SALISBURY — The mouth of the Merrimack River, where the 110-mile-long river meets the Atlantic Ocean, has earned a reputation among mariners as one of the most dangerous waterways in the nation.
It's a combination of effects that makes it so dangerous: a tremendous amount of water surging through a fairly small funnel-like area, notoriously strong currents during outgoing tides, and unpredictable eddies and swells caused by the currents, tides, winds, jetties and sandbars. "Standing waves" that can flip a boat can form, and when easterly winds blow, waves can stack up into virtual walls of water.
And the jetties — built over a century ago to hold the shifting sandbanks of the river mouth in place — have also claimed their share of boats. The jagged old granite blocks can rip open the hull of a boat in an instant.
According to files kept by the Massachusetts Board of Underwater Archaeological Resources, at least 70 vessels have come to an early demise in and around this area over the past two and a half centuries. Compared to a significant number of other Massachusetts river mouths, the Merrimack has more than three times the number of reported wrecks.
"Generally, the Merrimack is a dangerous place," Lt. Junior Grade Brett Gary of U.S. Coast Guard sector Boston said. "There are between three and five crashes or wrecks there per year."
The Daily News' files are filled with boating accidents at the mouth. One of the more frequent types of accidents is capsizing, particularly involving small boats with too many passengers aboard. Last year, nine passengers were thrown into the water when their overloaded 17-foot Stingray filled with water from swells, then rapidly sank. All nine survived, thanks to several nearby boats.
Boats have also struck the jetties. An accident in 2003 left a powerboat sitting atop a jetty after it hit it at high speed, catapulting the boat completely out of the water and stranding it atop the rocks.
Wednesday night's accident off the North Jetty involved a 36-foot recreational boat that sank after hitting the jetty around 10 p.m. At the time, there was a 2-foot wave swell at the jetty, and winds were measured at 10 knots. Water temperature was 61 degrees.
"A 2-foot ground swell is not too big," Gary said. "It's pretty small."
According to officials at the scene, the boat, filled with four experienced boaters — two cousins and two friends — was traveling in a thick fog just inside the North Jetty, an area known for swirling currents that can cause dangerous swells, particularly during shifting tides. High tide was at 9:24 p.m.
In that region of water, Gary said currents go in and out of the river, creating what's called a "breaking bar," a wall of dangerously churning water. That, coupled with the tendency for seas to build up between 6 to 20 feet high during strong winds, can create life-threatening conditions.
"There is a sandbar out there, and when surf goes over the bar, waves build off that," Gary said. "The high tide also made it difficult to see the jetties."
Gary said boaters without electric or paper charts can be lost and easily hit the jetties if they are not aware of their exact location.
"If it's nighttime and foggy and you're not familiar with the area, you wouldn't see it," Gary said. "A black rock jetty like that, and if you're not from the area, you could hit it."
According to an official at the scene, the boat had a GPS system, and the boaters may have been using it to navigate the channel, but the fog may have been too thick for them to spot the rocks.
A swell likely carried the boat onto the jetty, throwing two of the passengers from the boat. A search continues for the boat's captain — who had stayed with the boat — while the other three have been rescued.
Gary said the Coast Guard always alerts boaters via marine broadcast when conditions are risky.
"We alert boaters of the conditions when it is rough," Gary said. "We do routine patrols of the area and have a camera out there to look at the bar and see what conditions are like."
Newburyport Harbormaster Ralph Steele has said the narrow inlets of the harbor make for often treacherous conditions with strong waves battering boaters.
Steele has said waters along the coast can be risky, and boaters must use commons sense when navigating the mouth of the river.