Berlin lies between the two large water systems of the Elbe and Oder rivers. The most important natural watercourses in the Berlin area are the Spree and Havel rivers. Other natural watercourses are the Dahme, Straussberger Mühlenfliess, Fredersdorfer Fliess, Neuenhagener Mühlenfliess, Wuhle, Panke and Tegeler Fliess.
There are many man-made running waters – the canals. The most important canals in Berlin are the Teltowkanal (canal), the Landwehr canal, the Berlin-Spandau shipping canal, and the Hohenzollern canal.
The Spree river is especially important for the quality of waters flowing through Berlin. Berlin canals are fed primarily by water from the Spree river; its quality is decisive. The Spree river has much larger flow amounts than the Oberhavel (Upper Havel) river. Spree river water qualities are decisive for qualities of the Havel river below their point of confluence. Water qualities of the ‘urban’ Spree (flow within the city) are marked by the many small influxes of other waters.
The Spree river takes only a modest lower position in the ranks of German rivers. The Oder river has a long-term average outflow at Hohensaaten-Finow of 543 m3/s. The Elbe river long-term average outflow at Barby is 558 m3/s. The Spree and Havel together (converging at the Unterhavel) have an outflow 10 times smaller.
Discharges / Cooling Water
High loads in the Spree and Havel rivers become clear in a comparison of annual outflow and the sum of inflows it contains. The yearly sum of inflows in the Berlin area amounts to 400 million m3 (without rainwater in the separate sewer system). The mean annual outflow sum of the Spree and Oberhavel is estimated at 1.73 billion m3. Thus 1/4 of flow is composed of inflow water. About 3/4 of this inflow water comes from large public sewage treatment plants.
Cooling water withdrawals by thermoelectric power plants and industry are much greater than inflow volumes. Water withdrawals from surface waters in West Berlin alone average a total of about 1.3 billion m3 annually. The cooling water demand in dry years is greater than the total water content of the Spree itself.
This situation can intensify in view of increasing industrial development in the growing Berlin metropolitan area, for a long-term drop in flow amounts of the Spree is to be reckoned with. The influx of mine drainage water from brown coal surface mines in the middle Spree area has raised water availability in the lower Spree considerably, compared to natural amounts. Increasing reduction of brown coal mining will lead to lower amounts of outflow for the Spree.
The main problem for waters in and around Berlin is the increasing accumulation of plant nutrients, especially nitrogen and phosphorus compounds. Low available amounts of nutrients in unimpacted waters normally limit plant growth. The biogenic exchange of substances in waters with low nutrient inflow will, by means of the self-regulating food-chain, lead to a balanced distribution among all participants in the exchange of substances: the producers, consumers and decomposers. Algae are among the most important producers in waters. They are able to build organic substances from the inorganic nutrient salts, which then serve consumers (including zooplankton and fish) as a nutrient base. The microbial decomposition of dead algae, water plants and fish is accomplished by decomposers (bacteria).
In addition to previously existing (mainly low) pollution, public and industrial waste waters bring excessively high nutrient inputs of phosphorus and nitrogen into waters. The nutrient overload (eutrophication) enables phytoplankton to reproduce at a rate so high that animal plankton organisms are often unable to cope with this development. The normally self-regulating material cycle is disturbed. A mass breeding of algae (algal bloom) results. Algal blooms occur mainly in warm summer months and affect waters negatively. Massive amounts of algae affect light conditions, the oxygen supply (oversaturation or saturation deficit), the pH value, and thus the exchange of inorganic nitrogen.
High dissolved oxygen content is required for quick microbial decomposition of masses of dead algae. Oxygen contents in stratified lakes lessen with depth, causing most of the algae masses sink to the bottom. A considerably slower, mainly anaerobic, bacterial decomposition takes place here. This is linked to the formation of foul sludge.
The river lake (lake-like broadenings) areas of the Spree and Havel rivers have all the conditions that promote heavy algae growth and its negative consequences: large water surfaces with good light penetration and shallow water depths, extremely low flow speed and thereby long fallow periods, favorable water temperatures from the influence of power plants, and a continual supply of nutrients from discharges of large sewage treatment plants.