In Hasseler forest, between the Elbe and the city limits to Hilden, lies the nature reserve Dreiecksweiher. The former gravel pit is today a valuable refuge for waterfowl. Since 1984, the area is under protection and today covers 85 hectares.
Until 1949 gravel was dredged here. This valuable raw material for the construction industry is still mined today in the adjacent gravel pits. The protected dredged pits filled with groundwater and over time, nature took possession of the land again. The steep riparian zones created during the excavation were flattened as part of the renaturation so that aquatic plants and animals could settle.
Especially for ornithological reasons, the area has been designated as a nature reserve. With its sandy-gravelly shorelines, the water surfaces and the surrounding forest, the area forms a valuable alternative habitat for former biotopes on the Rhine.
From specially created viewing points, visitors can observe grebes, coots, gray geese or an impressive gray heron colony with up to 20 breeding pairs. A stately heron colony has settled here, but also rare ducks, geese and cormorants use the Dreiecksweiher and the headlands as a retreat area.