Why does sand and gravel build up near Bratislava on the Danube River?

Near Bratislava, Slovakia, the Danube River emerges from the narrow Hungarian Gates Gorge and enters the flatland of the Little Alfold plain. Upon reaching this flatter terrain, the river's current slows down abruptly, causing it to lose its transporting capacity. As a result, the river can no longer carry its sediment load, leading to the deposition of enormous quantities of sand and gravel on the riverbed.1

This accumulation of material has significant physical consequences for the river, including the formation of large islands and the frequent obstruction of navigation, which necessitates ongoing maintenance work to keep the channel clear.12