Here is some of the oldest advice ever given in angling (click it twice to get it full size):
See that second tip? "Also look that ye shadow not the water as much as ye may. For it is that thing that will soon fraye the fish. And if a fish be afraid he will not bite long after."
The first tip has the answer when you are on the west bank and the sun is setting behind you. "or else behind a bush that the fish see you not..."
It is surprising what you can get away with if you take some simple precautions. Here's a modern day version of following this medieval advice. Sneaking in using the tree to shield the angler from view, then sliding down under the tree to make a completely undetected approach to the fish rising in the run below the (west) bank.