SAN DIEGO — Just south of Mission Bay lies a long, narrow estuary that connects the San Diego River to the Pacific Ocean.

Photo by: Jack Innis Fatal Attraction? -- Boats often pass close to the south jetty at Mission Bay, an area lifeguards have called ’Disasters’ because of its long history of boating accidents.

While seemingly unrelated to boating activities in the nearby waterway, the river and associated estuary may play a significant role in boating accidents on Mission Bay’s notorious south jetty.

“Disasters”

Mission Bay Channel’s south jetty has gained a reputation as a “boat killer.” In any given year, about a dozen boats find their way onto the rocks in Mission Bay Channel — most of them near the tip of the south jetty. The area is such a magnet for boats that San Diego city lifeguards have a nickname for south jetty’s westernmost end: “Disasters.”

Disasters will likely claim more vessels in 2009 than any other year in history. Lifeguards have already responded to 18 boats on the rocks --and summer has not yet officially begun.

Many boaters wonder what makes the south jetty such a magnet for boat groundings. Certainly, when surf runs high, all jetties deserve wide berths. But even on the calmest of days, boats seem to wind up on the rocks at the tip of the south Mission Bay Jetty with surprising regularity.

While a host of factors undoubtedly come into play, part of the puzzle involves an innocuous looking waterway directly south of the jetty.

Disasters in the Making

More than a century and a half ago, U.S. Army engineer George Horatio Derby oversaw construction of a dirt berm that permanently blocked the San Diego River from dumping into San Diego Bay. In 1948, the federal government began construction on a massive jetty on the north side to prevent the river from flowing into Mission Bay. The two structures created a 3-mile-long, 200-yard-wide estuary that forces the San Diego River to run straight to the sea.

This long, narrow, estuary experiences the same daily tidal fluctuations as does Mission Bay, which is adjacent. As outgoing tides empty the 4,600 acre Mission Bay, they also empty the adjacent 3-mile-long estuary.

But when unusually large flood tides simultaneously fill both basins, a phenomenon occurs at the tip of the South Mission Bay Jetty — giving the spot known as Disasters its well-deserved reputation.

Deadly Eddies

Since the same jetty that forms the south Mission Bay Channel also forms the northern San Diego River Channel, water rushing toward the two basins at maximum flood (incoming) tide must split when it hits the tip of the jetty. The jetty behaves like an island in the middle of a river, dividing the water.

Those who make their living on fast-flowing bodies of water with islands in them know that water does not divide very effectively.

Steve Hadley is an authority on handling boats in fast currents and tricky eddies. The retired Coast Guard lieutenant commander has encountered just about every current, flow, eddy, vortex, countercurrent and undercurrent known to man during his 30-year career on the Mississippi, Missouri, Arkansas and Ohio rivers.

“On most river systems, there are islands and numerous channel control structures — such as weirs, dams and dikes. A lot of these can create confused and challenging current situations, depending on strength, depth, hydrology, and other factors of the waterway,” Hadley said.

In Mission Bay Channel, Disasters acts much like a river obstruction. Tide-driven water rushing toward it can create confusing, challenging and rapidly changing currents, even when the surf is low. Olympic swimming pool-sized slugs of water can flow toward the bay and then suddenly swirl toward the river, dragging any hapless vessel nearby onto the jagged rocks protruding from the jetty’s tip.

For boaters caught up in Disaster’s swirling eddies and cross currents, another surprise awaits.

When Rudders Fail

Current strength plays a major role in the way boats handle, according to Hadley. Vessels designed to operate in river systems generally have more powerful engines and bigger rudders than oceangoing counterparts. River boats need more power and control, even when eddies or tricky water flows are not a factor.

“If you’re running upstream (against the current) and you have enough power to move forward, the boat is easy to control,” Hadley said. “You can stop quicker, and it responds quicker to rudder commands. But if you’re going downstream (with the current), that’s a different story.”

The story is this: If a boat is moving at the same speed and in the same direction as the current, its speed relative to the water is zero. The boat may be moving relative to the sea floor and the scenery surrounding it, but in terms of rudder control, the vessel is essentially sitting dead in the water.

Mike Roberts, designer for San Diego-based Reichel-Pugh Yacht Design, sees how certain boats might lose control entering Mission Bay during an incoming tide.

“Generally, you have to be making way through the water for your rudders to be doing anything for you,” Roberts said. “If you’re in a 3 knot flood going into Mission Bay, you need to be going more than 3 knots over the ground. If you don’t have water flowing over your rudder, it’s not going to do you any good.”

Operators of slow vessels entering Mission Bay during a strong incoming tide may wish to heed Roberts’ advice and make sure their boats are moving faster than the currents — especially near Disasters, where confused currents and eddies have a habit of dragging boats onto the rocks.

“The problem ends up when you start getting sideways and you start overcorrecting with your rudder,” Roberts said. “You stall and lose all steerage.”

Keeled sailboats under sail in light winds are especially vulnerable. So are fishing boats that may be drifting or slow trolling.

Situational Awareness

It’s an unfortunate scene likely to be repeated throughout 2009: In low seas and light wind, a small sailboat ghosts into Mission Bay during flood tide. Transiting past the tip of the south jetty, the skipper senses no danger.

Outward signs indicate the boat is moving forward. Relative to fixed objects — especially the jetty itself — the boat is. But relative to the body of water in which it floats, it is not.

Suddenly, the sailor notices his boat is being drawn into the rocks. He tries to tack away, but the rudder doesn’t respond. It’s too late to raise more sail, start the engine or drop anchor.

Disasters has just claimed another victim.

“Experience is great, if you have it,” Hadley said. But a dose of caution might do in a pinch.

“If the current splits the jetty, you’d better not get near the tip of it, if you’re in a low-powered vessel,” Hadley said. “The eddy could turn you around, you could lose control of your rudder, you might not have enough power to overcome what’s happening and you might end up getting forced to port when you want to go starboard, or vice versa.”

Hadley shared a trick he learned on fast-flowing rivers: Before entering narrow areas with fast moving currents, stand off a bit to see what’s going on.

“Pay careful attention to other vessels transiting,” Hadley said. “They may offer the best clues to a safe passage.”

On a recent afternoon, we watched vessel traffic going in and out of Mission Bay Channel. All commercial vessels transited nearer the north jetty than the south. A 35-foot sailboat entering Mission Bay from the south gave Disasters a wide berth. But a cabin cruiser coming from the south at about 8 knots passed within 50 feet of the tip of the jetty.

The power boat made the transit without incident. But had there been a sudden loss of power, the skipper would have had little time to avert disaster.