ow moving in the opposite direction. The water level rises because of the shallowness of Neva Bay, flatness of its bottom and the narrowing of the <a href="//en.wikipedia.org/wiki/Gulf_of_Finland" title="Gulf of Finland">Gulf of Finland</a> near the delta. <a href="//en.wikipedia.org/wiki/Seiche" title="Seiche">Seiches</a>, <a href="//en.wikipedia.org/wiki/Onset" title="Onset">onsets</a> and another factors also contribute to the floods. Besides flooding as a result of tidal waves, in 1903, 1921 and 1956 fl