B'More Green: Noah's Bay - flooding adds to Chesapeake's woes - Going Green: Environment, energy, living green, conservation and more in Baltimore, the Chesapeake Bay and beyond

Hurricane Irene may have paradoxically breathed a little life back into the Chesapeake Bay, but the deluge that's caused flash flooding around Baltimore and forced evacuations along the Susquehanna River could well snuff out whatever spark of vitality the earlier storm brought to the ailing estuary.

That's the prediction of a pair of scientists I canvassed, who'd previously suggested that there was a silver lining to the havoc wrought on Maryland two weeks ago by Hurricane Irene. That storm's winds, which toppled trees and power lines across the state, roiled the bay's water and broke up its massive dead zone, they said, giving fish, crabs and shellfish a fresh infusion of life-sustaining dissolved oxygen at the end of what has been an extremely trying summer.

But the five-day downpour brought to us this week from the Gulf of Mexico by the remnants of Tropical Storm Lee is nothing but bad news for the bay, the experts say.

The Susquehanna, source of half of all the fresh water entering the bay, is rising to a level not seen since Tropical Storm Agnes in 1972, with projections that it will be pouring over the Conowingo Dam at more than 600,000 cubic feet per second when the flood peaks early Saturday morning. (UPDATE: The rising river may be cresting a day earlier and somewhat lower than previously predicted, according to the U.S. Geological Survey, but its flow was still projected to peak today at 777,000 cubic feet per second.)

That's well short of the 1.1 million cfs that raged over the dam during Agnes, wiping out grass beds and smothering oysters and clams down the bay. (UPDATE 09-12: Flow peaked Friday Sept. 9 at 778,000 cubic feet per second, third highest recorded, according to US Geological Survey data. The second heaviest flow reached 909,000 cfs in January 1996.)But it'll be more than enough flow to scour out the nutrient-laden sediment that's piled up behind the dam for the past four decades, according to Bruce Michael, director of resource assessment for the Maryland Department of Natural Resources.

"Scouring of sediments/nutrients trapped behind the Conowingo Dam occurs when river flows exceed 390,000 to 400,000 cfs," Michael emailed me, "so this event will result in a significant amount of sediments and nutrients being transported from behind the dam and deposited in the upper Bay."

In addition, the torrential rains have washed tons of nutrients, mud and organic matter (yup, sewage overflows, but also plenty of pet, farm and wild animal waste) into the streams and rivers now surging into the bay. All that pollution can feed a new round of algae blooms and suck right back out of the water the oxygen that Hurricane Irene may have brought. And the tide of fresh water itself can make matters worse by "stratifying" the water - salt water is heavier than fresh - preventing dissolved oxygen from mixing throughout the bay.

"In recent history, we have seen low oxygen events in the mainstem in virtually every month of the year given the right conditions," according to William Dennison, vice president for science applications at the University of Maryland Center for Environmental Science. "It will be an interesting autumn."

Michael said that even if oxygen levels do drop in the bay's waters - DNR plans to sample later this month - the dead zone is unlikely to reach the size or intensity it did in early summer. But it's just one more insult to a bay that in recent years has been showing some signs of recovery, especially with a resurgence of lush grass beds in the upper bay.

"Bottom line, this is not going to be a good year," Michael concluded, for the bay's water quality or the grass beds that provide vital shelter and nursery for fish and crabs.

Some might wonder if this just shows the billions of dollars spent on bay restoration so far have been wasted. The bay's condition is always going to vary to a degree with the weather, and this year's has been extreme, with unusually heavy spring flows of fresh water, then record heat followed by near-record storm flooding. The controversial pollution diet that Maryland and other bay states have been put on by the federal government can't negate extreme shocks like these, but Michael argues that the pollution reductions being required can reduce their impacts and give the bay more resilience to bounce back.

(Baltimore Sun photos: Susquehanna spilling over Conowingo Dam, flooding of Port Deposit, Sept. 8, by Karl Merton Ferron)