The question of the “dirtiest” river in the United States is complex, as contamination is measured by multiple scientific criteria rather than a single ranking. Federal and state agencies utilize various physical, biological, and chemical metrics to determine a river’s health and whether it is classified as “impaired.” This complex system means a river might be highly polluted by agricultural nutrients but clean of industrial toxins, or vice-versa. To understand US river pollution, it is necessary to first define the scientific standards by which water quality is judged.
Defining River Contamination
Federal agencies, such as the Environmental Protection Agency (EPA), establish water quality criteria based on a waterbody’s ability to support “designated uses,” including drinking, fishing, and swimming. When a river fails to meet the standards for one or more of these uses, it is officially classified as an “impaired water.” This determination often leads to the development of a Total Maximum Daily Load (TMDL), which sets the maximum amount of a pollutant a waterbody can receive while still meeting water quality standards.
One of the primary metrics for chemical contamination is the Toxics Release Inventory (TRI), a publicly available database that tracks the management and release of over 650 toxic chemicals by industrial and federal facilities. Beyond chemical analysis, biological indicators are used to assess long-term ecological health, as aquatic life cannot simply move away from chronic pollution events.
Scientists examine the diversity and abundance of benthic macroinvertebrates, organisms like insect larvae and crustaceans that live at the bottom of the river. The presence of pollution-sensitive species, such as certain mayflies, stoneflies, and caddisflies, indicates a healthy ecosystem. Conversely, a community dominated by pollution-tolerant worms and midges suggests poor water quality. Measurements of dissolved oxygen, temperature, and pH are also included in the overall assessment of river impairment.
Identifying the Most Polluted Waterways
When judged by the sheer volume of permitted toxic chemical discharges reported to the TRI, the Ohio River has historically been cited as one of the most polluted rivers in the US. The river, a source of drinking water for five million people, has long struggled with a legacy of heavy industrial pollution. The discharges include high levels of mercury and other toxic substances, which pose a severe risk to human health and aquatic life.
The Mississippi River and its tributaries consistently rank high for total toxic releases, largely due to permitted industrial wastewater discharge. Historically, a significant portion of the toxic chemical load in the Mississippi has come from fertilizer plants releasing substances like phosphoric acid and ammonia. The enormous scale of the Mississippi River basin means it receives pollution from a variety of sources across more than 40% of the continental United States.
Other rivers are cited for highly localized, severe contamination, often from legacy pollution. The Lower Passaic River in New Jersey, for instance, contains significant amounts of dioxins and polychlorinated biphenyls (PCBs) settled in its sediment from decades of industrial activity. Similarly, the Calcasieu River in Louisiana is burdened by industrial waste, chemical runoff, and oil and gas byproducts, reflecting the impact of chemical and petrochemical production in the region.
Primary Causes of Pollution
River pollution is typically categorized into two main groups: point source and nonpoint source. Point source pollution originates from a single, identifiable location, which is usually a pipe or ditch discharging into the waterbody. This includes discharge from factories, which can release heavy metals, industrial solvents, and toxic chemicals. Wastewater treatment plants are also classified as point sources; they release treated effluent or, during heavy rains, can overflow with untreated sewage.
Nonpoint source pollution is much more diffuse, coming from broad areas of land rather than a single pipe. This is the largest source of impairment for many US rivers, primarily consisting of agricultural runoff. Rain and irrigation wash fertilizers, pesticides, and animal waste from farm fields into streams and rivers, introducing excessive nutrients like nitrogen and phosphorus.
Urban runoff is another major nonpoint source, where stormwater carries sediment, oil, grease, and chemicals from streets and paved surfaces directly into waterways. Beyond these active sources, many rivers are also affected by legacy contamination, which is historical pollution embedded in the riverbed sediment and floodplain. These sites, often abandoned industrial areas, continue to leach toxic substances decades after the original source was shut down.
Ongoing Clean Water Initiatives
The foundation of US river protection is federal legislation, which mandates the development of water quality standards and programs to prevent pollution. This legislation requires the EPA to establish criteria for surface water quality based on the latest scientific understanding of pollutant impacts. It provides the legal framework for monitoring and enforcement, significantly reducing industrial pollution and improving sanitation infrastructure since its passage.
Federal funding mechanisms support local and state efforts to address contamination. The Clean Water State Revolving Fund (CWSRF) provides low-interest loans to states for water quality infrastructure projects. These funds finance the construction of publicly owned treatment works, nonpoint source control projects, and efforts to manage stormwater runoff.
Beyond federal mandates, restoration efforts are advanced through state-led watershed management programs and community initiatives. These programs focus on localized challenges, such as implementing best management practices for agriculture to reduce nutrient runoff. Restoration projects often involve developing green infrastructure like rain gardens and permeable pavements in urban areas to naturally filter stormwater before it reaches the river. These collaborative efforts aim to improve water quality and restore the ecological health of impaired waterways.