What is this?

Fish swimming in very clear water, I guess. I would say trouts because I see little points.

I think it is a stream or a river

Hello Matte5, Hmmm, this seems to me to be shallow water flowing over rocks, some of which seem to be beautifully moss covered. However, slightly left of bottom center there does appear to be a fish. If I'm seeing this correctly, the fish appears to be very large, considering the apparent depth of the water. Although the water might be much deeper, especially given it's clarity. If it is a fish, then 'ouch', swimming over those rocks much hurt!

rocks underneath a stream?

This is one of those discussions you have to respond to before reading the other responses. My first guess as to what it is, is that it is the bark on a tree trunk. Perhaps one that has a little moss or something on it. So am I close?

too small but It looks like a river.

It is a creek bed with water running over the rocks. I have not read the other responses so I will read them after I post this.

i am guessing it is your stream bed...or part of it perhaps...i hope you will get lots of fish out of it :-)