By the numbers, the effects of the Clean Water Act on the Mississippi River have been nothing short of amazing. Now a new survey looking at more than a century of river chemistry reports brings the full value of the act into view like never before.

As early as 1909, people were testing the Mississippi, the largest river in the United States, for contents of bacteria, sulfates, lead, and oxygen. The great waterway was shown to become filthier and filthier until 1980, when the effects brought about by the 1972 Clean Water Act (CWA) started to kick in.

Eugene Turner, author of the Louisiana State University study, examined water quality reports around four sites near the terminus of the Mississippi River where it drains into the Gulf: St. Francisville, the disturbingly named "Plaquemine," New Orleans, and Belle Chasse.

Following the implementation of the CWA, sewage treatment infrastructure became mandated and more advanced, resulting in a huge fall in the density of fecal coliform bacteria from raw sewage dumping over the last 50 years. "They're 1% of what they were before the 80s," remarks Eugene to Nola.

Oxygen concentrations—necessary for aquatic life to survive, rose in three of the four sites over the same period, though the site farthest from the sea changed little over the last 50 years.

Lead pollution could almost be described as non-existent, such was the effect of the CWA industrial runoff restrictions. They're 1000x lower than they were in 1979. In some places they're 2000x lower.

In 2011, environmental agencies actually stopped surveying the Mississippi for lead because the minuscule amounts in the water remained the same for a period of about 10 years.

In 1950 there was about 50 milligrams of sulfur dioxide per liter of water. Thanks to reduced sulfate emissions resulting from the Clean Air Act, the river now averages a measly 18 micrograms per liter. This reduction in sulfate has also led to a restoring of a far-more normal pH level in the water of about 8.4.

"All of these changes occurred over decades; they were not accomplished quickly after a few masterly reconfigurations of technology or rules, but through sustained attention at many locations, one smokestack or sewerage plant at a time," writes Eugene in his study.

"The Clean Water Act has been tremendously effective at decreasing the amount of industrial and urban pollution, as this study shows," Olivia Dorothy, a Mississippi River management expert with American Rivers, told Nola. "We need to protect the act and all of its authorities, [and] we also need to start looking at expanding it to cover the emerging public safety threats as they relate to water."

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