That is the genaral rule of thumb,and for the most part you can fish by it if you want to ,but I will tell a somthing. I have caught a many a fish up creek arms in winter and summer. Keep a open mind, try fishing up a creek on warm sunny day in the winter, you just might be suprised. Like now for example, I have been catching good numbers of crappie in creeks and sloughs off of the main river that feeds the lake, the water is no more than 4 feet deep.