Nymphing Depth

by Matt Walters

Photo by David Cannon

When I first began fly fishing one of the more difficult concepts was how deep to fish nymphs. Others may have had different struggles but for me it was this. I always questioned my fly fishing buddies as to why the flies needed to be so deep and have so much weight attached as they always snagged leaves, moss, trees, etc on the bottom and would cause me to have to re rig my whole set up when this occurred. The common response that I now have branded in my memory from those early days is "because that's where the fish are."

There is a reason for this.  Trout tend to stay deeper in water especially in faster runs.  Deep in the water the current is not as strong and this helps the fish exert less energy in their pursuit for insects/food.  The natural drift of an acquatic insect is also near the deeper portions of the water as their tiny frames have very little control over the direction they take when facing moving water thus pushing them deeper in the water column.   

Use judgment when selecting flies from your box.  The size and weight of the nymphs you use will also alter the fishing depth. I tie my #8 Kevin's Stone Fly with a wrap of lead wire around the shank of the hook so I don't have to use as much split shot to compensate for water depth. Obviously size #18-20 midges will require more split shot than the #8 Kevin's Stone Fly described above. You want your flies to reach the bottom where the fish are located as quickly as possible so that every drift in the run you are fishing is as efficient in covering the deepest water for the longest time frame possible.   Mending the line will also play an important factor in the depth and speed your fly will travel. The fly will travel at a greater speed and higher in the water column out of the trout's consistent striking range if your line is not mended properly causing the fly line to drag the leader too quickly through the water.  You want to make sure a dead drift is accomplished by mending appropriately.

It is also important to keep adjusting your indicator and weights for different sections on the water throughout the day. Sticking with the same depth that you rigged at your vehicle that morning and fishing it the same way all day will typically yield fewer fish.  Your nymphs should be bouncing off the bottom as a natural aquatic insect will do in its larva & pupa state.  The other variable that needs to be accounted for is water speed. You will need much more weight (split shot) added to your line if you are fishing faster moving waters as the current will not allow the fly to drop to the strike zone of a trout.  I have always heard "if you are not snagging the bottom you are not fishing the right depth and you are not putting your fly in front of the fish.”  

Sometimes the depth and speed of the water is a guessing game. A good way I have always used is to cast a short amount of line in front of you (near your feet) where you may still be able to see the flies under water using polarized sunglasses. The depth they reach will be a good indication of where they will be when you cast further into the run holding fish (similar to a practice run before the big show).  If you can see the flies near the bottom of the water you are fishing the correct depth if not you need to adjust your rig.  Another option is to dip the flies into slack non moving water in a spot you approximate to be the same depth as the run you will be fishing to determine the correct depth.  These "practice runs" may be difficult in very deep water where visibility will lack below the water's surface or in smaller tighter rivers that don't offer much "practice" area.  A stick or other depth gauging device can be used in this process as well if all else fails.  With time and experience and trial and error, determining the correct depth will become much easier.

How deep is too deep? If there is too much line between your flies and the indicator you may encounter missing strikes when attempting to set the hook on a fish as well as foul hooking some fish. If this is the case, try fine tuning your line length by shortening the distance between the flies and the indicator by removing some weight.  Use your best judgement based on the information above according to the water depth and speed of the current. If you find yourself not catching fish it may be time to lengthen the line below your indicator and/ or add some more split shot to your line. It may just make a major difference in your success on the water.

This is a simple yet very effective basic fly fishing technique.  I'm sure there are other tricks and techniques everyone uses out there to determine nymph depth if you want to share below we'd love to hear your insight.