Lakes produce powerful currents that drag swimmers underwater and offshore, but the force responsible is almost never a true “undertow.” The word undertow implies a steady sheet of water pulling you down beneath the surface, and while a thin return flow does exist near the lakebed under breaking waves, the real danger in lakes comes from rip currents, wind-driven circulation, and internal water movements that most people never see coming. The Great Lakes alone account for dozens of drowning deaths in a typical year, many of them linked to currents that swimmers did not expect to encounter in freshwater.
What People Mean by “Undertow” and What Actually Happens
When someone says they felt an undertow at a lake beach, they usually mean they were pulled away from shore or felt a downward tug on their legs. In nearly every case, what they experienced was a rip current, not an undertow in the strict sense. The two phenomena are related but different. Under breaking waves, water that washes up a beach slope has to flow back out somehow, and some of it returns along the bottom as a thin, seaward-moving sheet. That bottom return flow is the technical “undertow,” and it is typically weak, confined to the lowest few inches of the water column near the bed. Researchers studying lacustrine (lake) nearshore zones have documented these near-bottom offshore flows, noting that they contribute to sandbar migration by moving sediment seaward under wave action.1ScienceDirect (Marine Geology). Hydrodynamics and sediment transport within the inner surf zone of a lacustrine multiple-barred nearshore But in terms of danger to swimmers, this bottom return flow is not the main threat. The real culprit is the rip current.
A rip current is a narrow, fast-moving channel of water that flows away from shore, often through a gap in a sandbar or alongside a structure like a pier or jetty. Unlike the thin undertow layer creeping along the bottom, a rip current extends from the surface to the lakebed and can carry even strong swimmers hundreds of meters offshore in minutes. Rip currents measured near coastal structures on Lake Michigan reached speeds around 0.2 meters per second and persisted for several hours at a stretch.2ScienceDirect (Journal of Great Lakes Research). Rip currents near coastal structures in Lake Michigan: Characterization and assessment for warnings – Section: Rip current characteristics That speed may not sound dramatic on paper, but it is faster than many recreational swimmers can sustain, and the current’s persistence is what makes it lethal. People exhaust themselves fighting it before they realize what is happening.
Why Rip Currents Catch Lake Swimmers Off Guard
Ocean beachgoers are at least somewhat aware that rip currents exist, thanks to decades of public education campaigns and lifeguard signage. Lake visitors tend to have a very different mental model. Freshwater beaches feel calm, familiar, and safe. Many people assume that dangerous surf conditions are an ocean-only problem. Research on beachgoer perceptions at Great Lakes destinations has found that this false sense of security is widespread, and that few Canadian Great Lakes beaches even have lifeguards, let alone a regional safety strategy to warn swimmers of dangerous surf and currents.3Canadian Geographies / Géographies canadiennes. Perception of beach safety at a destination beach on the Great Lakes
The problem is compounded by the fact that rip currents on lakes can form quickly in response to shifting wind and wave conditions. On a large lake, a sustained onshore wind builds waves that look manageable from the beach but generate strong return flows through gaps in the sandbars just below the surface. Piers, breakwaters, and harbor walls focus these flows into even tighter, faster channels. A swimmer standing waist-deep may feel nothing unusual until they take a few steps into slightly deeper water and find themselves in the grip of a current they cannot overpower.
How to React if You Are Caught in a Lake Current
The instinct when pulled offshore is to swim directly back toward the beach. Mathematical modeling of rip current escape strategies confirms what lifeguards have been saying for years: swimming straight against the rip is almost universally the worst option. It can require several times more power from the swimmer than alternative strategies, making exhaustion and drowning far more likely.4Natural Hazards. On the swimming strategies to escape a rip current: a mathematical approach The recommended approaches are to swim parallel to shore until you exit the narrow rip channel, or to float with the current until it dissipates and then swim back at an angle. Most rip currents are only a few tens of meters wide, so moving laterally even a short distance puts you back into water that is flowing toward shore rather than away from it.
This advice applies equally well to lake rip currents and ocean ones, because the underlying physics is the same. The rip is a concentrated jet of water heading offshore, not a force pulling you to the bottom. Once you stop fighting it and move sideways, the current loses its hold.
Wind-Driven Currents and Upwelling
Rip currents are a nearshore hazard, but lakes also generate basin-wide circulation patterns that most swimmers never perceive directly yet shape everything about how the water moves. Wind is the dominant driver. When a strong, sustained wind blows across a lake, it pushes surface water downwind, piling it up against the far shore. That displaced water has to go somewhere, so deeper water flows back toward the upwind shore, setting up a large-scale circulation cell.
In lakes big enough for Earth’s rotation to matter, the returning flow gets deflected sideways by the Coriolis effect, producing what researchers call Ekman transport. Studies in elongated subalpine lakes like Lake Garda have shown that strong northerly winds in winter displace significant volumes of water laterally, producing intense downwelling along one shore and upwelling along the other.5JOURNAL OF LIMNOLOGY. Wind variability and Earth’s rotation as drivers of transport in a deep, elongated subalpine lake: The case of Lake Garda Observations in moderately sized lakes have also recorded upwelling both at the upwind boundary and along the coast to the left of the prevailing wind direction, driven by this same Ekman divergence.6Water Resources Research. 3D Flow Structures During Upwelling Events in Lakes of Moderate Size
Upwelling can be dramatic. In at least one deep lake, sustained winds exceeding about 10 meters per second pushed deep water upward by more than 70 meters, bringing cold, nutrient-rich water from the depths all the way to the nearshore zone.7Limnology and Oceanography. The setup and relaxation of spring upwelling in a deep, rotationally influenced lake For swimmers and boaters, the practical consequence is sudden temperature drops near shore when the wind shifts. Water that was pleasantly warm in the morning can become shockingly cold by afternoon as deep water surges up to replace the surface layer that blew away. Cold-water shock is itself a drowning risk factor, causing involuntary gasping and loss of coordination.
Seiches and the Bathtub Effect
Once wind piles water against one end of a lake and then dies down, that mound of water sloshes back. The resulting oscillation is called a seiche, and it works much like tilting a bathtub and watching the water rock back and forth. In large lakes the effect can be measured in meters of water-level change at either end, and the sloshing can continue for hours or even days after the storm that triggered it has passed.
Storm surge and seiche events on the Great Lakes are generally forced by severe storms, initially resulting in wind-driven elevation of water level on one or more sides of the lake, followed by a rebound and periodic oscillation between opposing shores.8PubMed Central. Statistical analysis of storm surge and seiche hazards for Lake Erie Lake Erie is particularly susceptible because of its shallow depth and long east-west axis. A strong westerly storm can raise the water level at the eastern end by a meter or more and drop it at the western end, then the oscillation reverberates for days. These water-level swings create strong horizontal currents as the entire lake volume shifts back and forth. They also catch people off guard: a beach that was calm and dry an hour ago may be knee-deep in rapidly moving water as the seiche pulse arrives.
Hidden Currents Beneath the Surface
Most lakes that are deep enough to stratify in summer develop a warm upper layer (the epilimnion), a cold lower layer (the hypolimnion), and a transition zone between them (the thermocline). These temperature layers create an internal structure that can oscillate independently of what is happening on the surface, producing what are called internal seiches. You cannot see them from above, because the surface stays flat, but beneath it the boundary between warm and cold water may be tilting and rocking by meters.
Monitoring in Lake Zurich found that the dominant internal response to wind impulses was a first-mode internal seiche with an average period of about 44 hours, meaning the internal boundary rocked back and forth on roughly a two-day cycle.9Limnology and Oceanography. Wind‐induced internal seiches in Lake Zurich observed and modeled In Lake Constance, the passage of basin-scale internal waves generated shear instabilities that produced large-amplitude density overturns of a meter or more in the thermocline, mixing warm and cold water together in bursts.10Limnology and Oceanography. Internal waves and the generation of turbulence in the thermocline of a large lake
These internal waves also drive horizontal currents that can flush entire embayments. In the long, narrow Kempenfelt Bay of Lake Simcoe, researchers found that internal wave-driven currents produced horizontal excursion lengths of several kilometers, effectively pumping water in and out of the bay far faster than river input alone could explain.11Limnology and Oceanography. Internal waves pump waters in and out of a deep coastal embayment of a large lake If you are swimming in a deep bay during one of these flushing events, you could experience a slow but persistent drift that carries you well away from where you entered the water, even on a day when the surface looks perfectly calm.
River Plumes and Density-Driven Flows
Lakes fed by large rivers have another source of strong subsurface currents that are invisible from above. When a sediment-laden river enters a lake, the incoming water is often denser than the surface layer because of its suspended particles and sometimes its temperature. Instead of spreading across the surface, the river plunge dives below the surface to a depth where its density matches the surrounding lake water, then spreads out horizontally as an interflow. Observations in Lake Geneva documented this process in the Rhône River plume, which flows as an interflow within the thermocline of the stratified lake.12Journal of Geophysical Research: Oceans. From Particles to Flocs: Revealing Where Flocculation Occurs in the Nearfield of a Negatively‐Buoyant River Plume in a Large Lake (Lake Geneva)
In fjord-type lakes with narrow, deep basins, river-induced currents can dominate the entire lake’s circulation. Modeling of the Thompson River’s entry into Kamloops Lake in British Columbia showed that the river water sinks to a matching density depth, spreads horizontally, and then flows along the lakeshore in a current balanced between pressure gradients and Earth’s rotational deflection. The conclusion was straightforward: river-induced currents can influence or even dominate circulation patterns within a fjord lake.13Journal of Geophysical Research: Oceans. River‐induced currents in a Fjord Lake For swimmers, the practical takeaway is to be cautious near river mouths. The turbidity reduces visibility, the currents are often stronger than the calm lake surface suggests, and the water may be significantly colder than what surrounds it.
How Lake Currents Differ from Ocean Currents
People sometimes assume that because lakes are smaller than oceans, their currents must be proportionally weaker and simpler. The reality is more nuanced. Lakes lack tides (or have only negligibly small ones), so the entire tidal component of ocean circulation is absent. They also lack the massive thermohaline circulation that drives deep ocean currents across entire basins. But in exchange, lakes are far more responsive to local wind forcing. A storm that would barely register on the open Pacific can completely reorganize the circulation of a midsize lake in a matter of hours.
The wave field in lakes also behaves differently. Fetch-limited shallow lakes produce waves that are steeper and younger than open-ocean waves, and these waves can collapse by white-capping at lower wind speeds and travel at lower phase velocities than their ocean counterparts.14Advances in Science Research. Observation of wave-driven air–water turbulent momentum exchange in a large but fetch-limited shallow lake This means that momentum transfer between wind and water in lakes does not follow the same rules derived from oceanographic observations. The practical effect is that waves on a lake can become dangerous at wind speeds that would produce only a moderate chop on the open ocean, and the associated nearshore currents ramp up faster as well.
Another difference is the thermal bar, a phenomenon unique to large freshwater lakes in spring and fall. Because freshwater is densest at about 4°C, warming nearshore water in spring can create a vertical front where 4°C water acts as a barrier between warmer inshore water and still-cold offshore water. Observations of the spring thermal bar in Lake Ontario found that this front inhibits horizontal mixing, effectively keeping two separate water masses side by side.15Limnology and Oceanography. Circulation and turbulent exchange characteristics during the thermal bar in Lake Ontario A swimmer crossing this front can encounter a sudden and extreme temperature change, with all the cold-shock risks that entails.
What Happens Under the Ice
Lake currents do not stop in winter just because the surface freezes over. Under ice cover, the driving forces shift. Without wind, the main circulation drivers become heat exchange with the lakebed sediment, solar radiation penetrating the ice, and density differences in the water column. Research on ice-covered lakes has documented a progression of circulation patterns: an early-winter convection cell driven by heat pulses from the sediment, a quiet middle period, then a radiatively driven mixed layer forming in late winter as sunlight warms the upper water through thinning ice. Just before the ice breaks up, rotational features briefly appear as the developing temperature structure interacts with Earth’s rotation. Once the ice finally clears, the resulting hydrodynamics are complex and can take nearly a month to settle into open-water patterns.16SpringerLink (Aquatic Sciences). Transitional circulation patterns from full ice cover to ice-off in a seasonally ice-covered lake
For anyone who swims, kayaks, or ice-fishes on lakes that freeze, the relevant point is that the water underneath is not still. Currents under the ice are typically slow compared to summer wind-driven flows, but they exist, and they matter for water quality, oxygen distribution, and the structural integrity of the ice itself. Localized currents near river inflows or warm springs can thin ice from below in ways that are invisible from the surface.
How Researchers Actually Measure Lake Currents
Understanding the invisible currents described above requires specialized instrumentation. Researchers deploy two broad categories of tools: fixed moorings and drifters. Fixed moorings anchor strings of current meters and temperature sensors at various depths, recording data continuously for weeks or months. The Lake Zurich study mentioned earlier used 31 current meters and 120 temperature sensors across 12 moorings.9Limnology and Oceanography. Wind‐induced internal seiches in Lake Zurich observed and modeled This kind of dense instrumentation network captures the full three-dimensional picture of how water moves at different depths.
Drifters take a complementary approach. These are floating devices released into the water that follow the current and transmit their position, essentially tracing the path a parcel of water takes. Lagrangian drifters designed for lake research consist of a low-friction surface float containing GPS and other sensors, tethered to a high-friction drogue at whatever depth the researcher wants to track.17Water Resources Research. An Economical Open‐Source Lagrangian Drifter Design to Measure Deep Currents in Lakes More sophisticated versions mount acoustic profilers on the drifter to record velocity at multiple depths simultaneously as the device moves, revealing vertical structure in the flow that a simple surface drifter would miss.18Limnology and Oceanography: Methods. A novel drifter designed for use with a mounted Acoustic Doppler Current Profiler in shallow environments This combination of fixed-point measurements and drifter tracks is what allows researchers to piece together the complex, layered circulation patterns that define how lakes actually move.
Sandbars, Piers, and the Structures That Focus Danger
The shape of the lakebed and any structures built along the shoreline play a huge role in determining where dangerous currents form. Sandbars running parallel to shore create a natural trough between the bar and the beach. Water pushed over the bar by waves fills this trough and seeks an exit, and it finds one wherever there is a gap in the bar. That gap becomes a rip current channel. The process is identical to what happens on ocean beaches, and in lakes with sandy nearshore zones and active wave regimes, the sandbars are constantly shifting in response to storms, making the location of rip channels unpredictable from one week to the next.
Piers and jetties make the problem worse by creating a hard boundary that concentrates the return flow. The rip currents measured near coastal structures on Lake Michigan were deflection rips, meaning the structures redirected longshore currents into focused offshore jets.2ScienceDirect (Journal of Great Lakes Research). Rip currents near coastal structures in Lake Michigan: Characterization and assessment for warnings – Section: Rip current characteristics Swimmers who like to stay close to a pier because it feels like a landmark or a safety feature are often positioning themselves in the most dangerous spot on the beach. The same applies to harbor entrances, where the narrowing between breakwaters can accelerate currents dramatically.
If you are choosing where to swim on a lake beach, look for even, unbroken wave lines rolling in parallel to shore. Gaps in the breaking waves, discolored or unusually choppy water extending perpendicular to shore, and debris floating steadily outward are all visual clues to a rip current channel. And give piers, jetties, and breakwaters a wide berth, even if other swimmers are clustered near them.