July 26, 2013
There's no such thing as still water. Drop food dye in a glass of water, and watch it mix. Water that contacts the sides of the glass warms and rises. Cooler water in the center of the glass sinks and moves toward the sides to replace the rising warmer water.
Similar mixing occurs in lakes, reservoirs, and ponds. Differences in water temperature, compounded by wind, inflow currents, bottom topography, shoreline topography, and other factors cause water movement. Moving water creates currents and currents affect catfish behavior. Anglers who understand how and where currents form in lakes understand how to find catfish.
Currents develop in bodies of water from farm ponds to oceans. Ocean currents are far more visible and dramatic, but even ponds have many sorts of currents. "Lakes and reservoirs are incredibly dynamic," says Chris Filstrup, post-doctoral research associate of aquatic ecology and limnology at Iowa State University. "Water movement is caused by inflows, outflows, wind action, and convective heating and cooling and other forces, so it can be difficult to grasp all their interactions. But because water movement is based on the physical properties of water and physics, it's predictable."
Brian Waldman, a multispecies angler from Coatesville, Indiana, works for Dow AgroSciences when he's not fishing. "I work in research and development, so I've always tried to combine science with fishing," he says. "If you understand the basics of current, you improve your chances of finding fish. It also helps when you fish a new lake. If you identify areas with current, you identify places that attract fish."
Let's examine currents commonly found in freshwater lakes and reservoirs. Most are subtle and often difficult to identify, but influence fish behavior.
Wind-Driven Currents: Waves due to wind initiate several types of currents. Individual waves don't move much water, but over time their energy can shift significant masses of water in the upper few feet of a lake in the windward direction.
Longshore Currents: When waves strike a shoreline at an angle, a subtle longshore current can develop that moves with the wind along and parallel to the bank. Longshore currents and wave action combine to create situations that attract and concentrate channel catfish. Lanny Miller, retired fishery biologist for the Iowa Department of Natural Resources, says that when he needed to collect channel catfish for displays or demonstrations, he often headed to North Twin Lake, a shallow, narrow natural lake that lies in a north-south direction.
"I'd pick a day in mid-summer when the wind was blowing hard out of the south, and in the evening I'd fish one of several long, shallow points that stick out into that lake," he says. Wave action over the points, combined with longshore currents moving along the shoreline, made the drop-offs on the downwind side of the points hot spots for channel cats. "I'd drift a chunk of shad or stinkbait into the flat water on the backside of the point, and I could generally catch as many channel cats as I wanted."
Channel Currents: When longshore currents and wave action are funneled between the shoreline and offshore obstacles, channel currents form. Picnic Rocks, in Lake Superior near Marquette, Michigan, are small islands several hundred yards offshore. Under certain conditions, the passage between the islands and the mainland funnels and accelerates longshore currents and wave action to create treacherous currents that have claimed the lives of many swimmers. In smaller lakes, channel currents attract catfish to necked-down areas between the shore and an offshore island or significant obstruction.
If the wind is strong enough or blows long enough squarely onto a shoreline, water rises along that bank. In the Great Lakes, the build-up can be several feet higher than normal along that shoreline — as much as 10 to 20 percent of the breaking wave heights. Gravity eventually forces the water built up along that bank to return to the lake's basin. Several types of currents can develop as a result.
Undertow: When accumulated water folds under and flows back toward the main basin in a relatively uniform layer along the bottom of the lake, undertows form. A shallow shoreline that tapers into an old creek or river channel parallel to the shore is a prime place for channel, blue, and flathead catfish to corral potential food drifted over the drop-off by an undertow current.
Rip Currents: When water built up along a shoreline due to wave action and longshore currents is deflected or concentrated by a small point, sandbar, or obstacle, and returns to the main basin via a distinct, narrow path at a right angle to the shoreline, a rip forms. Rip currents in freshwater lakes generally are subtle. But they exist, fish are aware of them, and smart anglers learn to look for veins of slightly flattened water or debris mysteriously floating against wave action that hints a rip current may be present that may attract catfish.
Not all currents in lakes are associated with shorelines. Langmuir circulation is the cause of long, thin parallel lines of foam commonly seen on lakes on windy days. There are complex physics involved, but Langmuir circulation is akin to long cylinders of water revolving beneath the surface, parallel to the wind. One cylinder revolves clockwise while adjacent cylinders revolve counter-clockwise, creating alternating areas between the slowly revolving cylinders where water is either being pulled upward from deeper in the lake, or pulled downward from the surface.
"Where the downwelling occurs, there's a zone of little or no water movement on the surface," Filstrup says. "Foam and light debris, such as insect exoskeletons, can become trapped in this zone, creating the streaks of foam between the long cells of water that rotate slowly below the surface."
Salmon anglers on the Great Lakes and trout anglers in New Zealand have for decades keyed on the "scum lines," "windlanes," "windrows," or "windstreaks" associated with Langmuir circulation. Do catfish respond to concentrations of baitfish that develop from Langmuir circulation? "That's a good question," Waldman says. "I've never heard of anybody paying attention to whether or not they caught more catfish under the windrows of foam. But I think if those windrows set up over or near shallow points or other structure, it might be something to check out."
Inflow, Outflow, and Full Flow Currents: Natural lakes develop complex currents related to wind speed and direction, water temperature, and other inputs. Artificial reservoirs, especially those on large rivers, have all those currents from the river that runs through them. The temperature of water flowing into a reservoir dominates that current's behavior in the reservoir because cooler water is more dense than warmer water.
"If cold, dense water flows into a relatively warm lake, the inflow may sink to the bottom and follow the submerged channel of the old river or stream at the bottom of the reservoir," Filstrup says. "I did my doctoral research on drinking-water reservoirs in central Texas. When water densities were right, water would flow down the old river channels and not mix much with the overlying lake water. Conversely, if the inflowing water was significantly warmer than the lake water, the inflow would flow over the surface until it began to cool and then it blended with the underlying water. Inflowing water enters a lake at the depth that matches its own density or temperature." Since inflowing water is a source of oxygen, nutrients, and other factors that attract baitfish, catfish often orient to subtle currents at the depths those currents form within a reservoir.
Many anglers have learned that summer thunderstorms create temporary currents that ring the dinner bell for channel cats. Drainage culverts and ditches that discharge stormwater into lakes flush insects, worms, and other delicacies into the mouths of waiting fish. "Foam and lightweight debris float on the surface of the lake," he says. "But the food catfish savor, like nightcrawlers and insects, is heavier and sinks to the bottom close to the discharge point, so that's where the fish hold."
Anthony Ford, catfish guide on Truman Lake and Lake of the Ozarks in Missouri (catfishingguide.com), says inflows, outflows, and discharges are triggers for blue cats on Lake of the Ozarks. "It's a river-run type of lake, and blues are incredibly sensitive to when they open the gates at Truman Dam, the head of the lake," he says. "It's 93 miles from Truman Dam downstream to Bagnall Dam. I've been at mid-lake and had all my rods go down at about at the same time as the blue cats suddenly went nuts. I called some friends fishing up by the dam and we figured that they started biting within minutes of when the gates opened. I don't know if there's a pressure surge the fish sense, but they know when a rise is coming."
During the warm season, lakes deeper than about 15 feet can stratify if there's no significant inflow to mix the water. Sun-warmed water stays near the surface, cooler water stays near the bottom, and a sometimes abrupt line of demarcation, defined by 10 or more degrees difference in temperature, delineates the thermocline. There's little water exchange between upper and lower layers of a lake once a thermocline develops, so water below the thermocline can become oxygen deficient. Fish and smart anglers avoid that deoxygenated zone.
"There's no use fishing deeper than the thermocline, once it sets up," Ford notes. "If a lake averages 30 feet deep, the thermocline in a normal summer sets up at around 12 to 15 feet. In a warm summer it might be deeper, around 18 or 20 feet."
"Turnover" occurs each fall in lakes with a thermocline, when surface waters cool and become close enough in temperature to bottom waters so that the layers mix, often in a short time. "Once surface waters begin to cool in fall, it usually takes one major wind event or storm to initiate turnover," Filstrup says. "The lake may become turbid as organic matter that had been trapped near the bottom during summer is brought up into the surface layers. Turnover is a good thing for the health of the lake because it replenishes oxygen near the bottom and recharges nutrients near the surface."
Thermocline boundaries aren't the only thermal features anglers may encounter in large lakes. In spring, salmon anglers on Lake Michigan look for "scumlines," zones of "flat" water, and floating debris parallel to the shoreline. Offshore scumlines form when spring winds push warm water into shorelines for days at a time. When conditions are right, cold water from deeper in the lake upwells in a narrow band parallel to the shore to replace the warm surface water that's pushed toward the bank. The upwelling of cold, nutrient-rich water forms a zone of slack water that marks a hot spot for predator species attracted by baitfish that feed in this nutrient-rich upwelling.
Waldman experienced a similar phenomenon one spring on Indiana's Heritage Lake. "I was near the dam, working a cove where the wind was blowing in and the surface water temperature was in the upper-50°F range," he recalls. "As I worked out toward the main lake, I noticed the water temperature dropped 10 degrees in a matter of yards. I started making circles and could map out what was like a wall of cooler water being pulled up from deeper in the lake. I'm learning to pay attention to things like that, because fish do."
Observant anglers are aware of how wind, water temperature, shoreline topography, and other factors cause currents in lakes. Sometimes currents are strong enough to see due to changes in water clarity, the presence of scum or debris, or because of changes in water temperature. Other times, currents are so subtle that they can be detected only with sensitive instruments. But just as we humans can detect the aroma of a steak on a neighbor's outdoor grill, wafted on a breeze so gentle it barely stirs cottonwood leaves, currents inform catfish of potential meals and create opportunities for anglers.
*Dan Anderson, Bouton, Iowa, is a frequent contributor to In-Fisherman publications and an avid catfish angler.