This is a view of the Seine River, the river that flows through Paris. This well-tamed watercourse crosses the city roughly from east to west along a curved path that looks like an inverted ‘U’ on a map. The average flow of the river is between one and ten cubic metres per second, and the depth averages six to eight metres. It's not exactly th e Mississippi, but it's a very pretty river, of appropriate size for this densely-populated city. The river is navigable for freight all the way to the Atlantic ocean and for a considerable distance above Paris, and barges are a common sight on the river at Paris. Two thousand years ago, thi s river was famous for the purity of its water and the abundance of fish that it contained; it is so-so today. There are still fish in the rive r (occasionally you'll see amateur fishermen catching them on the quays), but bathing in the river has been prohibted since 1923. The greenish color of the slow-moving water comes from silt and plant growth, not pollution. The principal sources of pollution are agricultural runoff upstream, and occasional overflows of water treatment plants during storms (if the systems are overwhelmed, untreated water escapes directly into the river).
This picture was taken from the pont des Invalides bridge in the early morning, and you are looking west in the photograph. The Left Bank is indeed on the left in this photograph (since we are looking in the direction of the river's flow), and the Right Bank is … on the right. The bridge in the distance is the pont de l'Alma.
Some people ask if the Seine provides drinking water to the city. In fact, roughly half of Paris' drinking water is obtained from natural springs east of the city, and is clean enough that no treatment is required beyond a squirt of chlorine to keep it pure during the trip to the city. The remaining half is obtained from the Seine and Marne rivers quite a distance upriver, after purification.
The Seine is eight metres deep today on average at Paris, but it is artificially maintained at a level higher than its natural level by dams and locks downstream. The river only drops about 30 metres on its 200-kilometre journey to the Atlantic, and in the days when it flowed at its natural level (hundreds of years ago), most of the riverbed was sand, with only a narrow stream of water flowing down the middle. Old pictures of the river in centuries past illustrate this clearly. In the nineteenth century, though, the aforementioned dams and locks were installed to raise the river level in Paris and elsewhere along its path, in order to make it navigable for boats. A side effect of this is that the river looks a lot prettier as well, since it always neatly fills the space between its banks as it flows through Paris now.
Incidentally, the name Seine is pronounced /sɛn/, and rhymes with the English word "ten" (and not with "rain," as many English speakers seem to think). The name comes from the Latin Sequana, which in turn comes from the old Celtic word squan, meaning "serpentine" (the Seine follows a very tortuous 776-kilometre route from its source on the Langres plateau to the ocean; 365 kilometres of this is below Paris).
I have a photograph taken from the Pont des Arts in the evening, if you are interested. I also have a photograph facing in the opposite direction, also from the Pont des Arts, towards Notre-Dame Cathedral. Finally, I have a daylight picture of the river on a dreary day, if you’d like to see what it looks like on those (frequent) days when it is overcast and drizzling in Paris. I also have a picture taken when the Seine was running particularly high (as it often does in winter and early spring), which you can see here.
In times past, the river occasionally froze over partially in the winter. It hasn't done that in a long, long time, however.
Click directly on the photo to see a larger version (twice this size). Photographed on July 15, 2008.