The FFH area, which covers 137 hectares, is located directly north of Flensburg and its western, northern and eastern outer borders border the Danish border.
It includes a distinctively formed ice age tunnel valley with the Krusau and Niehuus Lake, which are partly near-natural. It also includes the agricultural land adjacent to Niehuus Lake, the stream gorges and the lowland areas, as well as the lowland area of the Krusau estuary and the mill pond with adjacent areas in the Kupfermühle area.
The tunnel valley has a particularly diverse range of different biotopes and habitat types of flowing and still waters, grasslands and forests. The water habitats include the partially near-natural course of the Krusau with flooding vegetation (3260), the springs of the lowland and the naturally nutrient-rich Niehuus Lake (eutrophic lake, 3150). The surrounding grassland areas are characterized by semi-natural and near-natural, wet lowland biotopes and wet meadows. The lowlands north and west of Niehuus are characterized by sedge areas of varying types, but in particular the sedge (Carex acutiformis). The entire lowland area is the habitat of the moor frog. North of Kupfermühle, the former Kupfermühle pond and the lowland area adjacent to the south-west are occupied by extensive large sedge areas, moist tall herbaceous meadows (6430) and, in the slightly rising terrain, by (former) wet meadows. This part is particularly important for one of the main occurrences of the bulbous snail (Vertigo moulinsiana) in the area. Other occurrences are in the above-mentioned sedge areas and in the reed beds interspersed with giant swaths of Glyceria maxima.