Those who go for walks along the Vistula know that a thin layer of slimy loess clay carried by the current, called fen soil, appears above the banks that are still wet when the floodwaters subside and the level of water drops quickly. The loess banks of the Vistula around Sandomierz giving into side-erosion, submit to water currents and rinsed out flow downstream with Vistula’s current. In the Vistula’s freshet waters the loess clay comes up in the form of silt, hence the milk coffee colour of the water. The fen soil settled on the hard base makes it possible to slide on it like on packed snow.

The process that has lasted thousands of years led to the creation in Vistula’sdelta at the mouth of that section, called “Żuławów”, the fertile lands made up from loess soil. Until the 16th century there was no settlement here due to flooding of these regions. It was not until the arrival of the Dutch Mennonites banished from Holland, who thanks to their abilities to combat high floodwaters, put down their roots in places where others could not. Hence the presence of Mennonites along the Vistula right up to Warsaw, where there are still traces of their cemeteries. As a result of their painstaking work the whole Żuław in Gdańsk had been embanked. The furthest place to the east where they had stayed was Mościce Dolne nad Bugiem, at 338 km between Kodno and Sławatycze.

An artificial island had been created beyond Kazimierz Dolny at 354 km near Okale village on the right bank as a result of the careful regulation of the Vistula, so-called Puławska. The island, called Krowia Island (Cow’s Island) created from sand rubble is located on many hectares of land. Over several years it was covered by fen soil from floodwaters and overgrown with grass. Today, it is a grazing ground for cattle and a nesting reserve for water birds. The Narew River entering the Vistula at 550 km, before it joins the Vistula, along Twierdza Modlin up to Zakryczym, stands out thanks to its cleanliness and clarity. The milk coffee coloured Vistula waters clearly cut themselves off from the ‘black’ Narew waters. Half a century ago, the Narew waters were considered to be better than the water from the Warsaw water supply system. Barge trains on their way to Warsaw without fail deviated to the Twierdza Modlin bank so that the crew could fill up their barrels and containers with its drinking water. Today, that tradition has been forgotten. Anyway, the quality of that water is no longer what it used to be in the mid-20th century.