The surging rivers can simply uproot grasses from their bottoms, or bury them in a thick layer of new sediment. Oyster reefs likewise can be smothered as the mud clouding the water settles out, and the transformation of the bay to fresh water for prolonged periiods of time also can weaken or kill shellfish. 

In addition to sediment, such flooding washes countless tons of nitrogen and phosphorus off the land into the water, from farm and lawn fertilizer, animal waste and sewage treatment systems overwhelmed by the storm. Those plant nutrients feed massive algae growths in the water, which ultimately deplete the water of oxygen needed by fish, oysters and crabs.

Even in "normal" weather, a large oxygen-starved "dead zone" forms each summer in the bay, spreading out and up from the depths to the shallow edges.  A fresh infusion of nutrients washed off the land can feed a new round of oxygen deprivation.  (The storm-fed surge of fresh water also can help aggravate the situation, points out Mike Roman of UMCES.  It's lighter than salt water, so the bay "stratifies" into fresh and brackish layers, and with the water not mixing dissolved oxygen levels in the depths get depleted.)

There are reasons to believe the storm-caused ecological damage this time may not be as severe as in 1972. The rate at which water surged through Conowingo last week approached but did not equal the torrent during Agnes (nearly 800,000 cubic feet per second, versus 1.1 million cfs). The amount of sediment washed below the dam, though enough to turn the bay brown as far south as the Potomac River, was estimated to be a fraction of what Agnes pushed into the Chesapeake (4 million tons from Lee vs 20 million tons in 1972, according to the US Geological Survey).

And the timing could be key. Agnes hit in June, during the growing season for bay grasses and when the bay's dead zone usually begins to form. Lee has hit toward the end of summer, with the grasses gone or going dormant and the dead zone normally dissipating.

Some also think that the bay's in better shape now than it was in 1972, making it more able to handle a body blow from Mother Nature. They argue that the muti-billion-dollar cleanup that's been under way since the mid-1980s has done at least some good, cleaning up sewage discharges and getting many farmers to do things to reduce runoff from their fields and feedlots.  

If that's true, Tropical Storm Lee may prove to be only a minor or short-lived setback for the bay's health. If not, it could prompt some more to question the efficacy of what's been done so far, and whether the bay really can be saved without more radical measures - or maybe if it's even worth the effort.

How good are those expensive sewage plant upgrades if the sewer lines are prone to overflows whenever it rains?  And when you see the bay turn muddy all the way to the Potomac River, how effective really are all the "best management practices" farmers are being asked and paid to adopt to control runoff? 

So this is an unplanned pop quiz of sorts for the bay - and a test of the resolve of those who at least say they're committed to restoring it.  What's more, perhaps such extreme weather events should no longer be regarded as acts of God that can't be helped.  Scientsts have predicted that severe storms are likely to become more frequent as the earth's climate changes.  Seen in that light, bringing the bay back to ecological health just got even more challenging.

(Photos: NASA satellite image of sediment plume in Chesapeake after Tropical Storm Lee; floating plant material and debris on bay's surface, by Baltimore Sun's Kim Hairston)