Shipping companies prepared to halt the transport of goods on the Rhine as water levels in Germany’s biggest river neared a critically low point on Saturday.

An ongoing drought affecting much of Europe has lowered rivers such as the Rhine, preventing large ships with heavy loads from passing key waypoints and forcing them to use smaller vessels or to split cargoes into multiple shipments.

At one bottleneck, near the town of Kaub on the Middle Rhine, an official gauge measured the water level at 37cm centimetres (14.6 inches) on Saturday.

Big, heavy ships cannot pass if the level falls below 40cm (15.7 inches).

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A container ship passes Pfalzgrafenstein castle in the middle of the river Rhine in Kaub (Michael Probst/AP)

While the depth of the shipping lane in Kaub was still about 150cm (59 inches), experts say passage becomes tricky even for light or specially adapted cargo ships if water levels fall below 35cm (14 inches) at the gauge mark.

A reading below 30cm (11.8 inches) is considered unpassable.

Shipping authorities predict that point could be reached early next week, though it is unclear whether water levels will reach the record low of 25cm measured at the Kaub gauge in October 2018.

Logistics company Contargo said on Friday that it was preparing to halt shipping on the Upper and Middle Rhine for safety reasons and planned to shift some of its cargo on to trucks.

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A transport vessel passes the Rheinturm (Rhine Tower) in Dusseldorf, Germany (Christoph Reichwein/dpa/AP)

Road and rail freight capacity is limited, however.

Companies along the Rhine that rely on ships to receive raw materials and deliver finished goods are expected to face delays and shortages.

Coal-fired power plants and gas stations could also see supply shortages, if shipping on the Rhine is halted.

Meteorologists forecast rain in the coming days, though it was unclear whether it would be enough to keep water levels on the Rhine from dropping to a point that affects shipping.

In Italy, authorities allowed more water from Lake Garda to flow out to parched local rivers like the Po that farmers use to irrigate crops.

Combined with Italy’s worst drought in decades, the country’s largest lake is nearing its lowest-ever recorded water level.