AS THE RED River raged over makeshift dikes futilely erected against its wrath in North Dakota, drowning cities beneath a column of water 26 feet above flood level, meteorologists were hard pressed to describe its magnitude in human chronology.
A 500-year flood, some call it, a catastrophic weather event that would have occurred only once since Christopher Columbus arrived on the shores of the New World. Whether it could be termed a 700-year flood or a 300-year flood is open to question.
The flood's size and power are unprecedented. While the Red River has ravaged the upper Midwest before, the height of the flood crest in Fargo and Grand Forks has been almost incomprehensible.
But climatological records are being broken more rapidly than ever. A 100-year-storm may as likely repeat within a few years as waiting another century. It is simply a way of classifying severity, not the frequency. "There isn't really a hundred-year event anymore," states climatologist Tom Karl of the National Oceanic and Atmospheric Administration.
Reliable, consistent weather records in the U.S. go back only 150 years or so. Human development has altered the Earth's surface and atmosphere, promoting greater weather changes and effects than an untouched environment would generate by itself.
What might be a 500-year event in the Chesapeake Bay is uncertain. Last year was the record for freshwater gushing into the bay. The January 1996 torrent of melted snowfall into the estuary recorded a daily average that exceeded the flow during Tropical Storm Agnes in 1972, a benchmark for 100-year meteorological events in these parts. But, according to the U.S. Geological Survey, the impact on the bay's ecosystem was not as damaging as in 1972.
Sea level in the Bay has risen nearly a foot in the past century, three times the rate of the past 5,000 years, which University of Maryland scientist Stephen Leatherman ties to global climate warming. Estuarine islands and upland shoreline are eroding at an accelerated pace.
The topography of the bay watershed is, of course, different from that of the Red River. It's not just flow rates and rainfall, but how the water is directed and where it can escape without intruding too far onto dry land. We can only hope that another 500 years really passes before the Chesapeake region is so tested.
Pub Date: 4/22/97