Relying on the current E. coli test, which takes 24 hours to produce a reading, "is not a public service," Indiana Dunes National Lakeshore Superintendent Dale Engquist said.
The park began testing water off its beaches for E. coli bacteria in 1984, long before other parks in the region.
Now it will rely less on water tests and more on educating swimmers about potential risks, assistant resource manager Scott Hicks said.
Tests have shown that high-bacteria readings usually drop sharply the day after a high reading.
As a result, beaches often have been open on high-bacteria days - before the test results are ready - and closed on low-bacteria days.
"We think there's a false sense of security when we say beaches are open or closed (based on the test)," said Todd Webb, the Indiana Dunes State Park manager.
Scientists, including Greg Olyphant of Indiana University and Richard Whitman of the U.S. Geological Survey, have been developing systems to predict high-bacteria beach days based on factors like rainfall, wind direction and temperature.
Thanks to federal funding, more money is available for more frequent testing of beach water - and better systems to tell people about the results.