San Diego — Never say never, but a hurricane like the one that hit New Orleans is unlikely to flood San Diego anytime soon. The San Diego River, however, is another story. After a rainstorm last winter, the river's waters blew out a section of Fashion Valley Road that has yet to be repaired. Local weblog The Indepundit titled its January 7 response to that storm "Nature: Still a Bitch." It warned New Yorkers, in town for the weekend's Chargers/Jets playoff at Qualcomm Stadium, to travel to the game early, stick to the freeways, and avoid north/south roads in Mission Valley. It went on to mention the trolley but pointed out that "if you are staying at Hotel Circle, the nearest trolley station is Fashion Valley...conveniently located on the other side of the river."

None of last year's flooding on the San Diego River, however, came close to reaching the river's highest water mark. The biggest flood since records started being kept in 1914 occurred two years later, in January 1916, when 70,200 cubic feet per second gushed through Mission Valley. The top five floods came before 1938, and eight of the largest ten before 1942.

Floods on the lower San Diego River undoubtedly have become milder due to the El Capitan and San Vicente Dams, according to hydrology specialist and SDSU geography professor emeritus Philip Pryde. The City of San Diego built the El Capitan Dam on the San Diego River in 1935 and the San Vicente on San Vicente Creek in 1943. Both are situated in the hills above Lakeside.

The dams' main purpose has not been to control floods but to provide water for San Diego's burgeoning population. Most of the city's water comes from the Colorado River, but San Diego is "happy to get as much free water as it can," says Pryde. "So the local dams often are allowed to fill close to capacity. If a large storm strikes when El Capitan is close to capacity, water spilling over its top could inundate the floodplain below."

After El Capitan overflowed in April 1941, the flood in Mission Valley peaked at 9250 cubic feet per second, becoming the seventh-largest flood in the San Diego River's history. El Capitan overflowed again in February 1980, but a recording gauge was not in place that year. That flood's official maximum volume, 3420 cubic feet per second, is probably "understated," says Pryde.

As we've learned from the New Orleans disaster, the concept of "a hundred-year flood" plays an important role for flood-control designers. In a handout he uses for teaching, Pryde writes that the hundred-year flood "is frequently used as the maximum run-off size...for flood protection measures, and means a flood of this size or larger is expected to occur ten times in a thousand-year period (the intervals between these floods can, and will, vary greatly)." Pryde says that the hundred-year flood size in use in Mission Valley today is based on a complex calculation but approximates the 45,400 cubic feet per second of a February 1927 flood, the second largest on the San Diego River.

For years San Diegans have questioned the wisdom of developing in the Mission Valley floodplain. According to San Diego's municipal code, new development that will raise the projected flood level of the hundred-year flood is not permitted. This complies with standards of the Federal Emergency Management Agency, which has ultimate jurisdiction over flood control in the United States.

FEMA's code demands that "Communities shall prohibit encroachments, including fill [and] new construction...

within the adopted regulatory floodway unless it has been demonstrated [scientifically] that the proposed encroachment would not result in any increase in flood levels within the community during occurrence of the base (100 year) discharge."

But Randy Berkman of the River Valley Preservation Project tells me that the combined effects of the trolley line, a Fashion Valley parking-structure expansion, and reconstruction of the Stardust Golf Course, which required 200,000 cubic yards of fill, have raised the floodway baseline by a foot in central Mission Valley. (To comprehend the effect on the river of, for instance, the columns that support the elevated trolley line, he suggests you add ice cubes to a glass of water. As each one is added, the water in the glass rises.) Neither the city nor FEMA required developers to mitigate the effects of these projects, he says.

Instead, in 2002, FEMA issued a Preliminary Letter of Map Revision for the floodway in west Mission Valley. The effect of the letter, according to Berkman, is to accept the one-foot-higher level as the new floodway baseline.

Even worse, writes Berkman on his website, "the trolley east of Highway 163 has not been included in the mix! This is over one half of the entire project! The [letter] from FEMA violates the very definition of such a letter, which requires analysis of a whole project, not part of it!" FEMA should have required the Metropolitan Transit Development Board, says Berkman, to complete "a flood hazard study for the whole [project]" before any trolley construction in Mission Valley began.

Berkman has been fighting the trolley's effects on the San Diego River floodway since the early 1990s. "I have stacks of e-mails from FEMA officials," he tells me. "And Walter Gefrom of the city's engineering department has been especially candid about what needs to be done. Berkman says that Gefrom told him several years ago that the elevated trolley platform at the 24 Hour Fitness on Ward Road would require a $1 million mitigation project. An engineering firm suggested, as possible mitigations, lowering the station parking lot, moving an abutment, dredging a triangular area in the river east of Ward Road, widening the channel, and other measures, according to Berkman. But FEMA did not insist on any of them.

I try calling Gefrom at the city's engineering department. "He's no longer on the San Diego River floodway assignment," a spokesman tells me, routing me instead to "a woman who now handles it" but who says she'll ask her boss, senior civil engineer Jamal Batta. According to Batta, the west segment of the Mission Valley trolley to Highway 163 required no mitigation because the project did not raise the floodway level. Berkman figures FEMA's 2002 Letter of Map Revision is the only thing allowing Batta to make this claim.

For the Green Line extension beyond Ward Road, says Batta, an engineering firm issued a "no-rise certificate," stating that the extension would not cause a rise in the hundred-year floodway baseline, so mitigation was unnecessary. In the trolley's middle section, between Highway 163 and Ward Road, a one-foot rise in the floodway level did result from the trolley construction. But, says Batta, in that area no flooding of "an insurable structure" in the floodway is ever expected to occur. So no mitigation needs to take place there either.

Still, Batta admits, the city has its requirement that the San Diego River floodway level can't be raised by construction. So it is incumbent on the Metropolitan Transit Development Board, he says, to ask the city council for "a variance" from that regulation.

Randy Berkman thinks FEMA has abdicated its responsibility to force the transit board to mitigate the trolley's effects on the middle section of the river in Mission Valley. Having its own jurisdiction, the transit district does not need to consult the city when making decisions, but it cannot break federal law, says Berkman.

The middle section of Mission Valley that Berkman worries about does have levees on each side of the river channel. And SDSU's Philip Pryde, concerned about the same section, more specifically the section between Highway 163 and Qualcomm Way, notes that during the late 1970s, property owners and the city jointly built a "soft bottom" channel in the area. But in his book, San Diego: An Introduction to the Region, Pryde says, "[That channel] would safely convey a flood the size of the 1980 one, but not the size of the 1916 flood."

A particular danger in the area, and in such other low-lying sections as Grantville, says Pryde, is that rushing floodwater often rips out vegetation and sends it downstream, where it blocks culverts. Last winter a fire department crew found the body of a drowned man in just such a bundle of bushes and small trees logjammed behind Fashion Valley. If blockages occur in the river's flow, serious backflow results, exacerbating any flooding that is already taking place.

Last winter's 22.49 inches of rain recorded at Lindbergh Field was the third-highest annual rainfall in San Diego record books. Annual rainfall has been increasing in San Diego in the past 20 years, says Pryde. But wet winters don't correlate well with flooding. Single-storm intensities are bigger factors in the occurrence of floods, large amounts of rain in short time spans creating the most dangerous situations. And a sudden intense storm is the very thing it would take to cause an already full El Capitan dam to spill over.

According to Pryde, the Arizona and Southern California region has the greatest variability in rainfall patterns of any region in the country. "And it should be emphasized," he writes, "that we have a very poor idea (because of the brevity of historical records) of what the actual hundred-year flood size is for any Southern California river."

Pryde goes on to warn about increasing urbanization "within the San Diego River watershed." Buildings, cement sidewalks, and asphalt streets and parking lots, allowing no soil for seepage, send runoff flowing farther downstream.

"We've built too much in Mission Valley," Pryde tells me. "Earlier building was more flood-conscious." As an example, he cites the I-5 bridge over the 800-foot-wide San Diego River channel to the ocean. Old photos show the bridge's destruction in the 1916 flood. Later "it was designed for a full El Capitan spillway capacity."