An undertow is a steady, offshore-directed compensation flow, which occurs below waves near the shore. Physically, nearshore, the wave-induced mass flux between wave crest and trough is onshore directed. This mass transport is localized in the upper part of the water column, i.e. above the wave troughs.

While the term "undertow" is often used colloquially, what people usually encounter in large lakes (like the Great Lakes) are actually rip currents or longshore currents. A lake "undertow" typically refers to the "backwash" of a wave; as a large wave breaks on the shore, the water must return to the lake, creating a brief, strong pull near the bottom that can knock over a small child. Unlike the ocean, lakes don't have tides, so these currents are entirely driven by wind and wave action. Rip currents in lakes occur when water "piles up" between the shore and a sandbar; it then rushes back out through a gap in the bar, creating a narrow "river" of water moving away from the shore. This current won't pull you under the water, but it will pull you away from the shore into deeper, colder water. The best safety rule in 2026 remains "flip, float, and follow": flip onto your back to conserve energy, float to keep your head above water, and follow the current until it weakens, then swim parallel to the shore to escape.