At water level, some 30 feet below street level, the river is bordered--at least on those portions not transformed into expressways--by cobbled quays graced with trees and shrubs. From street level another line of trees leans towards the water. Between the two levels, the retaining walls, usually made of massive stone blocks, are decorated with the great iron rings of a past ages commerce and sometimes pierced by mysterious openings (water gates for old palaces or inspection ports for subways, sewers and underpasses). Here and there the wall is shawled in ivy.