Why was it decided to reduce the runoff from Lake Zurich?

The measure was taken due to the persistent drought, which causes the lake level to fall steadily. The decision, taken by the canton of Zurich together with the FOEN and the cantons of Aargau, Schwyz and St. Gallen, aims to curb the lowering of the lake level to avoid disruption to shipping, the ferry and the lifeguard service, as well as to prevent interruptions in the work at the Rathausbrücke construction site.

What are the consequences of reducing the flow rate?

Reducing runoff from 30 to 20 cubic meters per second allows more water to be retained in the lake, slowing its decline. However, this leads to a lower flow rate in the Limmat, with possible effects on depth and aquatic habitats. Although the risk of die-off for trout and grayling is currently lower than in summer due to lower temperatures, the authorities do not rule out negative consequences for the ecosystem and report impacts on hydropower production, although without providing precise numerical estimates.

How long will this measure last?

The measure enters into force on 1 October 2026 and has a limited and provisional duration, initially set until the end of October 2026. At the end of this period, the hydrological situation will be reassessed by the competent authorities, also taking into account the upcoming trout spawning season to define the next steps.

Lake Zurich: Reduced Outflow Starting in October 2026

Di Redazione Frontaliere Ticino · Published on 1 October 2026

To combat the drought, starting October 1, 2026, the outflow from Lake Zurich will be reduced from 30 to 20 cubic meters per second to ensure safe navigation.

Context

In brief

The runoff drops from 30 to 20 cubic meters per second
The measure starts on 1 October 2026
FOEN and four cantons approved the cut
The reduction lasts, for now, until the end of October

Key Facts

Decision → 30 September 2026
Lake → Lake Zurich
Outflow → 30 to 20 cubic meters per second
Effective → 1 October 2026
Duration → until the end of October
Coordination → the Canton of Zurich, the Cantons of Aargau, Schwyz and St. Gallen, FOEN
Effects considered → navigation, ferry, rescue, ecosystem and hydropower

On 30 September 2026, the Canton of Zurich, in agreement with the cantons of Aargau, Schwyz and St. Gallen and the FOEN, ordered the reduction of the runoff from Lake Zurich from 30 to 20 cubic metres per second. The measure will come into force on 1 October 2026 and will remain limited, for the time being, until the end of October.

The measure is intended to curb the lowering of the lake level and reduce the disruption to shipping, the Lake Zurich ferry and the rescue service. The operational downturn is an even lower flow into the Limmat, with consequences for the ecosystem and hydropower production.

A drought-related decision

Upstream there is persistent drought: the water flowing into the lake is less than that flowing from it and the level continues to fall. According to current weather and runoff forecasts, no significant improvement is expected in the short term.

A further decline would be likely to jeopardise the ferry, the Zürichsee Schifffahrtsgesellschaft shipping company and the rescue service in particular. The construction site of the Rathausbrücke, organised by the City of Zurich, could also be affected: if the level was too low, it would no longer be possible to use the necessary floating pontoons. The work would therefore have to be stopped, with delays and additional costs.

In mid-July, in view of the impending heat wave, Zurich had already increased the runoff to 30 cubic metres per second. In August, in order to contain the risk of fish die-offs in the Limmat, the FOEN and the cantons involved confirmed the decision, although they took into account that the lake level could fall below the lower limit set by the Water Regulation Regulation.

Since then, the lake has fallen again. However, the temperatures in the Limmat have dropped and are no longer critical for psychrophilous species such as trout and grayling. At the request of Zurich, the FOEN and the cantons involved have therefore approved the new runoff level. The duration remains temporary: after the end of October, the situation will be reassessed, also in view of the upcoming trout spawning season.

Operational details

Why the reduction has opposite effects

The change from 30 to 20 cubic meters per second creates a balance between the lake and the Limmat. In the first case, it retains more water and slows the decline; in the second, it reduces the available flow. The press release therefore describes an operational benefit for the lake along with possible environmental and energy-related effects downstream.

The Before-and-After Comparison

Area
Area	Before	With 20 cubic meters per second
Lake	level falling	slower decline
Navigation	risk of disruptions	measures to mitigate them
Limmat	higher flow rate	reduced depth and potential habitats
Fish	more critical summer water conditions	lower risk of mass mortality, though effects cannot be ruled out
Hydroelectric	production dependent on flow rate	reported consequences, without estimates

The practical implications vary depending on the perspective. On the lake, a slightly higher water level should mitigate the difficulties indicated for ferries, navigation, and rescue operations. In the Limmat, however, reduced flow may decrease water depth and the number of habitats. The text does not translate these effects into a numerical forecast or an operational timeline.

The situation for fish remains twofold. The Zurich Cantonal Office for Fisheries and Game Management estimates a lower risk of fish mortality compared to the hot summer weeks, as temperatures are now less critical for trout and grayling. However, the lower flow rate may adversely affect other aquatic organisms, and negative consequences cannot be ruled out.

Regarding hydropower, the source mentions consequences but does not specify their nature or extent. It is therefore not possible to attribute a quantified reduction in production to the decision. Regarding the work on the Rathausbrücke, however, only the scenario of disruption, delays, and additional costs are mentioned if the pontoons cannot be used.

To distinguish this local effect from a measure affecting household budgets, the reader may consult costo della vita in Svizzera: the press release does not provide figures on household expenses. Regarding transportation, no new schedule has been announced; updates on mezzi pubblici remain separate from the hydrological measure.

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Key points

How to follow the measure until the re-evaluation

The press release does not indicate forms, requests or documents to be submitted. The practical procedure for the reader is therefore a verification of the dates and activities directly mentioned, without adding obligations that the source does not provide.

Four operational steps

1. Effective date — mark 1 October 2026 as the start of the reduction. Until then, the new outflow is not indicated as having entered into force.

2. Navigation — if you have to use the Lake Zurich ferry or follow a navigation activity, check the operational communications of the service concerned: the measure was also created to contain possible inconvenience. The text does not communicate an alternative timetable.

3. Work on the bridge — those who follow the Rathausbrücke construction site must bear in mind the condition described: too low levels can prevent the use of floating pontoons, with possible interruption, delays and additional costs.

4. End of October — mark the end of October as the provisional deadline. The situation will be reassessed after the limited duration of the measure, also in view of the upcoming trout spawning period.

The check to be done

The assessment does not only concern the level of Lake Zurich. The two effects indicated must be read together: more water retained in the lake on the one hand, lower flow into the Limmat on the other. For the latter, the source points to a possible reduction in depth and habitats, while the risk of die-off is now lower due to lower temperatures. The picture therefore remains open until the new assessment.

The press release does not quantify either the effects on hydroelectric production or the additional costs of the works. To accompany this verification with an estimate of household disposable income, use calcolatore stipendio.

Source: admin.ch

Frequently Asked Questions

Why was it decided to reduce the runoff from Lake Zurich?: The measure was taken due to the persistent drought, which causes the lake level to fall steadily. The decision, taken by the canton of Zurich together with the FOEN and the cantons of Aargau, Schwyz and St. Gallen, aims to curb the lowering of the lake level to avoid disruption to shipping, the ferry and the lifeguard service, as well as to prevent interruptions in the work at the Rathausbrücke construction site.
What are the consequences of reducing the flow rate?: Reducing runoff from 30 to 20 cubic meters per second allows more water to be retained in the lake, slowing its decline. However, this leads to a lower flow rate in the Limmat, with possible effects on depth and aquatic habitats. Although the risk of die-off for trout and grayling is currently lower than in summer due to lower temperatures, the authorities do not rule out negative consequences for the ecosystem and report impacts on hydropower production, although without providing precise numerical estimates.
How long will this measure last?: The measure enters into force on 1 October 2026 and has a limited and provisional duration, initially set until the end of October 2026. At the end of this period, the hydrological situation will be reassessed by the competent authorities, also taking into account the upcoming trout spawning season to define the next steps.