Currently, there are 37 bridges going from one side to the other but for over 1,000 years, there were only four! Bridges were not only a way to cross the river but they also served as plants to supply the hydraulic power for operation of watermills and various other mechanisms. Bridge building must have been a lucrative occupation in those early days because the bridges were so flimsy, they were regularly swept away by floods or collapsed due to the weight of houses built upon them.When there were only two bridges in Paris (800 A.D.), the Big Bridge and the Small Bridge, the expression "monkey money" (worthless money) was coined (no pun intended). The Small Bridge was originally the toll bridge and only street performers could cross it free of charge--but only if they were able to get their performing monkeys to make funny faces...
The Pont Sully (named after one of the ministers of King Henry IV) is one of the most recent bridges; “recent” is relative—it was completed in 1876. It is located at the tip of Ile Saint Louis, a residential island behind Notre-Dame which is not only one of the oldest parts of Paris but one of the most exclusive.
The Pont Neuf (“new” bridge) was originally called the Bridge of Tears because when Henry III laid the first stone in 1578, according to legend, he was crying about two of his beloved courtiers who killed each other in a duel the night before. The bridge spans the widest part of the
river and has 12 arches. It is actually Paris’ oldest standing bridge which wasn't completed until 1607 (Henri III did not live to see its completion). Construction took so long because the French kings were using the country’s money for war during that time and taking it out of the bridge’s “construction fund.”But upon its completion, Parisians were duly impressed because its design marked the end of the Middle Ages. It was 66 feet wide and unlike other bridges, it didn’t have buildings or houses set atop it which allowed for a view of the river; it had real sidewalks; and there were 384 mascarons or stone mask sculptures on each side. The turrets you see while crossing the bridge were originally for vendors and street performers; now those curved comfortable cutouts are meeting places for lovers and stages for painters.
Right now the bridge looks pretty new because of a fresh cleaning and renovation in the 1990s. The famous now-closed department store, La Samaritaine, sits at the end of the Pont Neuf on the Right Bank. The store owes its name to the old water pump that used to be at bridge level and supplied water to the Louvre and Tuileries palaces. It was so named because it included a sculpture of the Samaritan pouring Christ a drink of water. Carved somewhere in the stone below the bridge (I don't know where) there is a reminder of the famous floods of 1910--something marking the high-water mark. I'll have to look for that on my next trip.
My favorite is the very ornate and elegant Pont Alexandre III, which is a bridge with wide sidewalks starting at the end of the esplanade from Les Invalides’ gold dome to the Grand Palais. Built to celebrate a treaty between France and Russia and named after the Russian Tsar Alexandre III, it was built in concert with the Trinity Bridge in St. Petersburg to honor of the Franco-Russian Alliance of 1892. Its first stone was laid in October 1896 in the presence of Nicholas II, current Tsar of Russia and opened for the 1900 Universal Exhibition. The first prefabricated structure was constructed with a single arch, albeit lower so that it didn’t obstruct the view of the Champs-Elysees or Les Invalides. It is lavishly decorated with pillars, sculptures, and lamp posts in beaten copper and guilded bronze.
Upstream, hammered brass belles represent the Nymphs of the Seine with the Paris coat of arms, while downstream, the Nymphs of the Neva carry the Russian coat of arms.The Pont du Change stands at a place in the river that has been the site of a bridge since before the Romans reached Gaul. The number of structures built on this site since the first one was destroyed in 50 BC is nearly impossible to tally. The name comes from the fact that in 1441, Louis VII ordered all the money-changers of Paris to move to this bridge—in the days when bridges had buildings built across them. The current bridge has big N’s carved on it—would lead you to think it was the Pont Neuf. But the N, of course, is for Napoleon III because it was constructed under his regime between 1858 and 1860.
Pont du Carrousel was built in 1833 and then demolished and rebuilt in 1936 because it was too low for boats to pass. Because it passes through the three arches into the Louvre’s courtyard and pyramid location, it is sometimes called the pont du Louvre. 
Walking across the bridges or on the footpaths along the river, you’ll see the famous tourists’ boats plus commercial barges which carry 20% of Paris’ transported goods. You’ll also see house boats parked along the banks—I read somewhere that there are 2,000 of them. Last year I read a book written by a guy who lives in one of these—a very interesting life style and not one that I would enjoy. But it’s interesting to get a peek into their lives as you walk along the river. It is very murky—looks gray on some days and army green on others. Even so, once in awhile, you’ll see a fisherman trying to catch who-knows-what from the shadowy waters.
I vowed to learn the names of all the bridges while I was there. Unfortunately, that didn’t happen….
