The best running trails around Hemmelsdorfer Lake

According to the European Water Framework Directive (WFD), Lake Hemmelsdorf is in an unsatisfactory ecological state. The lake receives heavily to very heavily nutrient-polluted water from the larger tributaries of the Mühlenau, Thuraubek and Spannau as well as other tributaries. The visible consequences in summer are often blue-green algae and foam formation on the banks. In terms of water quality, Lake Hemmelsdorf is one of the most endangered lakes in Schleswig-Holstein.

The fish population in Lake Hemmelsdorf includes perch, roach, bream, ruffe, zander, pike, eel, carp, and, to a lesser extent, rudd, minnow, and tench.[5] The northern part of the lake and the adjacent Aalbeek lowland were declared a nature reserve "Aalbeek Lowland" on 31 December 1984[6] and a European Bird Sanctuary in 2000, covering an area of 310 hectares.

Breeding birds at Lake Hemmelsdorf include the mute swan, greylag goose, gadwall, coot and great crested grebe. During migration, the little gull, osprey, black tern, great egret, tufted duck and pochard are present. The kingfisher and the white-tailed eagle are regular feeding visitors. The marsh harrier, reed warbler, great reed warbler, bluethroat, penduline tit and bearded reed can be found in the reeds.

Lake Hemmelsdorf is divided into two completely different basins in the area of the so-called Möveninsel. The larger northern basin is flat and four meters deep (four meters below sea level). The smaller southern basin is a large cryptodepression (the result of ice age glaciation). On August 31, 2007, the HafenCity University of Hamburg carried out a survey in this southern part of Lake Hemmelsdorf using sonar technology (side scan sonar, subbottom sonar and multibeam sonar) to determine the deepest point on land in Germany. The deepest point is 39.5 m and was marked with a buoy.

With a catchment area of 36.4 km², Lake Hemmelsdorf has numerous small tributaries, including the Aalbeck, which drains Lake Ruppersdorf, and the Mühlenau, which drains the Överdiek.

The lake was formed after the last ice age (the Weichselian glaciation), when the channel carved out by glaciers connected to the Baltic Sea and the Hemmelsförde was formed. This fjord was subsequently cut off from the Baltic Sea by the sand masses eroded from the Brodten shore by the Baltic Sea and moved westward by the current, thus forming the lake we see today.

In 2013, a 0.5 km long riverside path was created in the Hemmelsdorf district to allow direct observation of the riverside vegetation. The footbridge was built entirely from oak wood to blend in with the landscape. The oak posts on which the footbridge is founded were not stripped to maintain their durability. Other posts were prepared in different ways to observe the course of their decay.

The Hemmelsdorfer See in Schleswig-Holstein is located north of Lübeck in the immediate vicinity of the Baltic Sea in the area of the communities of Ratekau and Timmendorfer Strand and was formerly a fjord.