When people consider water safety, the term “undertow” often surfaces as a primary fear, usually conjuring images of being pulled beneath the surface. For lakes, however, the answer to whether they have a true undertow is no. Lakes are not immune to danger, though, as they possess a different set of powerful and sometimes invisible currents that swimmers frequently mistake for the ocean-based hazard. These lake currents are equally capable of creating life-threatening situations.
What an Undertow Actually Is
The term “undertow” in physical oceanography refers to a broad, steady return flow of water that moves offshore along the sea floor. This phenomenon is fundamentally driven by the mass of water pushed toward the beach by breaking waves. As each wave crashes and runs up the beach face, the water level temporarily builds up near the shore. Gravity then pulls this excess water back out to sea, primarily along the bottom, to compensate for the continuous onshore transport. This process is dependent on the consistent energy of breaking waves in the surf zone. Without the constant influx of water from large, organized breaking waves, the true undertow mechanism cannot be sustained.
Specific Lake Currents Mistaken for Undertows
Swimmers often mistake several distinct lake hazards for the ocean’s undertow because they feel a strong pull into deeper water.
Wind-Driven Return Flow
One common danger occurs when strong, sustained winds push surface water toward the shore. This water must return to the main body of the lake, creating a strong, subsurface current that mimics the sensation of being pulled under. These flows are particularly noticeable near steep drop-offs or underwater shelves where the water is channeled and accelerated.
Structural Currents
Structural currents are permanent hazards near man-made structures like piers, jetties, and breakwalls. Water flowing parallel to the shore (longshore current) is suddenly deflected and funneled outward around these solid objects. This creates a narrow, powerful, river-like stream that can quickly sweep a swimmer away.
Inlets and Outlets
Currents are always present near lake inlets and outlets, where rivers enter or dams release water. This produces a steady, strong flow that can overpower a swimmer trying to cross the boundary.
How Wind and Water Movement Create Lake Hazards
Wind stress acting on the water’s surface is the main force generating dangerous lake currents. Sustained wind pushes the top layer of water downwind, causing it to pile up against the leeward shore. This piling effect creates a pressure gradient, forcing a compensating current to flow back along the bottom or through narrow channels, such as sandbar cuts, to escape offshore. These channeled return flows are the mechanism behind many lake rip currents, which are distinct from ocean rip currents because they are not driven by tide or wave action alone.
Another significant factor is the lake’s thermal structure and topography. Lakes often develop distinct temperature layers, called a thermocline, especially during the summer. The inflow of colder river water or sudden temperature shifts can create density currents where water layers move independently. The shape of the lake bottom, or bathymetry, also intensifies currents, as shallow areas or sudden drop-offs accelerate water movement. Large, rapid changes in atmospheric pressure can also cause the entire lake basin to oscillate, a phenomenon called a seiche, resulting in unpredictable surges of water.
Swimming Safety When Facing Strong Lake Currents
If you find yourself being pulled away from the shore, the most important action is to remain calm and conserve energy. Do not attempt to swim directly against the current, as this will quickly lead to exhaustion.
The proper response is to swim parallel to the shore until you feel the current weaken and release its hold. Once out of the main channel of the current, you can then angle your swim back toward the beach.
Always avoid swimming near structures like piers and outlets, where currents are predictably strong and permanent. Wearing a United States Coast Guard-approved life jacket is a simple, effective precaution, especially when swimming in deep or unfamiliar lake environments.