Dead water

Dead water is the nautical term for a phenomenon which can occur when a layer of fresh or brackish water rests on top of denser salt water, without the two layers mixing. A ship powered by direct thrust under the waterline (such as a propeller), traveling in such conditions may be hard to maneuver or can even slow down almost to a standstill. Much of the energy from the ship's propeller only results in waves and turbulence between the two layers of water, leaving a ship capable of traveling at perhaps as little as 20% of its normal speed.

The phenomenon was first described by Fridtjof Nansen, the Norwegian Arctic explorer. Nansen wrote the following from his ship Fram in August 1893 in the Nordenskiöld Archipelago near the Taymyr Peninsula:

This phenomenon is observable where glacier runoff flows into salt water without much mixing, such as in fjords.

...
Wikipedia