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current, a river primer

Norm M

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We as river fishermen are affected by current in all aspects of our fishing experience. How many

of us really understand how it functions and how it affects our fishing? This will be the first in a

series of articles I plan to write about current, current situations and objects in the water that

affect current flow. The fisrt article will be the primer, so to speak, of the series.

To start with, gravity moves water downhill, this movement is what we call current. The steeper

the gradient, the faster the current flow. Riffles and runs are what we associate with this faster

flow. What we think of as pools are places where the gradient decreases and/or there is an

obstuction that slows the current. The gradient is the inclination or slope of the land's surface in

the direction of the flow.

The velocity of the flow is affected by several other factors. They are discharge, which is a

quantity of water passing through any cross section in a given unit of time and the form-ratio,

which is the proportion of the depth to the width of the stream. Water temperatures have some

effect as warm water is more fluid than cold water. In straight flows the maximum velocity

occurs in midstream near the surface. In bends the maximum velocity occurs near the surface

on the outside of the curve.

There are two types of flow recognized in limnological texts. Laminar flow is more or less

defined as all the water moving in a parallel course in one direction. These texts state that

laminar flow is seldom found in nature. Turbulent flow is defined as multiple eddies in a

variety of sizes with all the water moving in all directions as the main flow goes downstream.

This mixing of the water is caused by friction with the air-water interface, the bottom of the

river, the shoreline and any obstructions in the water's path. This friction that slows the flow

and the upper water that moves faster rolls and mixes the water which aids in the distribution

of oxygen from top to bottom. The difference in flow rates from top to bottom contributes to

the bow in your line as you fish.

Fish hold on the bottom of rivers because the drag of friction on moving water significantly

lowers current velocity. This creates an area you can think of as a slack water zone. The

slackwater zone is a place where fish can hold and conserve energy. They can hold until

they need to move for reasons of survival, i.e. predation, changing water levels or spawning.

This bottom slack zone is where most of the fish will be found most of the time. Where do

you think you should be fishing? The larger the objects on the bottom of the river and the

faster the current flow the larger this slack zone will be. These two factors- size of objects

and current velocity- can cause distrubances on the surface of the water that we note when

reading the water.

The Law of Fluid Dynamics also affects current flow. This involves Conservation of Momentum

if I understand the texts correctly. Basically what I got from this was that water does not like to

bend as it goes around corners and when the velocity of water decreases it's pressure increases.

Please don't ask me to go into great detail as that was rwo aspirin reading.

When water hits the front of an obstruction it stops and moves sideways toward the current flow.

It also moves both toward the bottom and the surface at this point. You may get a visible bulge

on the surface if when the speed decreases the increase in pressure is enough to push water

up in the surface. You can also get a slackwater pocket at the base of the obstruction due to

these changes. This pocket, if it forms, is a key feeding area for the most dominant predator

in that spot.

On the downstream side of the obstruction you get another slackwater area. This is because

water does not like to bend. The size of this slackwater area is limited only because the

pressure of the faster water going around the obstruction is lower than the higher pressure of

the slower water in the slack area. this causes a mixing of the waters as it moves from the

slacker water to the faster water. The turbulence from this mixing is what we think of as a

current seam. The slackwater area gets narrower as you look downstream because it's

losing it's energy to the lower pressure of the faster downstream flow.

A slackwater pocket forms at the base of a riffle[lift] on both the upstream and downstream

sides of the riffle. This pocket is an excellent feeding area especially in warmer water. As the

faster moving lower pressure water moves over the top of the slower moving higher pressure

water mixing occurs limiting the size of the slackwater area. This causes the turbulence

which is a familar sight to river anglers. On the upstream side of the riffle the bulge that is

present is a result of water being pushed up by the change in water pressure.

When a river bends the current picks up speed on the outside edge and a current seam

forms on the inside edge where the current slows. The faster current on the outside moves

sediment and carves away at the bed and bank of the river. This is why you have deeper

water on the outside of a river bend. The slower current allows sediment to be deposited

on the inside which is why the water is shallower and you get that familar point bar at

that location. if the angle of the bend is sufficient an eddy is formed. In this eddy friction

with the bottom and the shore further slows the current. This variety of current speeds,

depth changes, changes in the direction of the current and slackwater zones make this

a place for holding out of current and for excellent feeding opportunities.

Turbidity in a river is the presence of suspended solids in the water. Turbidity reduces the

transmission of light either thru scattering or absorption. This reduction in light can affect

water temps, the amount of aquactic vegetation, fish location and fish activity levels. From

what I read the amount of sediment load can affect current velocity in different ways due to

a number of different factors. The consensus seemed to be that with a higher sediment load

there would be a slight reduction in velocity once everything was factored in. Once again

this was two aspirin reading that would require lots of specific details to go any further.

Now for a brief primer in reading water. Start by looking at the surface for differences in

speed and the seams created by those changes. Look to changes in curent direction,

turbulence such as boils and bulges, slicks and changes in water color that denote

differences in depths, You should look for areas where the current narrows forming

feeding lanes that concentrate food washing downstream. If you can see a foam line

that is an excellent way to track feeding lanes. If multiple foam lines or feeding lanes

converge that can be a key spot for the most dominant predator in the area. Look for

objects in the water or in the shoreline that can deflect current influencing current

direction. Next look for objects in the water that can serve as ambush points for feeding

or for cover when fish are spooked by predators or careless, heavy footed fishermen.

Remember that boils on the surface indicate the presence of underwater objects

that can hold fish. These boils will be downstream of the object, just how far depends on

the size of the object and the current velocity.

I know this was a little technical but hopefully it will help those new to river fishing. It may

also offer new insights to crumudgedy old river rats.


Peace Be Unto You

Norm Minas aka Creekyknees



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