After weeks of hot, dry weather, the Loire River in Loireauxence, western France, now runs low, exposing its bed. Extreme heat across Europe is turning landscapes into arid expanses. DAMIEN MEYER/AFP

River tours on the Danube and Rhine face mounting risks as concern grows that prolonged drought conditions will continue to trouble Europe's main waterways.

After many weeks of hot and dry weather across the continent, Danube levels at Budapest have hit record lows, and vessels on lengthy sections of the Rhine are struggling to carry freight and passengers.

Low flows have turned navigation into a headache for cruise operators, fueling worries that ever-hotter summers will disrupt the easygoing trips travelers expect, The Telegraph newspaper reported.

Viking, the world's largest river-cruise brand, said last week that more than half of its departures from July to mid-August had been hit by low water levels in Europe. The situation is forcing shortened itineraries and bus transfers for passengers.

Analysts caution that if shallow conditions persist, travelers may abandon river cruises for more predictable vacation options.

According to financial ratings agency S&P Global, the low water levels "pose a risk to river cruise operators and could affect future river cruises, resulting in vouchers, cancellations, and higher costs".

S&P Global said in a report: "If low water conditions are prolonged or become more volatile over future years, this could affect future bo