October 18, 2016
Fishing Navigation Structures In Big Rivers
The U.S. Army Corps of Engineers calls them "training structures" because they train a river's current to follow desired channels. These structures include wing dams, weirs, chevron dikes, revetments, and a dozen other rock-covered, artificial structures. Training structures come in many shapes and sizes but share one thing in common — they alter flow and current speed, creating habitats catfish use.
These structures are important to anglers because catfish relate to changes in flow created by them. Often it's to find a place to rest out of a river's current. Other times catfish orient themselves to intercept food carried by current. In some situations it may not be as much about the velocity of the current as much as it is about its consequences — scour holes or channels carved by strong currents, or logjams that develop where reduced currents deposit woody structure.
"Some wing dams hold catfish year in and year out, and others rarely hold fish," says Captain Ryan Casey, catfish guide based in St. Louis. "The dikes may look the same to us, but there's definitely something about some dikes that fish like, and other dikes that they don't like."
Ever-changing water levels in rivers add the "when" to the process of figuring out why and how catfish relate to training structures. Higher or lower water levels increase or decrease current velocity over and around them.
"Fish prefer certain velocities for resting and feeding," says Kirk Hansen, fishery biologist for the Iowa Department of Natural Resources. "They move to find the velocity that fits their needs, so at low river levels we see more fish out toward the tips of wing dams. At high water levels that create faster current, they tend to move closer to shore along the wing dam to avoid the stronger currents."
Many training structures, especially wing dams, are built in multiples. Catfish sense the differences in a river's current between the first and last structure in a series. "In low-water situations with slower current speeds, catfish move to the upper wing dams," Hansen says. "As river levels rise, current velocity increases and the fish tend to reposition to downstream wing dams where the cumulative effect of the dams reduces the overall current."
Many wing dams are on the outside bends of rivers and positioned to deflect current to reduce bank erosion and broaden the navigable channel. At times wing dams are built on the inside radius of major bends to deal with unique erosive currents that develop on tight bends. Wing dams on inside bends offer catfish alternatives when rivers run high and fast.
"Structures on inside bends are definitely worth paying attention to," Casey says. "People don't realize how much the bottom changes when a river is rising or falling. When the river is falling, it scoops out the holes on the upstream side of dikes on inside bends. When the river is rising it tends to silt-in holes on the upstream side of dikes on inside bends. That's the opposite of what you might expect, but that's the pattern I've seen."
Many anglers focus on holes and troughs associated with wing dams. Big blue catfish love the strong current and deep water at the tips of wing dams. Flatheads lurk out of the main current in trees and debris washed into scour holes, or on the lee side of logjams lodged on the tops of wing dams. Channel catfish favor the milder currents and current breaks associated with turbulence on top of or just downstream from submerged wing dams. At steady river levels, all species when feeding frequent the trough that develops parallel to the upstream side of many wing dams.
The potential for finding catfish around wing dams makes them primary targets for many anglers — which makes them secondary targets for Casey. In his quest to put clients on top of the biggest catfish, he's learned to target areas near training structures often overlooked by other anglers.
"Everybody thinks deep water is the secret to catching big catfish, so everybody fishes the holes around wing dikes," he says. "I look for good places where other people don't fish. I've had good luck fishing shallow feeding flats near wing dikes, and not just in the summer. Two of the biggest blue cats I've put in my boat came from less than 5 feet of water. We caught a 105-pounder last winter from 4 feet of water and an 85-pounder in March in 5 feet of water.
"Those fish broke the rules about how big blues are supposed to be in deep water, especially in cold weather," he says. "That's why I like big flats behind or between wing dikes. Any fish out on those flats are feeding and aggressive. I watch my side-imaging sonar, and if I see a lot of big fish scattered across a flat, pointing various directions, they're probably carp, buffalo, or other rough fish. But if I see a bunch of fish on a flat that are all oriented upstream, they're probably blue cats, big ones, and it's time to put out baits."
Casey also uses experience to eyeball the river and its surroundings to identify potential hotspots associated with training structures. He says adjacent shorelines tell a lot about the river's bottom between wing dams. "Sandy shoreline is okay, a vegetated shoreline is okay, but what I really like to see between dikes is dark mud bank," he says. "Catfish don't like a silt bottom because that silt gets stirred up easily and I think it bothers their breathing as they suck water across their gills. But a firm mud bottom between dikes is good. One thing I've noticed on flats is that cats hold tightly behind any little sand dune or small pocket. An 80-pound catfish can tuck behind a 2-foot lip and be happy."
Similar small breaks in current also attract catfish to some of the largest training structures in rivers: riprap rock revetments. The engineering theory behind riprapping shorelines with large boulders is to armor the bank and prevent erosion. Novice anglers often dismiss long riprap shorelines as "rock deserts," but veteran catmen know better.
"Riprap banks have all sorts of nooks and crannies for invertebrates and crustaceans, little points and pockets that create eddies, and back-currents that attract baitfish, so the entire face of an armored bank can be great fishing," says Joe Jordan, environmental biologist with the U.S. Army Corps of Engineers and avid multispecies angler. "Any place there's an eddy or especially a short stretch of back-current can hold fish. And the junction of where riprap meets the normal bottom of the river is a highway for fish. They move up- and downstream along the base of the riprap. I experiment to find that rock-to-bottom transition, and that's where I want my bait to be."
Transitions between rock and mud, currents and slack water, and deep and shallow water are focal points for catfish associated with river training structures. Here are some of the most common training structures used on rivers, and rules of thumb on how they alter current direction and speed, river bottom topography, and other variables that affect catfish behavior.
Wing dams/dikes: Wing dams jut out perpendicular from a river's shoreline, often along outside bends. Their purpose is to deflect the river's current away from the outside bank toward the center of the river to help maintain a deep navigation channel. They produce excellent catfish habitat.
Some wing dams are designed to rarely appear above the river's surface. Others are submerged only during floods. Either way, there is usually a trough carved parallel to their upstream side. Major scour holes develop at the tip of a wing dam as currents rush around the end. Large holes develop on the downstream side of wing dams either due to water moving across the top and then diving down the backside, or because of water flowing around the tip and back toward the shore.
"Never assume what you see on the surface around a wing dam is what the current is doing at the bottom," Casey says. "The surface water might be flowing downstream, but at the base of the wing dike it may be moving sideways, toward, or away from shore. Rig and place your baits to match how catfish relate to currents on the bottom."
Notched wing dams: Placing an extended notch in the surface of a wing dam doesn't change its value as a training structure but dramatically enhances its value to aquatic life.
"Biologists requested them," Jordan says. "A notch adds more areas of altered velocity and creates more eddies and areas of reduced current, so there are more opportunities for invertebrates and crustaceans and everything up the food chain to find conditions they like. There generally seems to be more fish associated with notched wing dams than conventional wing dams."
Spur dikes: Similar to wing dams, spur dikes are short rock structures that jut from the shoreline perpendicular to current flow. Spur dikes create holes, troughs, and flats similar to wing dams, but also encourage the development of a meandering main mid-river channel. Depending on the distance between the dikes, the channel may pass around the end of each dike and then veer toward the bank between dikes, creating a series of subtle submerged inside and outside bends in the main channel.
That meandering main channel is emphasized in situations where spur dikes alternate on opposite sides of the river. The goal in fast moving rivers is to create more side-to-side movement to moderate the river's tendency to create a narrow, deep, high-velocity channel. One of the results of alternating spur dikes is a main channel that meanders back and forth subtly between opposing dikes, with resulting submerged inside and outside bends.
L-dikes: Adding a short leg on the downstream end of a wing dam creates an L-dike, which produces larger scour holes and significant downstream flats.
"There are a lot of L-dikes on the lower Missouri River all the way up to Kansas City," Casey says. "They are good year round, but really good for winter fishing. There's usually a big hole at the tip and a trough around the end of the "L," and a nice flat on the backside. Those flats warm fast in the winter or spring so they're prime places for cold weather fishing. If there's a blowout where part of the dike has washed out (akin to a notched dike), those usually have eddies and back-currents that are good any time of year."
Weirs: Weirs come in several sizes and designs, but are generally submerged rock structures placed in main channels or associated with channels along outside bends. They're often W-shaped and are used to redirect primary currents. From a catfish's point of view, they're bottom structure that provides current breaks, ambush points, feeding flats, scour holes, and just about everything a catfish wants.
"We have W-dikes below Hannibal on the Mississippi River," Jordan says. "They add a lot of changes in current and velocity. We've seen strong increases in biodiversity associated with them, and biodiversity attracts gamefish like catfish."
Multiple scour holes develop on the downstream side of W-shaped weirs, creating hidden mid-river holes, deep-water rock bars, and rocky shoreline structure if they're connected to the shoreline.
Chevron dikes and blunt-nosed chevrons: Chevrons are V-shaped or U-shaped dikes generally placed on the shallow side of a river's channel, with their closed ends pointing upstream, to direct water toward the navigation channel. In the lower Mississippi, where strong currents have eliminated many natural islands, the Corps of Engineers has dredged material from the river's bottom to create artificial islands then installed blunt-nosed chevrons upstream from the islands to protect them from erosion.
"There are chevrons in Pool 18 on the Mississippi River that are above water at normal pool," Jordan says. "I see catfish anglers fishing at the upstream point of the structures, in the area of slack water where the current is deciding if it's going to go to the left or right side of the structure. That position lets them fish both current seams from the same location."
Revetments: Shorelines riprapped with large boulders or concrete chunks attract a variety of species. The myriad nooks and crannies provide a smorgasbord of invertebrates that feed all levels of the food chain. The irregular surface creates ever-changing current breaks, eddies, and back currents that provide shelter and ambush points for catfish of all sizes.
"Bigger rocks create bigger cavities that attract bigger fish," says Greg Gelwicks, fishery biologist for the Iowa DNR. "Areas of big, broken slabs of concrete are especially attractive to big flatheads during the spawn, when they're looking for nesting cavities. Smaller flatheads up to 14 inches long use riprap areas a lot because the size of the cavities matches their body sizes."
Closing Structures: In the complex island-slough-backwater systems on the upper Mississippi, closing structures are low rock dams or sills placed across the upper ends of side channels of the river to direct flow toward the main channel. Closing structures are generally below the surface at normal pool, with a wide notch to allow recreational boat passage. The turbulence created by water moving over the shallow dam, the presence of logs and structure captured on its surface, and other hydrodynamic qualities make closing dams prime prospects when looking for catfish.
Which is true about nearly every training structure used to alter flow in rivers. While some hold more fish than others for a variety of known and unknown reasons, all wing dams, W-weirs, riprap revetments, chevrons, and other training structures consistently hold more catfish than open stretches of rivers.
"There's a reason a lot of my waypoints are around wing dikes and weirs and other navigation structures," Casey says. "They flat-out attract and hold fish."
Dan Anderson, Bouton, Iowa, is a freelance writer who regularly contributes to In-Fisherman publications. Contact Capt. Ryan Casey at 314-477-8355, showmecatfishing.com.