When storms as severe as the deluge of recent days sweep through the Susquehanna River Valley, residents downstream from the Conowingo Dam look at the massive concrete structure with trepidation.
They know that Exelon Power - Conowingo's operator - might have to open some of the dam's gates to relieve the pent-up pressure of billions of gallons. Already, 13 gates are open - enough to cause the river to threaten to spill over its banks and cause minor flooding near towns such as Port Deposit, only 11 miles to the southeast.
And by tomorrow, as the volume of water flowing down from Pennsylvania surges, dam operators expect to double the number of open gates to 26 and send 3 million gallons a second coursing through. And that is likely to cause flooding in Port Deposit itself, power officials say.
"The people downstream of the dam pay very, very close attention to when there are floodwaters headed their way," said Susan Obleski, a spokeswoman for the Susquehanna River Basin Commission. "The residents have become very familiar with this dam and recognizing that it does play a role in their lives during these flood periods."
Officials at Conowingo said projections resemble water flows that the dam sees two or three times per year, and while the National Weather Service is warning of flooding upstream, severe flooding is not a certainty here.
"It's not out of the ordinary - it's just out of the ordinary for this period of time," said Benjamin Armstrong, an Exelon spokesman, of the rising water.
Water levels on par with this week's typically occur as ice and snow melt in the spring, or during tropical storm season in the late summer and early fall. June is typically a time of drought.
Despite the heavy flow of recent days, the dam is not generating any more energy - rather, Exelon is losing the opportunity to process water that could have passed through turbines under less frenetic conditions.
"This is our fuel draining down the stream," said Ron Smith, an environmental specialist for Exelon.
Citing security concerns and maintenance being performed on two malfunctioning generators, Exelon officials would not allow reporters to tour the control room of the dam, which plunges 10 stories and runs across the river between Harford and Cecil counties, linked by U.S. 1.
But the same sort of decisions as those confronting Exelon were being made by the operators of the hulking T. Howard Duckett Dam in Laurel, where the waters of the Patuxent River back up into the rain-saturated Rocky Gorge Reservoir along the Howard and Prince George's county lines.
Unlike the Conowingo, the Duckett dam dispenses water to customers of the Washington Suburban Sanitary Commission.
After the first in a string of storms struck the region Sunday night, Karen Wright had a difficult call to make. The 52-year-old engineer could either order workers to manually lift the dam's metal gates and potentially flood homes and businesses in North Laurel, or do nothing and risk the water spilling uncontrolled over the tops of the gates, potentially damaging the structure and paralyzing her ability to control flooding for days.
"If we let that happen and lose control, there is a greater risk of failure," Wright said yesterday from inside the dam's control room at the arching corporate headquarters of the Washington Suburban Sanitary Commission on Sweitzer Lane in Laurel.
Aside from windows in small offices just off the control room, the nerve center for three area dams does not have windows. A row of computer work stations sits atop desks shaped like the top half of a hexagon and facing a floor-to-ceiling schematic of the commission's complex water system - pumps, tanks and dams marked inside color-coded shapes. The thin black lines connecting the boxes and triangles represent the pipes.
Before the recent rains, engineers at the Laurel dam had artificially lowered the Triadelphia Reservoir upstream for repairs, and Rocky Gorge was at a natural low because of the recently dry weather. The reservoirs filled up during Sunday's heavy storms and, had it not been for the low levels, Wright would have had to act earlier.
By Monday, however, the reservoir reached 100 percent capacity. To lessen the impact on communities downstream, Wright timed the release carefully. She watched a chart of downstream water levels, where sensors showed a heavy burst of rain had raised the river's level exponentially. Once that receded, Doppler radar showed another small shower passing through the area. Once the river flushed that out to the Chesapeake Bay, Wright acted - waiting any longer could increase the risk for a spillover.
"As soon as the water begins to go down, you know you have room in the river to put more water," she said.