# Czech reservoir cyanobacteria crisis continues

> Live situation record from CLSTR: https://clstr.news/situations/czech-reservoir-expansion-and-drought-response
> Updated: 2026-08-05T03:20:31.000Z. Sources: 12. Developments: 7.

The drought‑driven cyanobacteria outbreak at the Nové Mlýny reservoir remains acute. In June, Agriculture Minister Martin Šebestyán announced a national programme to build about twenty new reservoirs, prioritising the New Heřminovy, Vlachovice and Kryry projects to secure water supply and flood protection over the next two decades.

In early July the ministry proposed raising the Nové Mlýny water level by 35 cm to improve flow, oxygenation and groundwater stability, but the plan faces a lawsuit before the Czech Supreme Administrative Court. The same period saw a sudden foam formation on weir walls as cyanobacterial concentrations spiked, triggering a drop in dissolved‑oxygen that killed roughly one tonne of fish – far less than the 30 tonnes lost a year earlier.

Povodie Moravy responded with emergency aeration pumps, floating barrier walls and continuous oxygen monitoring; oxygen levels stayed within safe limits and no further fish deaths were recorded. On 17 July officials debated draining the lower Novomlýnske tank to curb the bloom, noting a phosphorus load of about 160 t yr⁻¹ and reporting 1.5 t of dead fish.

By 21 July the basin authority reiterated that full water releases remain technically unsafe, while limited releases modestly improved oxygen levels. Regional health authorities warned on 5 August that cyanobacteria concentrations in the upper reservoirs pose a health risk (water‑quality level 3), urging swimmers – especially vulnerable groups – to shower after bathing and avoid prolonged exposure.

Authorities continue to monitor the situation with drones, consider flushing the Dyje River and explore converting stagnant zones into flowing or wetland habitats to limit future algal growth.

## Timeline

### 2026-08-05: Cyanobacteria risk persists in Nové Mlýny reservoirs, health officials warn swimmers

Health officials warn that cyanobacteria remain in Nové Mlýny reservoirs, posing a risk for vulnerable swimmers, while other South Moravian sites are deemed safe.

2 sources. https://clstr.news/cluster/cyanobacteria-risk-persists-in-nov-mlny-reservoirs-health-officials-warn-swimmers

### 2026-07-21: Nové Mlýny reservoirs face ongoing cyanobacteria crisis as water release deemed unsafe

Cyanobacteria blooms in the Nové Mlýny reservoirs threaten fish and water safety; officials say releases are unsafe, while limited water exchange improves oxygen but risk stays high.

4 sources. https://clstr.news/cluster/nov-mlny-reservoirs-face-ongoing-cyanobacteria-crisis-as-water-release-deemed-unsafe

### 2026-07-17: Czech Ministry Considers Draining Lower Novomlýnske Reservoir Amid Cyanobacteria Crisis

Czech officials weigh draining the lower Novomlýnske reservoir to curb deadly cyanobacteria blooms that have killed 1.5 tonnes of fish, amid calls for expert debate and higher wastewater treatment.

2 sources. https://clstr.news/cluster/czech-ministry-considers-draining-lower-novomlnske-reservoir-amid-cyanobacteria-crisis

### 2026-07-13: Cyanobacteria surge at Nové Mlýny reservoir triggers emergency response

Cyanobacteria bloom in the Dyje River and Nové Mlýny reservoir forced authorities to aerate water, install barriers and monitor oxygen, averting fish deaths.

2 sources. https://clstr.news/cluster/cyanobacteria-surge-at-nov-mlny-reservoir-triggers-emergency-response

### 2026-07-11: Cyanobacteria bloom threatens fish in the Dyje River near Nové Mlýny, Czech Republic

Cyanobacteria bloom in the Dyje River near Nové Mlýny dam caused a fish kill of about one tonne in early July, prompting ongoing monitoring by the Morava River Basin Authority and local fishermen.

2 sources. https://clstr.news/cluster/cyanobacteria-bloom-threatens-fish-in-the-dyje-river-near-nov-mlny-czech-republic

### 2026-07-04: Czech Minister Martin Šebestyán pushes plan to raise Nové Mlýny reservoirs after fish die‑off

After a heat‑induced fish die‑off on the Dyje, Czech Minister Martin Šebestyán advocates raising Nové Mlýny reservoir levels by 35 cm, a plan stalled by an environmental lawsuit.

2 sources. https://clstr.news/cluster/czech-minister-martin-ebestyn-pushes-plan-to-raise-nov-mlny-reservoirs-after-fish-dieoff

### 2026-06-19: Czech Minister Martin Šebestyán Calls for New Water Reservoirs to Avoid Future Shortages

Czech Agriculture Minister Martin Šebestyán warns current reservoirs will be insufficient in ~20 years and proposes three new dams—New Heřminovy (2033), Vlachovice, and Kryry—funded by national and EU sources,

2 sources. https://clstr.news/cluster/czech-minister-ebestyn-calls-for-new-reservoirs-to-address-future-water-shortage

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Cite as: Czech reservoir cyanobacteria crisis continues. CLSTR, https://clstr.news/situations/czech-reservoir-expansion-and-drought-response