Oberalsterniederung, Protected area in Henstedt-Ulzburg, Germany.

Oberalsterniederung is a 907-hectare nature reserve in the Segeberg and Stormarn districts of Schleswig-Holstein, protecting part of the roughly 2,500-hectare lowland of the upper Alster and its tributary the Rönne. It stretches along the Alster from its source near Henstedt-Rhen to the B 432 south of Nahe, and along the Rönne from the former Elmshorn–Barmstedt–Oldesloe railway to its mouth. The open lowland holds wet meadows, fallow and extensively used grassland, alder woods, three former raised bogs and boundary hedges.

The lowland formed at the end of the Weichselian glaciation from an extensive ice-dammed lake in which water draining eastward was held back at the glacier front. Over millennia the lake silted up, creating a wide wetland of fens, carr woodland and swamp. Three raised bogs later developed: the Alsterquellmoor, the Schlappenmoor and the Wakendorfer Moor. From the early Middle Ages the area was settled and became a farmed cultural landscape.

The reserve covers part of the roughly 2,500-hectare lowland of the upper Alster and its tributary the Rönne, which joins the Alster within the protected area. Settlement from the early Middle Ages created a farmed landscape of grassland, arable fields, bog remnants, alder carr and hedgerows along the edges. The meadows are periodically flooded and are mostly wet grassland, some of it fallow or grazed only occasionally.

Several paths run throughout the protected area, allowing exploration on foot or by bicycle with information boards at various points along the routes. The area is accessible year-round, though paths can be muddy in spring and after rain, so appropriate footwear is recommended.

The Schlappenmoor, a former nature reserve designated in July 1971, was absorbed into the Oberalsterniederung when the larger reserve was established in 2004. Its wet meadow holds numerous orchids, and the reserve shelters birds such as the great curlew, snipe, lapwing and corncrake.