Understanding Drag-Free-Drift: A Vertical Profile of Stream Velocity

“Drag-free-drift” was repeated to me like a mantra in my early years fly fishing. The goal when fishing with a fly (other than catching lunker trout!) is to present your pattern in such a way that it imitates an insect suspended in drift and traveling the speed of the water. The challenge with this is keeping your fly-line from catching the current and skating your fly through the water. This challenge is multiplied when fishing wet patterns as the stream velocity changes from the surface down to the substrate.

As a rule, the closer the water gets to the side of the channel, the slower its velocity.

At the surface of the stream we find the greatest velocities. There is minimal friction between the air and water so your fly and fly-line will always travel the fastest on the surface. As we move down within the water column we will reach a point of average velocity at 60% the total stream depth. At this pivotal depth we are beginning to experience the dampening effect of the turbulence and friction created by the channel bottom. Above this depth is the Zone of Greatest Velocity Gain, while below it is the Zone of Greatest Velocity Loss.

As we move towards the bottom of the channel the water expends energy against the substrate due to friction. Cobbles and boulders on the channel bottom reach up into the water column obstructing the flow and further reducing the stream velocity. The effect of friction reduces stream velocities down to 0 meters per second at the substrate-water interface (the boundary layer). This sheltered, low velocity layer is where we find the majority of aquatic invertebrates sheltering.


Here are a bunch of Simuliidae (black fly) larvae clinging to a rock and filtering food out of the slow moving boundary layer.


These Baetidae mayfly nymphs (Blue-Winged Olive) are able to maneuver through the boundary layer on this boulder due to their flattened bodies.

Fish also take of this zone of decreased velocity to conserve energy and make forays into faster water to grab insects that come off the bottom and into the drift.


Note how this Brook Trout (Salvelinus fontinalis) is sheltering in the low velocities next to the substrate.

To achieve drag-free-drift, specifically when fishing wet patterns, we need to compensate for the greater surface velocities so that we don’t skate our fly through depths of lower velocity. This calls for constant mending of your line against the prevalent current to maintain a vertical profile of your fly-line over your fly.

I hope helps you get down to where the lunkers and holding and catch more fish. So until later, keep you r fly on the water and I hope to see you on the river!

Invertebrate and fish photos taken from troutnut.com