2020 traced the faecal bacteria to 32 stormwater overflows from treatment plants upstream. Outside rainfall the water is usually very clean, but Hessian law prevents limiting the ban to periods after rain.