The lowest summertime flow on the Minnesota River in 24 years is providing a rare opportunity: to compare water quality under similar conditions two decades apart.

The Minnesota Pollution Control Agency has been testing the metro end of the river to see whether the oxygen content may have reached dangerously low levels after seven recent months of drought and the second-hottest July on record in the Twin Cities. It's the first test of its kind since 1988, the last time the river flow in July and August was so meager.

"We still haven't seen a fish kill, so that's good news," said Glenn Skuta, water monitoring manager for the MPCA.

Workers were testing 21 miles of the river last week, from where it enters the Mississippi River at Fort Snelling to near Valley Fair in Shakopee. Test results, which won't be known for several weeks, will be compared with those from 1988, another legendary hot and dry year.

Skuta said the MPCA has been waiting since 1988 for the river to drop to a late summer weekly average flow of under 1,500 cubic feet per second. On Friday, the river flow at Jordan was about 1,000 cubic feet per second, about half the normal for the date. It dropped below 300 cfs temporarily during the summer of 1988.

Low oxygen threatens fish and other aquatic life. And while oxygen is the testing target, the analysis is also a way to measure whether wastewater treatment strategies introduced in 2004 and aimed at reducing phosphorus have been effective.

Since then, treatment plants in the Minnesota River basin have been required to reduce phosphorus in the treated water they dump into waterways, because phosphorus, a nutrient, spurs the growth of algae. When algae die, microorganisms that digest them consume oxygen, reducing the amount available to fish and other creatures.

The thinking behind this month's tests is that if there's more oxygen in the river than there was in 1988, that will indicate that the phosphorus-reduction efforts have been beneficial, since excess phosphorus, by nourishing algae, ultimately can deplete oxygen. Lake Pepin, part of the Mississippi River downstream from the metro area, saw a massive algal bloom in 1988, sparking phosphorus-reduction efforts.

The Met Council has tracked an 88 percent reduction in phosphorus discharges from metro wastewater treatment plans from 2001 through 2011. That's largely the result of adding phosphorus-eating bacteria to the discharge. Some of the treatment plants also add oxygen to their discharges in summer months.

Skuta said that while reducing the amount of phosphorus flowing from treatment plants into the river ought to have kept oxygen levels from dropping as far as in 1988, phosphorus also flows into rivers from other sources: in runoff from farms and cities, and in animal waste and grass clippings. It might also fluctuate because of changes in the river itself, including dredging and water temperatures.

While the basic one-week test was to conclude Friday, Skuta said it will continue as long as the river's flow continues to drop (and there are enough workers to keep testing). If a fish kill occurs, the extra testing might reveal the exact conditions at which that can be expected and prompt precautions, Skuta said.

Bill McAuliffe • 612-673-7646