Just in case you have never fished a spring creek of this difficulty, here is a short lesson on how to fish downstream on Silver Creek.
First of all wear colors that fit into the surroundings. Wade into position very carefully and stay low if you can. If the fish is feeding in a slot between weeds, stay out of that slot. If you enter it, the silt from your wading will drift down to the fish and it will stop feeding it. The lanes between weed beds help to confine the silt to the lanes you are in.
This is from previous posts:
"The downstream approach is the single best way to catch these super spooky fish. It is the method of choice in my namesake, Silver Creek.
Flyfisher's Guide to Idaho - Ken Retallic, Rocky Barker - Google Books
Here's how to do a parachute cast, my favorite method for delivering a fly absolutely drag free from the upstream approach.
If micro-drag is the problem, the goal is to remove all drag. The easiest way to remove drag and place the fly accurately into the feeding lane of the fish is with a parachute cast from the upstream position. It is also the easiest way to time a fly so that it reaches a rhythmically feeding fish.
You must be able to get upstream of the fish and into casting position without spooking it. This can be directly upstream of the fish but more often it is upstream and across so that you can stay out of the direct feeding lane but still do a reach mend to place the cast into the feeding lane.
From the upstream position cast downstream to the feeding fish so that the fly lands upstream and outside the window of the feeding fish. Stop the rod high so that the line drapes down to the water like a parachute cord (this is where the name of the cast comes from). You can jerk back on the rod tip as the leader unfolds to give you more slack line and leader and place the rod in a more vertical position at the end of the cast.
If you are a little off in your cast, you can skate the fly directly into the feeding lane because the fly, line and leader are still outside the window of the feeding fish. When the fly is in the right place and at the right time (for a rhythmically feeder), lower the rod tip as the current takes the fly to the fish and the fly will enter the fish's window before the leader and without drag. If you time it right, the fly will arrive just as the fish is rising to feed again, and the fish will choose your fly from amongst the others because yours is arriving at the right time. If you need more slack than just lowering the rod will provide, pop the rod tip up and down while releasing line to stack mend line into the drift.
When the fish takes, delay just a bit until the mouth closes and the head goes down before you lift the rod to set the hook. If you set too fast. you will actually pull the fly out of the fishes mouth before it closes. The hook set is delayed a bit when you are upstream of the fish compared to a downstream position.
If the fish does not take, gently move the rod to the side so that the leader goes to your side of the fish without disturbing the surface too much. Gently pick up and do your false cast to the other side of the river so that you don't spray the water over the fish and then try again.
For fish that are feeding selectively and rhythmically by holding close to the surface there is no better method. They have a small window so you can get closer than a fish that is holding deeper. Trying to remove all drag with a slack line, being highly accurate, and timing the fly for a rhythmical feeder is nearly impossible when casting from below the fish.
Nymphing - Gary A. Borger - Google Books
If the fish stops feeding when you do the parachute cast, it might have seen your rod. Then you will need to do a tilted sidearm cast to keep the rod out of his window. Then perform stake mends to feed line into the drift.
If this does not work, you can approach from below and try dropping a hopper pattern near the bank and BEHIND the fish. Hope for a reaction strike. Note that the fish's window extends behind him so aim the fly far enough behind the fish to land outside his window. A "splat" in his window may spook him.