A river catches fire, so polluted that its waters have "no visible life, not even low forms such as leeches and sludge worms." This could describe the mythological River Styx from Hades. Residents of Cleveland, though, may recognize the government's assessment of their own Cuyahoga River in 1969. While it is hard to imagine today, discharging raw sewage and pollution into our harbors and rivers was common practice for most of the nation's history, with devastating results. By the late 1960s, Lake Erie had become so polluted that Time magazine described it as dead. Bacteria levels in the Hudson River were 170 times above the safe limit.

In 1972, a landmark law reversed the course of this filthy tide. Today, four decades later, the Clean Water Act stands as one of the great success stories of environmental law. Supported by Republicans and Democrats alike, the act took a completely new approach to environmental protection. The law flatly stated there would be no discharge of pollutants from a point source (a pipe or ditch) into navigable waters without a permit. No more open sewers dumping crud into the local stream or bay. Permits would be issued by environmental officials and require the installation of the best available pollution-control technologies.

The waste flushed down drains and toilets needed a different approach, so the act provided for billions of dollars in grants to construct and upgrade sewage-treatment plants. To protect the lands that filter and purify water as it flows by, permits were also required for draining and filling wetlands.

This may seem like common sense today, but the idea of nationally uniform, tough standards against polluters was both original and radical. Thinking big, the Clean Water Act's preamble declared that the nation's waters would be swimmable and fishable within a decade, with no discharges of pollutants within a dozen years.

By many measures, the Clean Water Act has fulfilled this ambition. The sewage discharges that were commonplace in the 1960s are rare. The number of waters meeting quality goals has roughly doubled. Once a dumping ground for all manner of filth, rivers now represent an urban gem. Hartford, Conn.; Kansas City, Kan.; Cleveland, and other cities have based their redevelopment around now clean and inviting waters, with parks along the water's edge. More people have access to safe drinking water from their taps than ever before.

But major challenges remain. The Environmental Protection Agency estimates that about half of our rivers and streams, one-third of lakes and ponds and two-thirds of estuaries are "impaired waters."

The reason lies in the Clean Water Act's narrow focus. We have made great progress in controlling industrial pipes that discharge waste, but other major sources remain largely unregulated. To gain sufficient congressional support from farm states in 1972, the Clean Water Act was written to exempt most runoff from agricultural fields and irrigation ditches. As a result, pesticides, manure and other pollutants have flowed into our waters. To take the most frightening consequence, the Mississippi River basin, draining one-third of the country, empties nutrient-laden waters into the Gulf of Mexico. There, the "Dead Zone" regularly grows to 6,000 square miles or more, suffocating sea life that cannot swim away from its oxygen-starved waters. Storm-water runoff with oil and trash also threatens water quality around urban areas.

These "nonpoint" sources can be addressed, but this requires enhanced authority to regulate farm practices and major funds to overhaul storm-water infrastructure. Neither seems an easy option with a divided Congress and tight budgets.

And then there are threats that have emerged more recently. The bounty of natural gas made possible by hydraulic fracturing may provide energy security, but it raises concerns about contamination of groundwater by methane and fracking fluids. And recent studies have identified more than 50 pharmaceuticals or byproducts in the drinking water of metropolitan areas. Some of the contaminants included antibiotics, anti-anxiety drugs and hormonal medications such as birth-control pills.

When the Cuyahoga burst into flame in 1969, it was not a huge deal to locals. The river had burned before. Today, though, such an event seems inconceivable. There remains much unfinished business, but it is important to recognize the undeniable achievements of the Clean Water Act, particularly when the very role of government in environmental protection has been under challenge.

The Clean Water Act was not inevitable. It took vision and bipartisan commitment. Many forget that President Richard Nixon vetoed the law, concerned about the cost of treatment plants. The next day, members of Congress on both sides of the aisle came together to override his veto. At a time of high-stakes partisan wrangling and gridlock, when no major environmental legislation has been passed for more than 20 years, the Clean Water Act's anniversary gives us cause both to celebrate and to consider whether some shared environmental benefits - clean air, clean water, open space - can once again transcend partisan differences.

James Salzman is a professor at Duke Law School and the Nicholas School of the Environment and the author of "Drinking Water: A History." He wrote this for Slate.