Current is the engine of stream fishing. Fish position based on current — heading into it to face oncoming food, holding behind structure to avoid the heaviest flow. Food arrives via current; fish wait on it.
"Reading current" means seeing where the flow speeds up (over a riffle), slows down (entering a pool), splits (around a boulder), or reverses (in an eddy). Surface current and subsurface current can disagree — trout often hold in slow bottom water beneath a fast top, eating things drifting at depth while the surface flies past.
Almost every fish in moving water faces upstream. Cast accordingly.