Undercurrent is a flow of water below the surface: In an ocean, a subsurface current, a water current which flows beneath and usually independently of surface currents. In a river, a subsurface current (see whitewater)
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Rivers and streams can appear calm on the surface but there may be: Strong undercurrents that can pull under even a strong and experienced swimmer.
Swimming in rivers can be dangerous for a few reasons. Rivers can be very deep and have strong currents, which can make it difficult to stay afloat. Rivers can also be filled with debris such as branches, rocks, and even snakes, which can cause injuries.