what are rivers & streams made of?

Waterrr...Which is a mixture of two Hydrogen atoms and one oxygen atom. Btw the water in rivers and streams are usually mixed with alot of debris. Which is basically dirt, rocks, or chemicals that may have run off from the surrounding land. THat's all i can think of :)

Rain. Underground springs. Melting ice. Also river and stream water will contain various mineral sediments, aka dirt and sand.

Melted snow from the mountains,

Very Funny ^^

It also comes from oceans and lakes though, not just snow. I dunno if it helps, but it also has sediment from scraping away at the earth.

dihydrogen monoxide.