Learn French: Nous marchons le long de la rivière.

Le long de is a prepositional phrase that means "along" or "alongside." Literally, it translates to "the length of" (le long being a masculine noun meaning "the length"). You use it to describe moving or being positioned parallel to something, like a river, a wall, or a road.

What happens to le long de if the noun after it is masculine or plural?

Since the phrase ends with the preposition de, you must follow the normal contraction rules if the next word uses the definite article le or les. Walking along the wall (le mur) becomes le long du mur (de + le = du). Walking along the docks (les quais) becomes le long des quais (de + les = des). Here, rivière is feminine, so de la remains separate.

Could I use nous nous promenons instead of nous marchons here?

Yes, but it changes the nuance slightly. Marcher simply refers to the physical action of walking. Se promener specifically means taking a walk or strolling for leisure.

What is the difference between une rivière and un fleuve?