Sambre River Turns Fluorescent Green Amid Severe Drought

Residents of Namur, Belgium, have been startled in recent days by an unusual and alarming sight: the Sambre River, which flows through the historic city center, has turned a striking fluorescent green. Thousands of small green leaves now float on the water’s surface, forming a dense plant carpet in places. Authorities are investigating the phenomenon, which has raised both environmental alarms and public health concerns across the Walloon region.

The Cause: A Toxic Bloom

Water samples taken on Wednesday, July 22, at the confluence of the Sambre and Meuse rivers by biologist Dr. Guy Bouxin, a PhD in botany, identified several aquatic organisms responsible for the discoloration. According to RTBF, the samples contain duckweed, green algae, and several species of cyanobacteria — including Microcystis aeruginosa, a potentially toxic species.

Cyanobacteria, also known as blue-green algae, are photosynthetic microorganisms that occur naturally in aquatic environments. Under favorable conditions — warm, slow-moving, nutrient-rich water — they can proliferate rapidly, forming dense blooms. These conditions have been met in recent weeks due to a combination of record drought, high temperatures, and nutrient pollution from untreated wastewater.

Record Drought Conditions

The fluorescent green discoloration is a visible symptom of a much deeper ecological crisis unfolding across Wallonia. The first three weeks of July 2026 have been the driest on record for this period in Belgium. Only 1.4 liters of rain per square meter have fallen since July 1, compared to an average of 55.3 liters for the same period in other years — a staggering deficit of nearly 98 percent. Temperature anomalies have reached +7.1°C above normal in Saint-Hubert and more than +6°C in Dourbes and Uccle.

As RTL Info reported on July 23, following a meeting of the CORTEX crisis coordination center, new drought restrictions have been announced. Fishing in fast-flowing waters is now banned, kayak prohibitions on the Lesse and Ourthe rivers have been extended, and a fifth Walloon municipality — Saint-Hubert in the province of Luxembourg — has imposed water use restrictions on residents.

Nicolas Yernaux, spokesperson for the Public Service of Wallonia (SPW), told RTBF that river flow rates are currently at 26 cubic meters per second, well below the 30 m³/s threshold of concern. “What worries us is not so much the level as its earliness,” Yernaux said. “We have already experienced lower flow rates, but reaching such levels by mid-July is striking.” By comparison, during the catastrophic floods of 2021, flow rates reached 1,334 m³ per second.

Public Health Advisory

While most of the organisms identified in the Sambre are harmless, the presence of Microcystis aeruginosa is particularly concerning. This cyanobacterium can produce microcystins — potent hepatotoxins that pose risks to humans and animals through ingestion, skin contact, or inhalation of aerosolized toxins. Authorities recommend that pet owners not allow their animals to drink from the Sambre or swim in it. Swimming in the river is already prohibited.

Frédéric Mouchet, coordinator of the Haute-Meuse River Contract, noted that while the cyanobacteria call for caution, it is above all the drought that is causing the greatest concern. The low water levels and reduced current are the primary drivers behind the proliferation of these microorganisms.

A Fragile Ecosystem Under Stress

The drought is having cascading effects on aquatic life across Wallonia. As water warms, it holds less dissolved oxygen, putting cold-water species like trout under severe stress. Several Walloon rivers — including the Semois, Ourthe, and Lesse — are experiencing critically low levels, and some stretches are beginning to dry up entirely.

A portion of domestic wastewater is still discharged untreated into Walloon waterways. Combined with heat and reduced current, this creates a nutrient-rich environment that fuels the proliferation of algae and cyanobacteria. The problem is not limited to the Sambre; it reflects a systemic issue affecting multiple waterways simultaneously.

Analysis: A Climate Change Warning

The drought conditions driving this event are consistent with climate change projections for Western Europe. A related RTBF report noted that climate change has made soil drought in Belgium 11 times more likely. The SPW’s observation that reaching critical water levels by mid-July is unprecedented underscores the accelerating pace of climate impacts.

The economic implications are also significant. Restrictions on fishing, kayaking, and other water-based tourism affect local economies, while navigation on the Sambre and Meuse is impacted by low water levels. Agricultural water restrictions may further affect crop yields, and water treatment costs are likely to rise as quality deteriorates.

What’s Next

Rain is forecast for July 26, and the next CORTEX meeting is already scheduled for the following week. If the drought persists, further restrictions may be imposed. Whether the fluorescent green coloration will persist or require active intervention remains an open question.

For now, residents of Namur are left watching their river — a vivid, neon-colored warning of the ecological challenges that may become increasingly common as climate change intensifies drought conditions across Western Europe.