An act of sabotage by Mainz merchants in 1841 to blockade the Biebrich free port in order to defend themselves against shifting traffic flows is known as the "Mainz Fog Boy Prank".

There had been regular steamship traffic between Cologne and Mainz since 1827. As the Rhine ships were unable to navigate the Main due to their draught, the goods destined for Frankfurt am Main had to be transhipped in Mainz. The goods were taken over the ship's bridge to Kastel and transported onwards from there. Merchants from Mainz made money from this situation.

In 1831, the Rhine states signed the Rhine Navigation Act and agreed on the free movement of people and goods on the Rhine. Each riparian state was given the right to set up a free port where merchants could temporarily store their goods duty-free. The Duchy of Nassau and the Grand Duchy of Hesse were co-signatories. In 1831, the Duchy of Nassau declared Biebrich on the right bank of the Rhine a free port. The construction of a quay wall on the banks of the Rhine in Biebrich laid the foundations for the Biebrich port.

In 1840, the Taunus Railway from Frankfurt to Wiesbaden was completed. It had a branch to the Biebrich bank of the Rhine at Curve station. This branch line made the handling of goods in Biebrich interesting for Frankfurt merchants, as it saved the detour via Mainz and thus costs.

However, the Mainz merchants did not want to be deprived of their traditional source of income. Repeated appeals to the Grand Ducal government in Darmstadt were unsuccessful, as the new traffic situation was completely legal due to the Rhine Navigation Act. They therefore decided to take matters into their own hands and block access to the port of Biebrich in order to direct all shipping traffic back to Mainz.

They hired 70 to 100 Neckar barges in Mannheim and had them loaded with 50,000 hundredweights (approx. 2,500 tons) of quarry sandstones. On the night of February 28 to March 1, 1841, the cargo was dumped into the river between two Rhine islands off Biebrich. This made the northern channel of the Rhine between the tip of the Petersaue Rhine island and that of the "Biebricher Wörth" (today part of the Rettbergsaue Rhine island) impassable. From then on, ships could only navigate the southern arm of the Rhine. Once again, they had to unload their cargo in Mainz. From there, they were transported by horse and cart over the Rhine bridge to Kastel and on to Frankfurt from the Taunus Railway station there. A direct rail connection from Mainz across the Rhine did not yet exist.

This illegal action, which hindered free navigation on the Rhine, led to a protest by the Nassau State Ministry to the Grand Ducal Government in Darmstadt, demanding that the obstacle be removed immediately. As the Darmstadt Minister President Karl du Thil refused, the ducal state ministry lodged an official protest with the Federal Assembly in Frankfurt am Main. Grand Duke Ludwig II of Hesse then ordered at least one passage to be cleared.

The stone wall was opened on March 18. Three months later, it was largely demolished with the help of Austrian and Prussian soldiers from the Mainz garrison. However, the last remnants remained in the river until 1844.

The Mainz Fog Boy's Prank even found its way into literature. In 1844, Heinrich Heine had "Father Rhine" describe the event himself in "Deutschland. A Winter's Tale" in 1844:

"At Biberich I swallowed stones,

Truly, they did not taste delicious!

but heavier in my stomach

the verses of Niklas Becker."