Gauges at the Conowingo Dam registered 5 trillion gallons of discharge during the three-month gusher that ended in May, enough to replace the water in the upper bay every 30 days, according to the National Oceanic and Atmospheric Administration. The spring total is surpassed only by 1993, when 5.5 trillion gallons gushed from the river's mouth.
The drastic change has put federal and state officials on notice.
"This is going to be an unusual year," said Lynn Fegley, assistant fisheries director for the Department of Natural Resources.
The near-freshwater conditions in the bay's main stem and tributaries above the Bay Bridge has put oyster planting on hold and has federal and state biologists bracing for the possibility that invasive species such as the northern snakehead, zebra mussels and blue catfish might be able to use the change in salinity to gain a foothold in new waters.
On the other hand, biologists note that juvenile striped bass often flourish in less salty water and jellyfish might not be able to venture as far north this summer.
"It's not estuary Armageddon," said Fegley. "Part of the deal, if you're a critter living in the Chesapeake Bay, is being asked to put up with an incredibly wide range of conditions, including great swings in salinity."
For example, just last year salinity was higher than normal and fishermen found black drum off Pasadena and bullhead sharks in the Potomac River.
Recent dry conditions have allowed salinity levels to begin their creep toward normal levels, and unless there's a deluge from a tropical storm to add more fresh water, the upper bay might return to a more normal level.