Mending the line is to reposition it on the water's surface to eliminate drag on the fly. In certain types of fly fishing (like drifting nymphs or dry flies for trout) you're normally trying to achieve a drag-free drift of the fly. When a real bug is drifting down the river it's not connected to anything, so it floats naturally at the same speed as the current it's in. Your fly needs to do the same thing, but it's a bit more challenging since it's connected to a leader and fly line that can get pushed around by multiple current speeds between you and the fly.
When trying to drift a fly without drag, you're trying to compensate for the various currents by repositioning the line to prevent it from pulling on the fly. You may have to mend any direction, but an upcurrent mend is what most folks think of when it comes to mending. Have a look at this simple series of graphics showing an upstream mend of a floating line from an overhead perspective.
Knowing how to efficiently manage and manipulate your fly line is a pretty big deal. All too often anglers waste time on the water with poor presentations and other frustrations. While it's difficult to never encounter trouble on the water (we certainly have!), various annoyances can be greatly reduced with sound techniques and mechanics. Our Fly Fishing Basics section wouldn't be complete without going over a few crucial extras!
When threading your fly line through the rod guides, first double over the fly line then push it through each guide. That way, if you lose your grip on the line during this process it won't slide back through all of the guides forcing you to start all over again.
Stripping the line is simply the very basic technique of retrieving/pulling in the fly line. While we've seen a few variations of how folks like to accomplish this task, using the index finger is both an easy and secure way to help strip line.
With the fly line running under the index finger of your casting hand, grab the line immediately behind your finger with the non-casting hand and pull in the desired amount. Let the stripped portion of line fall to your side and repeat the process as many times as desired.
Smaller fish can often be fought by simply stripping in the line while never actually utilizing the reel. When a fish is hooked up, maintain a proper bend in the rod and strip the line to bring the fish towards you. To maintain pressure on the fish in between strips, your index finger will need to trap the line against the rod handle to prevent line slippage.
Threading Rod Guides
Stripping the Line
The swiftest current is represented by the 4 arrows. Imagine you make a short cast straight ahead into a likely looking area with a bit less current.
Since nothing is being done to manage the line, the strong current immediately pushes a belly into the line and the fly begins to get pulled down and across current unnaturally.
The basic mend is often done in one motion by lifting the rod tip high and over to the left or right, then lowering it to place the line back on the surface. The rod tip is often said to travel in an upside down "U" shape using this method. Mending may also involve other tricks like a flick or flip of the line to either side to achieve the desired result. However you mend, make sure you're actually moving enough line on the surface to make a difference. An ideal mend will move just the line and not the fly, but sometimes the fly/leader may need repositioning. Achieving proper mending technique comes with experience and feel, so get out there and practice!
In many cases the mend would be made immediately after the cast hits the water. Moving the line upstream of the fly's position helps compensate for the speed of the faster current down the middle. Try to mend before a belly can develop and mending more than once per presentation can often be necessary.