“If you’re not hanging up on the bottom occasionally, you’re not in the zone of the fish.” This is a common sentiment I have heard frequently from fellow fishers and read in several books. Recently, several innovative fly anglers advocated for fishing “in the strike zone” a few inches above the stream bed sediments. Understanding hydraulics, the science of flowing liquids, can help us understand what life may be like at the bottom of a river. 

Anglers describe conditions near the bottom of a river where water velocity is reduced significantly from what we see on the surface. Water flowing through a channel is influenced by the slope, depth, and boundary conditions. Boundary conditions consist of the stream bed and the substrate as well as the banks, and any vegetation that extends into the water. The relative roughness of the boundary condition interacts with the water changing flow paths and energy. 

Irregular Water Surface Indicative of Turbulent Flow

Channels with smooth boundary conditions act similarly to a hose, where the velocity of water is furthest from the boundary, experiencing laminar flow. In a natural channel, the high maximum velocity position occurs normally near the surface of the point with the greatest depth. Conversely, channels that are rough, like those with large boulders irregularly spaced in the stream bed and lots of overhanging vegetation cause the water to mix vertically. Flow in rough channels is referred to as turbulent flow and is evident anywhere the water surface is irregular. 

Laminar and Turbulent Flow (Adapted from Crowe et al)

Regardless of the boundary roughness, a physical phenomenon known as the law of the wall applies. The law of the wall is a hydraulic theory which describes the change in velocity through a transition from a stationary boundary to a flowing liquid, whereby the very thin layer liquid touching the immobile boundary is not moving. If the water is attached to a stationary feature, it must not be moving but very quickly particles will accelerate toward the velocity of the surface. This transition zone, known as the viscous sublayer, can be millimeters to centimeters in depth with a larger buffer zone above. In total this area can be up to 20% of the total depth in very rough channels. 

Velocity Distribution Curve (Adapted from Crowe et al)

These transition zones are important to fly anglers because they are where the insects live. Many aquatic insects scrape algae from the surface of rocks or attach themselves to the rocks to filter organisms and organic matter suspended in the water. The body shapes of the insects are flattened in the dorsal ventral directions to allow them to cling to the curved surface of each rock and to stay within the hydraulic transition zones. An aquatic insect is essentially weightless and neutrally buoyant. Any perturbation of the water can lift them from the rock and propel them into the water column above or downstream, into an unsafe space, the realm of the fish.

With turbulent flow in the transition zone, velocity is moving in many different directions, vertically upward, downstream, and laterally around obstructions. In addition to accidental dislodgement, certain insects purposefully release themselves from rocks as part of their life cycles in a process known as behavioral drift. This adaptation can distribute the insect’s population or identify beneficial habitat for other life stages. It also presents vulnerable insects as a food source. 

Near this lower velocity transition zone is an optimal combination of food and resting areas for the fish. Identifying this sweet spot in the water column is a challenge as water depth and substrate size change rapidly throughout a riverbed and often you cannot see through water with slight turbidity. Even in clear water, depth is difficult to estimate visually, as water bends light and magnifies submerged items. Calibrating your sight and your movement of a fly line takes practice, attention, and likely some luck. Overweighting a rig and losing flies that become lodged between large rocks or embedded in submerged logs and branches is frustrating, expensive, and time consuming. 

Measuring the Depth of Water Using Your Leg is Easier Than a Visual Estimate

While reading the water in attempts to find feeding fish, it is important to remember the law of the wall and the velocity transition zone. In very rocky stream beds, soft spots will form between the obstructions and create larger depths of slower velocities. The greater the irregularity, the greater the height of the transition zone. Smoother stream beds will require flatter slopes and deeper water to create large enough transition zones to be comfortable for feeding fish. These transitions are “the strike zones”, where sublayers create habitat for insects and fish.

 In college, I had a hard time visualizing and accepting the law of the wall, but the more I fish, the more I see its importance and application. Open channel hydraulics can be a bit nerdy and challenging, but it is helpful for reading a stream. 

 Keep Mending…  

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