I spent years carrying around a variety of sinking lines on spare spools and never ended up using them. I packed them around mostly for lake fishing, but rarely got them out for the river. My home court is the upper Madison River where a "deep" hole is 3 feet at the most. You don't need sinking line to get down 3 feet right? Wrong. You can fish a sinking line in almost any depth. It is less about depth and more about how a sinking line makes the fly behave. A fly on the end of a sinking line stays flat in the water. On the other hand, stripping streamers on a dry line especially flies with a heavy head tend to go up and down.
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I want my streamer down and dirty a couple of inches off the bottom, running flat just like a live bait fish. I think the best way to accomplish this is the slow swing with an intermediate or full sink depending on the depth of the water. This doesn't mean that sinking lines are always the way to go.
Sometimes we fish the Madison River early in the morning when the fish are still stacked on the banks. In this situation the fly needs to land right next to the bank behind every little rock, bush, log or whatever. Two twitches and one strip and that thing needs to be back in the air and on its way to the next spot. When fishing this aggressive rapid fast moving style, sinking lines are to slow to get out of the water because you have to strip most of the way in to reload and fire. By doing this, you end up missing every other spot and the name of the game is show the fly to as many big fish as possible. So if you're fishing fast staccato style, keep your sinking line in your bag.
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