First of all, what is a mend? according to websters, it's:

Verb. mend, repair, patch, rebuild mean to put into good order something that is injured, damaged, or defective. mend implies making whole or sound something broken, torn, or injured. mended the torn dress repair applies to the fixing of more extensive damage or dilapidation.=

It's a fix, a patch, a repair on something broken.

Now let's translate that into fly fishing. More specifically, fly fishing from a drift boat and more specific again, let's say fishing a foam fly to get the best, longest, drag-free drift possible.

So, Where does the mend come in? I'm going to say that the mend is going to fix or repair the reach cast that you missed or do not possess. The reach cast is a matter for another time. Not every drift or presentation lasts forever and there can be several contributing factors such as boat speed, conflicting currents or line stick/drag. A mend can "fix" or repair these usually, but it needs to be done correctly.  

You break a handle off a Coffee cup at home. The broken cup is unfortunate; the one thing that would have avoided this problem is not dropping the cup in the first place, but hey shit happens. You decide to use super glue to repair it. It should work fine and you'll get lots of more use out of the cup if you do a proper repair. 

Maybe you didn't do the best job of prepping the surface by drying it off, let's say. You make your repair and it breaks again. The odds of a better repair the second time around are reduced, right?  

The broken cup is your reach cast. You missed the opportunity to catch it when it fell. It's broken, but what you can do to fix it is prep your surface and get a good repair on it by doing it correctly. The repair is your mend. This can all be fixed relatively easily, but it needs to be done right. Just like the repair on the cup, if you miss the opportunity to do it right the first time, you have reduced your chance at success with a second mend. Like many things in life, your first chance is your best chance.  

Ok, so if someone were to ask me to define a mend as simple as possible. I would say it is two parts. 

1. You lift the rod tip to get the line off the water. 

2. You move your rod tip over and down to put the line where you want it.  

Pretty simple right. . . . yes and no. It takes practice and experience. It takes practice and experience. It takes practice and experience. It takes practice and experience., This means you need to practice it and just like in casting practice can be done off the water.

It's tricky to describe in words, but when drifting, a good mend is one which is done after casting 45' downstream of the boat. 

Let's say, the boat's 12 o clock is 90' and directly to the shore is 0 45' is in the middle of those two. Seems easy right . . . . Wrong. "45' Down" "45 Down" This is one of the most repeated things that you will hear me say during a day of foam fly fishing. It takes a while to really really get in the habit of throwing downstream.

Ok So now we have a cast that has landed 45' down and relatively straight — starting w a low rod tip. Raise the rod upward and towards the bank slightly until the fly line and leader lift off the water. Bring your rod tip down and toward the bank slightly to create a sort of upside-down V or upside-down check mark w the long stroke of the checkmark being the second stroke. With any luck, your line is now straight and ALL your line and leader is upstream of your fly. If your line is not ALL upstream of your fly, your system has a belly. This belly is about to grow and your drift either does or is about to suck.

After you do your up, at least when fishing foam flies, pull back when you do your "over" do not be afraid to pull the fly slightly to make sure everything is tight. It's beat into our heads to not move the fly during the mend, but this fear often causes us to go to light on the mend and it is performed incorrectly. I am looking for long term results here. I'm not worried about the fish under the fly right this moment. I'm setting this drift for the long haul. I am wanting for the fish under the fly 15ft, 50ft, 100ft down the river. If your drift is set up correctly, you may get him.

In the end, I want the fly to be drifting the same speed as the river. My job as the oarsman is to keep the boat travelling at the same speed as the fly in order to give you the best chance at getting the longest drift. Over time I have seen that it is not always the best casters who catch the most fish in my boat, but it is the best menders and folks who buy into the long drift program. You can’t catch any fish when your fly is air born in a false cast. I can tell the efficiency of anglers by how often they are casting. Watch the boat that isn’t casting too often and you will find that they are also the boat with the bent rods.