Lake Greifen

Lake Greifen occupies the flat upper Glatt valley, in a landscape shaped by the Linth glacier. Its catchment contains densely settled areas as well as farmland. This sets it apart from a remote mountain lake: its ecological condition depends not only on weather, but also on the origin of inflows and how nutrients are managed across the catchment.

The Mönchaltorfer Aa and Ustermer Aa are among the main inflows. The latter brings water from Lake Pfäffikon to Niederuster; the Glatt leaves Lake Greifen at its northern end. The two lakes are connected, but are not controlled in the same way. Changes upstream can influence the downstream system, without Lake Pfäffikon's level or temperature being directly transferable to Lake Greifen.

The canton of Zurich is explicit: Lake Greifen's level cannot be regulated. Unlike Lakes Zurich and Pfäffikon, it has no weir operation that can deliberately bring its level to a seasonal target. Rainfall, evaporation, inflows and the conditions for discharge into the Glatt determine its course. A rise should not be read as a regulation decision, nor a low summer level automatically as deliberate drawdown.

A lake-level station on Greifen shows the elevation of the water surface. A discharge station on the Glatt instead measures the volume passing a particular cross-section each second. The curves are connected but do not respond identically. To ask whether a level is high for the season, long-term lake records are useful; comparing only with recent weeks describes a different, short-term change.

Summer can squeeze the habitat available to fish from both sides: warm water above and low oxygen below. AWEL's January 2026 assessment still identifies a need to reduce phosphorus inputs. The aeration installation creates a local refuge with more oxygen; it does not aerate the entire lake. A surface thermometer shows neither that refuge nor deep-water oxygen levels, so temperature and ecological quality must be read separately.