Im still a beginner but I tend to find that my best days fishing are when the water isnt too low or too high. Usually if its very low it will be pretty clear and the fish will be very wary and will spook easily, Remember if you can easily see them then they can just as easily see you and your leader and your wrong choice of fly.

At the other end, usually if a river is very high it will be carrying quite a bit of colour and so they will be harder to spot, this also means that you can usually get much closer to the fish if theyre active. Also if its high then the flow will be faster so they might not get the chance to inspect any flies on the surface or might not want to if its flowing too fast.
Somewhere inbetween is good for me but since I only really get to fish on weekends then I just have to put up with whatever the conditions are and try and learn what I did right or wrong.
These are not set rules though unfortunately, Ive been out when the river has been very high and coloured and caught fish and alternatively ive blanked when the conditions look ideal.
The more you fish the river the more you will learn and realise that no two days are the same.
Daryn