Casting is crucial, sure. But what happens after your flies hit water—the presentation—is more important. You want to avoid the drag created by currents pulling on your line, which will cause your bugs to drift unnaturally. Mending, the lifting and placing of your fly line upstream from your flies, negates that problem. As a rule of thumb: Mend when your fly, or strike indicator, is even with your position in a run, and not before. And remember that one solid mend is always better than many halfhearted mends.