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Sunken Line Simulator

Fly Depth Results

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Fly Depth Formula

Sink Tip Length Formula

Estimating Water Velocity

Fluid Dynamics Principles

Rule Number Lookup Tables


Estimating Water Velocity

When putting rule numbers to use (see fly depth formula, fly depth calculatorsink tip length formula, sink tip length calculator), it's best to make a rough estimate of water velocity.  While 3 mph is the speed of a brisk walk, much good steelhead water is slower.  Some is faster.  In addition, there is a natural tendency while wading to overestimate water velocity, which could lead to overestimating the amount of sink tip needed.  Overall, it's better to make a quick, simple measurement before applying these formulas.

The table below makes measurement easy -- especially easy if you are wading and using a Spey rod.  Mark your rod (e.g. with a small piece of tape) at point that is 13 ft 2.5 inches from the tip.  For most Spey rods, this point will be well down the rod near your hands.  Hold your rod out from your body far enough that the water disturbance from your body is not affecting the current, and point it directly downstream.  Now pick out a bubble, leaf, insect, or other oddment floating by and measure by stopwatch the time it takes to go 13 ft 2.5 inches.  That's it: read the water velocity from the table.  For example, if it takes 4.5 seconds for a leaf to move this distance, then water velocity is 2 mph.  Also notice, for ease of memory, that 3 seconds equals 3 mph.

Tip: For unusually clean water conditions (nothing floating downstream), carry a few twigs in your pocket and drop one in as needed.

Fly depth results depend not only on surface water velocity (as discussed above), but on subsurface velocities as well.  Under typical river flow conditions, water velocity is fastest just under the surface and much slower near the bottom -- especially when boulders or other obstructions are in the streambed.  It is often better to assume that the average water velocity over the entire water column is slower than the velocity measured at the surface.