The Mill Creek has been one of the most polluted waterways in the US for years. But after decades of hard work, the ecosystem starting make a come back. (Photo Credit: Science Over Everything)

The Rise of the Mill Creek – How Cincinnati is restoring its dirtiest waterway

Next Generation Science Standards: 

  • MS-ESS3-3. Apply scientific principles to design a method for monitoring and minimizing a human impact on the environment.*
  • MS-LS2-1Analyze and interpret data to provide evidence for the effects of resource availability on organisms and populations of organisms in an ecosystem.
  • MS-ESS2-2.Construct an explanation based on evidence for how geoscience processes have changed Earth’s surface at varying time and spatial scales.

Key Vocabulary: aquatic, habitat, restoration, headwaters, pollution, invertebrates, ecosystem, floodplain, food web, waterways

Mill Creek Article Guide

Lab_Erosion and Deposition

Few waterways in the United States have been as critical to the growth of a city has the Mill Creek has been to Cincinnati. The river runs through the city’s center until it meets the Ohio River just west of downtown. Just a few hundred years ago, the river was pristine and supported a growing city on what was then the American frontier.  

But after nearly a century of pollution from unregulated industries along its banks, the Mill Creek became a dumping ground for chemicals and waste. By the 1960’s, the river was devoid of almost all aquatic birds, fish, mammals, and invertebrates. In 1997, the Mill Creek was named “the most endangered urban waterway in America”, an embarrassment for the city. The national recognition spurred a coordinated clean-up and after twenty years of restoration, the Mill Creek’s ecosystem is finally starting to make a come back.

A Dirty History

The Mill Creek runs for about 30 miles towards the southwest from its headwaters in Bulter County, draining the entire city of Cincinnati and most of the surrounding areas. The river itself formed from the meltwater of glaciers retreating at the end of the last Ice Age several thousand years ago. For centuries, the Native Americans who lived in the area relied on the Mill Creek for their livelihood.

As European settlers arrived at the turn of the 19th century, Cincinnati’s population grew quickly, displacing native peoples and developing land for industries. The creek became an instant draw, providing fresh water and a transportation option at a time when moving goods and people more than a few miles was a real challenge. Slaughterhouses, breweries, and mills developed along the banks of the Mill Creek. The EPA wouldn’t be established until decades later, giving businesses little to no accountability over how they used the local water supply. As a result, the creek became a dumping ground. Factories released chemicals and industrial waste into the waterway with no oversight and the city allowed for untreated sewage to enter the creek.

The Mill Creek became more or less an open sewer as industry developed. (Photo Credit: Mill Creek Council of Communities)

By the 1990’s, the problem was so bad that when the Ohio EPA conducted its first comprehensive survey of the Mill Creek, bacteria levels from raw sewage exceeded federal and state standards at almost every single sampling site. Heavy metals like lead and cadmium, pesticides, and ammonia contaminated river sediments and polychlorinated biphenyls, a chemical used as an electrical coolant, were found in the tissues of fish. The survey found sludge worms, bloodworms, and leeches were the only animal species living in the inner city segment of the Mill Creek due to the high pollution levels. The ecosystem was more or less dead and in 1997 the conservation group American Rivers named the Mill Creek the most endangered urban waterway in America, a dubious honor and a black-eye for the city of Cincinnati.

Getting Clean

In the fallout of the national embarrassment, the Ohio EPA become a more stringent enforcer of water quality levels and levied fines on industries that exceeded pollution levels. No longer could businesses dump their waste in the creek without consequences. Several community groups such as Groundwork Cincinnati and the Mill Creek Watershed Council of Communities formed to organize river clean-up days and helping to enforce water quality regulation with industries in over 40 communities. In July 2018, these groups will merge to form the Mill Creek Alliance, which will allow for better organization and deeper impact. 

Cleaning up the Mill Creek has taken a lot of hard work from volunteers and staff. (Photo Credit: Mill Creek Council of Communities)

A wide variety of habitat restoration projects have been completed to begin restoring the Mill Creek’s natural habitat. City sewer lines were built across the Mill Creek, acting like dams and preventing fish from getting upstream. Fish ladders have allowed for fish to move from the Ohio River to smaller creeks and waterways to lay their eggs. Tree planting days and wetland restoration grants have brought back native plants and slowed the flow of the river. As the pace of river water slows down, it gives fish a place to breed and birds, reptiles, mammals, and invertebrates a place to live and food to eat. Floodplain benching projects have also been completed in populated areas, giving a permeable surface for rainwater to go during periods of flooding.

Restored Floodplain at Caldwell Park (Photo Credit: Mill Creek Council of Communities)

However, the biggest restoration project to date has been the Greenway Trail, a 3 mile bike path along the river that provides a route through neighborhoods with limited transit options and restores river habitats. The Alliance will work to connect completed sections of the Greenway, and has funding to build another mile of the the bike trail. The hope is to one day have the Greenway follow the entire length of the Mill Creek.

The results have been astounding. Water quality has dramatically improved, so much that American Rivers has taken the Mill Creek of their endangered list. Invertebrates such as insects, crustaceans, and snails, which form the base of aquatic food webs, have returned. These species are sensitive to pollution, so their come back is a good indicator of ecosystem health. With more invertebrates to eat, nearly 50 species of fish have also been documented as returning to the watershed. The population of catfish in the watershead increased by over 500%. River mammals such as muskrat and beaver have also made their way back. Perhaps most encouraging of all, in 2017, an Osprey was seen fishing along the river’s banks for the first time in 30 years. If top predators can find enough food in the Mill Creek, it’s a good sign that the rest of the ecosystem is doing better.

The Greenway has provided a transit option for resident while at the same time protecting the natural habitat of the Mill Creek (

The future of the Mill Creek

The Mill Creek is substantially healthier than it was 20 years ago, however, major ecological problems persist, the biggest of which is the city’s sewer system. Cincinnati has what is called a combined sewage overflow, an older design in which excess stormwater gets mixed with the city’s sewage, and together, is dumped in the Mill Creek. Cincinnati has experienced several exceptional floods in the last few years, putting a lot of untreated human waste and trash into the Mill Creek. An updated system would be both environmentally friendly and cheaper to operate, but extremely expensive to build. Sewage lines that cross the river would have to be rerouted and water lines would have to be dug up. Cost estimates would be close to $2 billion and would likely take a decade or more to complete.

This picture shows how combined sewage overflow systems (CSO) work. Photo Credit: Science Over Everything

And while the current Greenway has helped restore a lot of natural habitat, the vision for extending the line along the entire Mill Creek would be challenging indeed. Multiple railroads run along the river, the right of way on which are owned by the rail companies, an asset they will not likely give up easily. To extend the Greenway the 6 miles to where the Mill Creek meets the Ohio River would be very costly and legally challenging, despite the ecological benefits.

Clearly, hard work remains. But hope in the future, perhaps predictably, is vested in kids. Groundwork Cincinnati, which will soon become part of the Mill Creek Alliance, has had a robust youth outreach program. Classes from around the Cincinnati area bring students into the field to take water quality data and learn about the river’s history and ecology. High school students can also join the Green Team and work on river restoration projects and data collection while getting paid. Students can continue to advance their skills by moving to the Green Corps after graduation and be prepared for high demand environmental jobs.

Getting students involved in taking care of the Mill Creek is an important part of the Mill Creek Alliance’s mission. (Photo Credit: Groundwork Cincinnati)

Building a sense of environmental stewardship in the next generation will be the critical if the Mill Creek is to continue its resurgence. The efforts of the last 20 years have made for an impressive come back, but an enormous amount of work remains before the decades of pollution and abuse are finally cleaned. That effort will fall on the shoulders of today’s students, as their task will be to rebuild so many of the world’s ecosystems.

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