Origin of the "Discolored Water"

Much has been made about the notation on the chart that USS San Francisco was using during her grounding last week that had been marked "discolored water". This area was apparently charted 3 miles south of where the San Francisco collided with the undersea mountain. This article on the Navy Times website shows the actual satellite photograph that, in retrospect, may have shown the seamount, and could have been used to update the chart. An important thing to remember is that the "discolored water" notation was not based on the satellite evidence; it was based on a single report from the Japanese from the 1960s or earlier. The potential misplotting of the discolored water is probably therefore not due to incompetence, as the Navy Times article seems to be trying to imply. Rather, it is probably more likely due to navigational accuracies available in the 1960s, before GPS. Probably some Japanese surface ship had noted discolored water, and conscientiously reported it to their authorities along with their best estimate of their position when they saw it. Hopefully the San Francisco grounding will act as a spur for the cognizant authorities to investigate these reported anomalies that litter the charts and determine once and for all if they're accurate.

Going deep...

Update 2144 22 Jan: Here's the New York Times' take on the same story. (Will probably require registration soon.) Excerpt:

"David Sandwell, a geophysics professor at the Scripps Institution of Oceanography in San Diego, said it was also possible that the danger zone - an oval area described as containing "discolored water" - was a mistaken and poorly located reference to the undersea mountain.
"Defense Department officials have said that the notation dated to the early 1960's, and that it probably came from a surface ship that had spotted murky water. The discoloration could have been a temporary problem, like an oil slick, or a hazy indication of an undersea structure...
"The exact location of the crash remains classified. But the undersea mountain shows up on the satellite images at 7 degrees, 45.1 minutes north latitude and 147 degrees, 12.6 minutes east longitude...
"Besides relying on charts, submarines also receive fixes from navigation satellites and take soundings of water depths. According to officials, the San Francisco's officers have said they took a sounding just four minutes before the crash, and it indicated that the vessel was still in 6,000 feet of water."

This one piece of new information, that the ship took a sounding four minutes before the collision, will be very important in possibly exonerating the Captain and crew of any dereliction, if the sounding was properly evaluated (i.e. verified to match the expected water depth shown on the chart).

Update 0727 25 Jan: Here's another copy of the story above, from The Seattle Times, that probably doesn't require registration.