Europe's energy traders have spent recent weeks glued to maps of the Persian Gulf. They should also be watching the Rhine.
While the renewed confrontation between Washington and Tehran has pushed oil prices higher and revived fears over the security of shipping through the Strait of Hormuz, another threat is quietly emerging much closer to home.
Water levels at key inland shipping gauges including Cologne and Kaub on the Rhine and Budapest on the Danube have fallen towards levels rarely seen outside major drought years, forcing barges to slash cargo loads and driving up transport costs.
By themselves, low rivers are an inconvenience. Combined with rising geopolitical tensions in the Middle East, they become something more dangerous: a multiplier of energy insecurity.
Europe's rivers are the hidden arteries of its energy system. Fuel imported through Rotterdam, Antwerp and Amsterdam still has to move inland to refineries, power stations, chemical plants and industrial consumers.
When drought reduces barge carrying capacity, every ton of coal, diesel, gasoline, chemicals or biofuel requires more vessels, higher freight rates and longer delivery times.
As renewed US-Iran hostilities once again threaten oil flows through Hormuz and push energy prices upward, Europe's shrinking waterways risk turning an external supply shock into a much broader internal logistics crisis.
And the timing could hardly be worse.
The Rhine is Europe's most important inland commercial waterway. German industry alone relies on it to transport roughly 200 million tons of goods annually, including fuels and industrial raw materials.
At Kaub, the river's most critical bottleneck, water levels have dropped to exceptionally low levels for July, forcing vessels to sail only partially loaded. Some operators report freight reductions of 80 per cent depending on vessel type and route.
The Danube tells a similar story.
In Budapest, water levels have approached lows more commonly associated with late summer droughts. Shipping companies have already reported vessels operating at only a fraction of normal capacity, while freight surcharges have surged as operators attempt to compensate for reduced cargo volumes.
This matters because Europe's energy system has become increasingly dependent on logistical flexibility.
Following the loss of much Russian pipeline gas, Europe rebuilt its energy security around seaborne imports of LNG, oil products and alternative fuel supplies. The assumption was that as long as cargoes could reach European ports, markets would remain adequately supplied.
But ports are only the beginning of the journey.
The Rhine connects North Sea import terminals with Germany's industrial heartland. Coal for power plants, feedstocks for chemical manufacturers and petroleum products for inland consumers all depend heavily on river transport.
When river levels collapse, cargo must be shifted towards rail and trucking networks that are often already congested and more expensive. The consequence is not necessarily an outright shortage. More often it is a sharp rise in delivery costs that feeds directly into energy and industrial prices.
The economic damage is far from theoretical.
During previous Rhine droughts, notably in 2018, Germany experienced measurable industrial disruption and lost output.
The lesson from that episode was that river levels can become macroeconomic variables. When barges stop moving efficiently, factory production, fuel distribution and industrial profitability all suffer.
Now add the Middle East.
Renewed military exchanges between the United States and Iran have once again focused attention on the Strait of Hormuz, the world's most important oil transit chokepoint.
Attacks on shipping, military strikes and escalating threats have already driven oil prices higher and reignited concerns about interruptions to tanker movements. Even without a complete closure, heightened risk raises insurance, freight and commodity costs.
For Europe, the danger lies in the interaction between these two risks.
Higher oil prices can usually be absorbed. Temporary shipping disruption can usually be managed. Low river levels can usually be worked around.
But when all three arrive simultaneously, the system becomes significantly less resilient.
European refiners may face higher crude costs because of gulf tensions, while distributors simultaneously struggle to move fuels inland because of restricted barge traffic. Chemical producers could confront elevated feedstock prices at the same moment that logistics costs jump.
Utilities may discover that alternative fuel supplies are available in ports but harder and more expensive to move to where they are needed. Each problem reinforces the others.
In effect, Europe's energy supply chain begins to resemble a narrowing funnel.
The irony is that policymakers increasingly view "climate change" and geopolitics as separate challenges requiring separate solutions. Recent events suggest they are becoming deeply intertwined.
The same heatwaves and drought conditions that are lowering Europe's rivers are also boosting electricity demand and increasing pressure on energy infrastructure.
Meanwhile geopolitical conflict is reducing the margin for error in global fuel markets. What looks like a weather problem in Germany can quickly become an energy security problem across Europe.
That is why traders should pay attention not only to missile launches in the gulf but also to river gauges at Kaub and Budapest.
One measures geopolitical risk. The other measures whether Europe can physically move the energy it already has.
For decades, Europe's energy security was shaped by pipelines, tankers and diplomacy. Increasingly, it may be shaped by rainfall.
And as rivers shrink and conflicts multiply, the continent is learning an uncomfortable truth: sometimes the most dangerous energy chokepoint is not halfway around the world, but running through your own backyard.
(Reporting by Gavin Maguire; Editing by Sonali Paul)