Strabo Geography 5.1.5

[5] The whole of this country ¹ is full of rivers and marshes, especially the district of the Heneti, which likewise experiences the tides of the sea. This is almost the only part of our sea ² which is influenced in the same manner as the ocean, and, like it, has ebb and flood tides. In consequence most of the plain is covered with lagoons. ³ The inhabitants have dug canals and dikes, after the manner of Lower Egypt, so that part of the country is drained and cultivated, and the rest is navigable. Some of their cities stand in the midst of water like islands, others are only partially surrounded. Such as lie above the marshes in the interior are situated on rivers navigable for a surprising distance, the Po in particular, which is both a large river, and also continually swelled by the rains and snows. As it expands into numerous outlets, its mouth is not easily perceptible and is difficult to enter. But experience surmounts even the greatest difficulties.