Scientists, watermen and recreational users of the Chesapeake Bay are monitoring the now-brown water coursing down the Susquehanna River, hoping that history doesn't repeat itself.
Since Tropical Storm Agnes dumped millions of tons of sediment and pollution into the bay in June 1972, devastating bay grasses and aquatic life, flood- waters in 1996, 2003 and 2004 also have made their mark.
But while this week's deluge will not come close to matching Agnes in terms of water volume, it shares one ominous characteristic that could set it apart from more recent storms: timing.
"Timing is everything," said Dr. Beth McGee, senior water quality scientist with the Chesapeake Bay Foundation. "1996 was a winter storm, and Isabel and Ivan came through in September. This one is coming at the height of the growing season for plants and animals," a bad time for a storm.
Too soon to tell
Scientists are quick to point out that it might take weeks or even months to determine the precise damage this week's rains will have on the bay. The U.S. Geological Survey, which calculates water levels at the end of every month, won't know how much poured into the bay until next week, when gauges are read. But based on history, this massive infusion of fresh water has given everyone pause.
"Can this be good? In terms of this much water coming down this fast, no," said Harley Speir, a Department of Natural Resources fish biologist.
At its peak, Agnes' floodwaters gushed over the Conowingo Dam near the mouth of the Susquehanna at a rate of 1.13 million cubic feet per second. Isabel's peak was 139,000 cfs. At 8:45 a.m. Wednesday, water poured over the Conowingo at 145,000 cfs; it is expected to peak this afternoon at about 400,000 cfs.
Floodwaters carry sediment, nutrients and other pollutants downstream. When sediment pours into the bay, it blocks light needed for the growth of filtering bay grasses. The nutrients - such as farm fertilizer - create favorable conditions for algae blooms, which eventually deplete oxygen at the bay bottom.
Last year was the worst on record for so-called "dead zones" in the bay, with an average of 30 percent of the estuary falling below the oxygen level needed to sustain fish.
Damage to the submerged vegetation would be a setback for bay restoration. During the past two summers, thick beds covered more than 10,000 acres - about three times the size of Baltimore-Washington International Thurgood Marshall Airport - at the confluence of the Susquehanna and the bay. But June is a vulnerable time of year for the grasses because they haven't reached full canopy yet.
DNR biologist Mike Naylor said he hasn't assessed the damage, but state data show the water is murky - especially in places such as the Bush River in the upper bay.
"A prolonged storm event at this time of year has the potential to be detrimental," he said. "We're all hopeful that the rainfall won't reverse the positive things we've been seeing."
The influx of fresh water from the rainfall can also assault the bay on the surface, said Dr. Robert Wood, head of the Cooperative Oxford Laboratory.
Fresh water, which is lighter than salt water, creates a lens that prevents any meaningful turnover, or circulation, in the water.
"Fish are able to move downstream or up and down in the water column, but crabs have a harder time," said Wood. "And for oysters, the fresh water and the low oxygen are a double whammy. They can survive one or the other for a certain period, but rarely both."
Larry Simns, president of the Maryland Watermen's Association, called the storm waters "a mixed bag."
"It depends how long it lasts, how long they have to keep the [Conowingo Dam] gates open," said Simns. "Oysters can go dormant for eight to 10 days, and if it doesn't last any longer than that, we might skim by."
The flip side of the coin, Simns said, is that fresh water lowers bay salinity, creating an inhospitable environment for the diseases that attack oysters.
Speir said the freshwater gusher means that anglers might be seeing some species, such as catfish, farther south in the bay while others, such as flounder, won't venture as far north.
And, said McGee, bay swimmers will probably see fewer jellyfish.
Sun reporter Rona Kobell contributed to this article.