Nymphs, for the most part, are nature spirits that inhabit a number of region in nature. Most popularly known are river nymphs.
Did the Greeks believed nymphs were present in every river?
Homer, Iliad 20. 4 ff ff (trans. Lattimore) (Greek epic C8th B.C.) : "But Zeus, from the many-folded peak of Olympos, told Themis to summon all the gods into assembly. She went everywhere, and told them to make thier way to Zeus' house. There was no River [Potamoi] that was not there, except only Okeanos (Oceanus), there was not one of the Nymphai (Nymphs) who live in the lovely groves (alsea) [i.e. Dryades], and the springs of rivers (pegai potamon) [i.e. Naiades] and the grassy meadows (pisea poiêenta), who came not. These all assembling into the house of Zeus cloud-gathering took places among the smooth-stone cloister walks."
Every river had a river god [Potamoi].
The Naiad nymphs were sometimes classified by domain:-- (1) Pegaiai (Pegaeae) were the nymphs of springs; (2) Krenaiai (Crenaeae) were naiads of fountains; (3) Potameides presided over rivers and streams; (4) Limnades and Limnatides inhabited lakes; and (5) Heleionomai were the nymphs of marshes and wetlands.