It is best not to collect your rocks from a streambed. These rocks have been rounded and polished. Of course, some of the prettiest rocks can be found in streambeds or at the beach. Just remember that these rounded rocks probably come from rock formations that outcrop far upstream. The best place to collect your rocks is from outcrops.
Outcrops are the visible parts of the rock formation that makes up an area.
For instance, a rock that is not attached to the underlying rock formation is not an outcrop. A boulder is not an outcrop. A cliff face is an outcrop. Rock formations exposed by an excavation are outcrops. Don't collect gravel from a driveway . . .