There are numerous fjords on in the Salish Sea—river valleys carved by glaciers and drowned by the rising sea—where summer freshets superimpose a layer of brackish water upon the dense salt water lying beneath. (Salt water has a higher density than fresh and consequently sinks.) Ships transiting these fjords have sometimes been dramatically slowed, losing headway with no opposing current and no explanation. Ships under sail and power were equally affected. Sailors, often as poetic as they were superstitious, called the phenomenon "dead water." The name suggested the sea itself clung to the ship like the dead hand of a drowned sailor. It was an oddly appropriate name.
Oceanographers began to unravel the mystery of dead water a hundred years ago but still the explanation seems counter-intuitive and oddly unsatisfying.
First, some basic marine architecture. A significant amount of the energy needed to move a ship through the water is translated into its wake. The more power applied, the faster the ship travels, the larger the waves generated—to a point. Eventually the vessel digs a hole in the water and becomes entrapped by its own bow wave. No amount of additional power will move it faster unless it changes the nature of the game, rises above the water and planes.
Second, some basic physical geography. Fjords are valleys first formed by a glacier, then filled by the sea as the glacier withdrew. Glacial valleys are typically deep, narrow, and steep-sided The same water that once formed glacial ice now enters the fjord from rivers and streams. In Northern Europe and the Pacific NW, that’s a lot of water. Constrained by the narrow walls of the fjord, the fresh water tends not to mix readily and floats upon the dense salt water. A boundary layer forms between the different densities of water.
Ships with a draft deep enough to span the fresh water layer and penetrate the dense, salt water below generate internal waves—in effect, a vertical wake. This is the part that’s hard to get your head around.
Unlike wind-generated waves, the crests of internal waves don’t distort the water’s surface. Instead, crests flatten near the surface, troughs deepen, and the wave’s most pronounced vertical displacement of water molecules occurs within the water column.
Just like a surface wake, an internal wake is powered by a ship’s motion through the water. The internal wake leeches power from the ship, dramatically slowing its forward motion. Dead water.
In 1972 the research vessel R.V. Endeavour created an internal wake in Orford Bay, British Columbia, by steaming astern, adjusting her speed to maintain maximum dead water. She then steamed ahead, reversing her course, studying the wave train she had created.
The internal waves created by Endeavour formed a pattern of parallel surface slicks often characteristic of internal waves. They require a bit of explanation.
As internal waves propagate, the motion of water molecules at the surface converge over the troughs and diverge over the crests. Driftwood, eel grass, and debris is drawn toward the troughs and pushed away from the crests. Drift logs entrained by the waves align parallel to the slick and advance with the waves. It’s typically a stately advance since internal waves travel slowly and dissipate more quickly than surface waves.
The troughs of internal waves often look like tide lines—the intersection of opposing currents where debris collects—except that they occur in parallel series.
This surreal photograph of the Endeavor making internal waves on Orford Bay was published in Richard Thomson’s excellent book Oceanography of the British Columbia Coast.