On an average day, about 24 billion gallons of water flow down the Susquehanna River past John McSparron’s office — enough water to cover a square mile 115 feet deep.

But increasingly, McSparron, whose office is in the Susquehanna River Basin Commission building in Harrisburg, is worried that future dry summers will leave the the East Coast’s largest river as little more than a trickle.

Historically, the Susquehanna has been more notorious for floods than droughts, but as more users have tapped into what seems an inexhaustible water supply, that situation is changing.

River flows are normally low in the summer, frequently dropping to about 2 billion gallons a day at Harrisburg — one-twelth of the average. And more people and businesses are drawing that water out consumptively, especially during the hot months. “You project that out 40 to 50 years,” said McSparron, the SRBC’s chief engineer, “and we would have nothing but a dry river bed.”

Because it is an estuary — a unique and highly productive system where fresh and salt water meet — the Chesapeake depends on freshwater flows to create suitable conditions for many species. While droughts occur naturally, their frequency, duration and severity are expected to increase as a growing population consumes more water for everything from watering the lawn to electric generation to crop irrigation.

This isn’t a new concern. In 1984, the Army Corps of Engineers studied the impact of future freshwater flow decreases to the Bay and concluded that the grass beds, oysters, waterfowl, soft clams and other resources could suffer severe setbacks as a result of predicted salinity changes.

The report said that during future severe droughts, more than half the water that would normally reach the Bay could be siphoned off for other uses before ever reaching the Chesapeake.

Not much has changed since then, except more water is drawn out of Bay tributaries each year.

“Water quantity is the forgotten issue for the Bay Program,” said SRBC Executive Director Paul Swartz, who warned that future dry spells could mean problems, some of which may be unpredictable, for the Bay.

“I can’t tell you that if flows from the Susquehanna diminished to ‘x’ that it is going to have catastrophic results to the living resources of the Bay,” Swartz said. “We don’t know that. But intuitively, we think given some of the problems that seem to be associated with flow, like pfiesteria, and given some of the salinity-related phenomena, like oyster diseases, that it’s an important issue to learn more about.”

The flow issue is most closely tied to the Susquehanna because it is the Bay’s largest tributary, typically supplying about half of the freshwater that enters the estuary. Or, to look at it another way, the Chesapeake is the Susquehanna. The Bay was formed over thousands of years as the Susquehanna carried off eroded sediment from the mountains and built up what is now the Delmarva Peninsula.

But low flows are not just a Susquehanna issue. While the big river has the greatest impact on the Bay, protracted low flows in other tributaries can similarly affect resources within tidal portions of those rivers, and even parts of the Chesapeake. “That brings up the need for for managing consumptive water use to be done throughout the Bay watershed, not just in the Susquehanna,” Swartz said.

Total water use, on a per capita basis, has actually been declining largely because of water conservation efforts taken by industry and homeowners — things like water-conserving toilets or low-flow shower heads. That’s good news for water authorities and others who must build reservoirs or find other ways to slake the thirst of urban areas.

But consumptive use has been increasing. Consumptive use refers to water that is taken out and not returned. It is lost through evaporation, absorbed by plants, incorporated into products, or diverted out of a particular basin.

It’s often hard to think of water discharged from an industry or flushed down the toilet as a “good” thing — but at least it goes back into the river where people downstream can use it.

By late summer, it’s not unusual for flows in the Susquehanna to drop to 2 billion gallons a day during a mild drought. But water use in the basin is about 5 billion gallons a day. “By the time it gets to Harrisburg and the Bay,” McSparron said, “it’s been used two or three times. Peoples’ wastewater becomes the next peoples’ supply."

So water that goes down the drain is, in effect, recycled. But “consumptive use” water isn’t — for the most part, it vanishes into thin air or is absorbed by plants.

On an annual basis, about 90 percent of the water used for public water supplies is returned to the river, with the remainder lost to consumptive use. But the percentage lost to consumptive use can grow to 25 percent in the summer, when the river is lowest — and water demand highest.

Where does the water go? In the Susquehanna, the breakdown is like this:

  • Public water supplies consumptively use about 200 million gallons a day. Consumptive losses include things like watering the lawn, washing the car, evaporation from backyard pools and other outdoor water uses, as well as leaks in water lines.
  • Industry consumptively uses about 30 million gallons a day. This includes water incorporated into products, such as concrete, lost through evaporation, or used outside.
  • Agriculture consumptively uses about 120 million gallons a day. This is the fastest growing water use category and reflects the increased use of irrigation as farmers try to improve the quality, and productivity, of high-value crops. The growing number of large, animal feedlots are also large consumptive users.
  • Electrical generation consumptively uses about 130 million gallons a day. Almost all power plants use water to create steam, resulting in large evaporative losses. A coal-fired or nuclear power plant evaporates a half gallon of water through its cooling tower to create enough electricity to burn a 100 watt light bulb for 10 hours.
  • All others consumptively use about 60 million gallons a day. This includes hospitals, prisons, institutions and golf courses that are not hooked to municipal water supplies. An 18-hole golf course may lose 200,000 gallons a day to evapotranspiration.

All that adds up to more than 500 million gallons a day. During the worst droughts in the past several decades, river flow has fallen as low as 1.2 billion gallons a day. If today’s consumptive use were subtracted from that, only 700 million gallons would remain in the river. And by 2010, with projected increases in consumptive use, only a little more than 400 million gallons would remain — something that would turn the Susquehanna into a mere trickle.

Those effects, ultimately, would be passed on to the Chesapeake, where they would greatly change salinity patterns in the upper Bay.

Right now, the Susquehanna is the only river basin within the Bay drainage where consumptive water users are routinely required to make up for water losses.

Since 1971, the SRBC has had to approve any consumptive use that removes more than an average of 20,000 gallons per day for any consecutive 30-day period. Users generally either have to provide makeup water to offset their consumptive use, discontinue consumptive use, or pay the commission during low flow events — at a rate of about 14 cents per 1,000 gallons.

The commission then uses the money to purchase water storage in upstream reservoirs. When the water drops to a certain level, the commission “turns on the tap,” so to speak, and has the water released to augment flows.

But not everyone contributes to the solution. Municipal water authorities are exempt from the regulations, and an SRBC attempt to charge for large agricultural withdrawals several years ago was abandoned after drawing stiff opposition. And many water users established prior to 1971 are exempt as well.

As a result, even as the SRBC has the most comprehensive consumptive use requirements, its program fails to fully offset consumptive losses. Consumptive water lost through municipal water supplies is expected to increase as the population grows, and irrigation is expected to expand as well.

And there are outside interests eying Susquehanna water: The SRBC has recently battled with the City of Baltimore — located outside the Susquehanna basin — which contends it has a long-stranding right to withdraw up to 250 million gallons of water a day from the river without replacing it.

Another concern is that the SRBC’s primary source of water storage is at the Conawesque Reservoir in northern Pennsylvania. Using funds from two utilities that were required to replace consumptive use water, the SRBC in the late 1970s contracted with the Army Corps of Engineers to purchase water stored at the reservoir during low flows.

While that can offset some of the consumptive use and help maintain flows to the Bay, it does little to relieve drought effects that may be spread throughout the Susquehanna’s six major subbasins, leaving tens of thousands of miles of tributary streams vulnerable to drought effects.

As the flow decreases, the amount of water available to fish and other resources in streams declines — sometimes dramatically. Full streams spread over wide, shallow banks, giving fish lots of habitat. But as the flow diminishes, they are restricted only to the deep channel of the stream.

Crowded together, the fish are more susceptible to stress and disease — not to mention anglers. And streams with less water are often more susceptible to pollution problems and low levels of dissolved oxygen. “It makes them easy pickings for the birds and the animals — and the fishermen,” McSparron said.

McSparron said studies in Europe have shown that each 1 percent decline in the average flow of a stream over the course of a year translates into 1 percent decline in total fish biomass in that stream.

It’s something that increasingly concerns SRBC biologists. Historically, the commission has planned for what is known as the seven-day, 10-year drought — the lowest flow over a one-week period that might be expected during a decade.

That flow was set as a design criterion for wastewater dischargers to meet water quality standards. “This is not really a fish habitat number,” McSparron said.

Recent studies indicate that maintaining adequate habitat for bass, trout as well as migratory fish the Bay Program hopes to restore — such as shad and river herring — requires more water than the 10-year-drought plans for.

To offset those impacts, SRBC planners — in what could be a model for the Bay watershed as a whole — hope to supplement water flow in coming years by converting a series of flood control dams, located in the headwaters of each of the Susquehanna’s four upstream subbasins, into storage sites that could be tapped during low flows.

The SRBC is studying the possibility of expanding the storage capacity at the Whitney Point reservoir in New York. As part of that study, biologists for the first time are looking at whether they can maintain flows based not just on water quality needs, but on the benefit for stream dwellers. “We’re no longer looking at a situation of replacing water one-to-one,” McSparron said. “If we can replace it two to one and three to one and improve the quality of the resource, we would like to do that.”

When water levels drop to the point where they are harmful to resources — but are still above the 10-year drought — water could be released to improve conditions in the future. If, for example, the seven-day, 10-year trigger would have been 500 cubic feet per second flow at a particular area, but studies show there is a huge habitat benefit if flows were augmented 1,000 cfs, future water releases might be modified.

Exactly how all this would be paid for is unclear. There is talk of having state legislatures appropriate money to pay for the agricultural use of water. And — if improving low flow conditions is shown to substantially benefit stream habitat — some federal environmental restoration funding may be available.

New consumptive water users — everyone from new co-generation power facilities to bottled water companies — usually opt to pay the 14-cent per 1,000 gallon fee rather than trying to find other options to offset consumptive use. Over time, that will gradually provide more funds as well. “We expect that revenue source to grow,” said Rich Cairo, SRBC counsel.

Still, the irony that dams originally built to provide flood relief in the most flood-prone watershed in the East may one day provide relief during droughts is not lost on McSparron. “It seems crazy that we would manage for the high flows,” he said, “and not do anything about managing for the low flows.”