A lot happens where the Chattahoochee River meets Atlanta's Peachtree Creek.

That's where the river abruptly transforms from metro Atlanta's drinking water source into a repository for its treated sewage. The entire transition happens within rock-throwing distance of the intersection of the river and the creek.

Stacy Shelton/AJC (ENLARGE)

Jason Ulseth, watershed protection specialist for the Upper Chattahoochee Riverkeeper, and intern Edward Sutherland (holding line) measure for dissolved oxygen in the river under Camp Creek Parkway, which connects Fulton and Douglas counties.

MORE ON THE DROUGHT: • Map: Heavy demands on our water • Photos: April showers bring ... • Photos: Lake rises dramatically in January RELATED: [an error occurred while processing this directive] • More on coping with the drought

Just upstream, the city of Atlanta withdraws as much as 100 million gallons of water a day from the river. Across the river, Cobb County's R.L. Sutton plant pipes in the first of the region's major treated sewage discharges. A little way downstream and around the bend is the city's massive R.M. Clayton wastewater plant.

Peachtree Creek is the spot, understandably, where more than 30 years ago the state set a standard for the minimum amount of water in the river. The standard, set at 750 cubic feet per second or 484.7 million gallons of water a day, is an important determinant of how much water is released daily into the river from Buford Dam at Lake Lanier.

The problem is, there's no gauge in the river at Peachtree Creek and thus no way to accurately measure the flow.

A dozen U.S. Geological Survey monitoring stations constantly measure the Chattahoochee's flow between Buford Dam and West Point Lake, but there has never been one at Peachtree Creek.

The measurement is important because not enough water at this juncture would mean dangerously high concentrations of contaminants sent downstream, jeopardizing drinking water sources and the river's wildlife.

It's also the starting point for forecasting metro Atlanta's growing water needs. The state uses it to calculate the river's ability to handle the stress of being the region's workhorse.

All those factors were considered this spring when the U.S. Army Corps of Engineers, which operates the dam, reduced the flow more than 13 percent at Georgia's request to conserve more water in the lake during the drought.

But while the standard is based on the water level at Peachtree Creek, the measurement is actually taken nearly three miles upstream.

The river flow is calculated by using the gauge at Paces Ferry Road in Vinings for the instantaneous flow, then subtracting Atlanta's daily withdrawals, which are not available until the next day.

"We need to have instantaneous monitoring in place to make sure we're attaining the flow," said Sally Bethea, executive director of the Upper Chattahoochee Riverkeeper, a river watchdog group.

"This is the heaviest used river in the state of Georgia. ... Millions [of people] depend on it."

Brian McCallum, assistant director of the USGS Georgia Water Science Center in Atlanta, said because of the technical difficulties at the site, it would be tough but not impossible to install a monitoring station.

"All the regulations point to this location on the map, and ... if water is going to be scrutinized to this degree, it would probably be helpful for them to have a gauge there eventually," McCallum said.

The cost for a standard monitor is $23,000, plus $13,600 a year to maintain it. He said a gauge at Peachtree Creek would cost more. USGS, the state Environmental Protection Division and other local partners spend about $3.4 million a year to measure the flow of rivers and streams, lake levels and rainfall.

Fred Cox, senior engineer for Southern Co., which has a Georgia Power dam on the Chattahoochee near Roswell, said the current system works.

"You're never going to get things perfect," Cox said. "There's a certain amount of error in any river gauge."