River pike fly fishing
River pike
In the picture: nice summer river-pike caught on a big streamer 

River pike are real game fish. On average stronger than their cousins in ponds and lakes, they are capable to put up very hard fights.
If you’re after wild big pike, big rivers are the places where to look for, but be aware of the a challenge: more water to cover and the places where pike could hide are not easy to figure. Given for granted that pike like to hide in water weeds and prefer still or slow-flowing waters, there is neither a repeatable absolute condition, nor any easy mathematical formula that can help sort the riddle of when the fish will bite. Based on personal notes from recent past catches, let us see when and how there was any success. The notes do not mean to be a statistic, nor pretend to be an example of how to catch fish. Fishing for pike in large rivers is far less than deterministic. A 10% may be knowledge, experience, and common sense. The rest 90% is reacting to events and situations.
Unless one comes across the stereotyped, perfect sunken tree, drop off, etc., which even then doesn't mean a sure catch, one will have to try to read the water, evaluate the stream conditions, the eddies on the spot, and cover as much water as possible.