In my studies, about a year ago, I found a website called Old Maps of Paris. This map was drawn in the 1700’s of how Paris may have looked in 360 AD.
I spent hours trying to cross-reference the map with Google Earth and verified three things as true.
1) There were four islands in the Seine River but now only two. In the 1600’s, three of the islands were merged.
«In 1585, work began on the Pont Neuf at the far west end of the island. In 1607, Henry IV gave up the royal garden on the island, which was transformed into the Place Dauphine, and the Pont Neuf was completed.
When the Pont Neuf was built, two small islands, Île à la Gourdaine and Île aux Juifs, which were just to the west of the island, were joined to it. The Ilot des Juifs had been the site of the burning at the stake of Jacques de Molay, the leader of the Knights Templar, in 1314, as the island was just below the towers of the Royal Palace. The junction of the small islands was completed in 1607, and the land was developed into the Place Dauphine and the Square du Vert-Galant.» (Source)
It seems Paris may have first originated on the islands, since the river is a natural barrier against barbaric invaders (which Paris did come to experience).
2) The street above the Seine River going North is Rue Saint Denis.
My first inclination was to think it’s Boulevard de Sébastopol, but that didn’t pan out. Boulevard de Sébastopol was built in 1854 during the renovation of Paris by Georges-Eugène Haussmann. However, adjacent to that road is Rue Saint Denis, which was completed in the first century AD. The ancient street has a long history with the world's oldest profession, but most interestingly there is an archway at the intersection with Boulevard Saint Denis called Porte Saint-Denis (built in 1672), which is where a gate of the Wall of Charles V was located.
3) In the bottom right of the map, there was a Bièvre river. Coming from the Latin word «biberia,» which means «beaver,» it once a very charming river, abundant with beavers, but which became so abused by industrial pollution that in the 1800's when Haussmann was doing his renovation of Paris, it was sealed underground (completed in 1912), and today flows into the sewer. If you walk above on the street, there are little markers commemorating the river.
What streets are above the river, you ask? Thanks for asking! Here is a map:
LA BIÈVRE À PARIS - Paris révolutionnaire
Plan du parcours de la Bièvre dans Paris place de l’ Alma Quartier : Champs-Élysées - Alma - Arrondissement : 8 - Lieu : Ancien (...)
www.parisrevolutionnaire.com
And here is a guided tour!
There are efforts to restore some portions of the river, including plans to create a walking trail along its banks.
«Today’s urban planners have no intention of digging a canyon or tearing down buildings in Paris to uncover the river. But the Bièvre is situated less than 10 feet underground in open spaces such as the Square René Le Gall. Occupying the former vegetable garden of the Manufacture des Gobelins, this verdant park is one of three spots identified for potentially reopening the river. (The Parc Kellermann and the Natural History Museum annex are also feasible targets.)» (Source)