How Clean Is the Tennessee River?

The Tennessee River is cleaner than it was half a century ago, but it carries a complicated mix of old and new pollutants that keeps it far from pristine. Industrial chemicals left over from decades of manufacturing sit in its sediments. A massive coal ash spill in 2008 introduced heavy metals that are still detectable in river-bottom mud. Trace pharmaceuticals, microplastics, and newer synthetic chemicals show up in water samples. Whether the river is “clean” depends heavily on what you are asking about and where along its 652-mile course you are standing.

A River Shaped by Industry

The Tennessee River drains parts of seven states and has been a workhorse for American industry since the Tennessee Valley Authority began building dams in the 1930s. Cheap hydroelectric power attracted heavy manufacturing, chemical plants, and federal facilities, including nuclear and weapons-related operations near Oak Ridge. That economic engine came with a cost: the riverfront and its tributaries became heavily contaminated with industrial discharge.¹ The Clean Water Act of 1972 and tighter state regulations eventually curbed the worst point-source pollution, but decades of effluent had already settled into sediments and entered biological food chains.

Habitat alteration has compounded the chemical problems. Dams, introduced species, toxic spills, mining, and agriculture have fragmented the upper Tennessee River system so severely that nearly 40% of the riverine habitat in major tributaries is either impounded or altered by tailwater discharges.² That fragmentation isolates fish and mussel populations and limits the river’s ability to recover from pollution events on its own.

The Kingston Coal Ash Spill

On December 22, 2008, a containment wall failed at TVA’s Kingston Fossil Plant, releasing roughly 5.4 million cubic yards of wet coal ash into the Emory River, which feeds into the Clinch and then the Tennessee. It remains one of the largest industrial spills in U.S. history, and its effects on the river system have been studied continuously since.

An 18-month investigation found that several leachable contaminants from the coal ash, particularly arsenic, selenium, boron, strontium, and barium, showed up in the surrounding environment. Downstream river water stayed below EPA drinking-water and ecological thresholds, but surface water in areas with restricted flow and pore water extracted from river sediments told a different story. Arsenic concentrations in pore water reached up to 2,000 micrograms per liter in some spots.³ That is many times above the EPA’s drinking-water standard for arsenic, though the contamination was concentrated in sediment pore water rather than the open water column.

Mercury proved to be an especially persistent concern. In the sediments of the Emory River near the spill site, total mercury concentrations were three to four times higher than in sediments upstream. More troubling, methylmercury, a form of mercury that accumulates up the food chain and acts as a neurotoxin, was elevated by up to a factor of three at certain locations. The coal ash appeared to have provided sulfate and other substrates that stimulated bacteria to convert inorganic mercury into this more dangerous form.⁴

Eight years after the spill, researchers checked on how the situation had evolved. The ash from 2008 had been buried under roughly 13 to 18 centimeters of natural sediment at downstream locations. However, elevated ash and metal concentrations were still present in shallow sediments close to the Kingston Plant, suggesting the ash was redistributing from upstream. A comparison of metals in raw coal ash versus river sediments that had sat in the water for two years showed similar concentrations, implying the metals were not dissolving readily into the water column. They were staying locked in the sediment, which is good news for surface water quality but means the contamination is still physically there, waiting.⁵

PCBs and the Fish You Catch

Polychlorinated biphenyls, or PCBs, are synthetic chemicals that were widely used in electrical equipment and industrial applications before being banned in the late 1970s. They break down extremely slowly in the environment and tend to accumulate in sediments. Because they are fat-soluble, they concentrate up the food chain: small organisms absorb them, fish eat those organisms and accumulate higher doses, and anything eating the fish gets an even larger dose.

In East Tennessee, the Oak Ridge Reservation is a major legacy source of PCBs. Long-term monitoring of fish in lower-order tributaries on the reservation and in the adjacent Clinch and Tennessee River reservoir system shows that PCBs are declining, but the pace varies widely. In small stream fish like minnows and sunfish, attenuation rates ranged from 0% to 8% per year. In catfish from the larger reservoirs, rates were somewhat faster at roughly 5% to 11% per year.⁶ That spread means some locations are still seeing PCB levels decline slowly or barely at all even 35 years after monitoring began. Fish consumption advisories remain in effect in several parts of the Tennessee River system, particularly near Oak Ridge, and are likely to persist for years to come.

Pharmaceuticals in River Water

When you take a pill, your body does not absorb all of it. Some of the active ingredient passes through and enters the wastewater stream. Wastewater treatment plants are not designed to remove most pharmaceuticals, so trace amounts end up in rivers. Researchers have looked specifically at the Tennessee River and its tributaries for these compounds.

A study sampling multiple points along the upper Tennessee River and at the inflow of major tributaries detected 14 pharmaceutical compounds at very low concentrations, measured in parts per trillion. Caffeine, carbamazepine (an anti-seizure medication), and sulfamethoxazole (an antibiotic) were among the most frequently detected, and their concentrations showed statistically significant trends related to season and proximity to sewage treatment plants.⁷

A separate investigation in a smaller East Tennessee waterway, the West Prong Little Pigeon River, found ibuprofen and triclosan (an antimicrobial once common in hand soaps) in river water near a wastewater outfall. No estrogen hormones were detected directly, but estrogenic activity was measured in river water using biological screening assays. The measured estrogenic activity was much higher than what could be explained by the known concentrations of detected compounds, suggesting the presence of other unknown estrogenic chemicals or additive effects from mixtures at low concentrations.⁸ The concentrations involved are vanishingly small compared to a therapeutic dose, but the ecological effects of chronic low-level exposure on fish reproduction and behavior are still being studied across the country.

PFAS and Microplastics

Per- and polyfluoroalkyl substances, widely known as “forever chemicals,” are among the newer contaminants worrying environmental scientists. PFAS are used in nonstick coatings, food packaging, firefighting foams, and countless other products. They resist breakdown in the environment and can accumulate in drinking water sources.

A statewide survey of Alabama’s rivers, which include stretches of the Tennessee River as it flows through the northern part of the state, detected PFAS in 65 out of 74 riverine samples. The average total concentration across six measured PFAS compounds was about 35 nanograms per liter, with the highest single-river concentration of 237 nanograms per liter found in the Coosa River rather than the Tennessee.⁹ The near-ubiquity of PFAS detection across Alabama’s waterways, including portions of the Tennessee River basin, fits a national pattern. The EPA finalized enforceable drinking-water limits for several PFAS compounds in 2024, which will force many utilities to add treatment technologies they currently lack.

Microplastics are another emerging concern. During and after the historic flooding in the Mississippi River system, researchers measured microplastic concentrations in several tributaries. The Tennessee River contained roughly 14 microplastics per liter during flooding conditions. That was the lowest concentration measured across the tributaries sampled; the Ohio River during low-flow summer conditions had roughly 83 microplastics per liter.¹⁰ Flooding dilutes concentrations because of the sheer volume of water, so the Tennessee River’s low-flow concentrations are likely higher than that 14 per liter figure. Still, the data puts the Tennessee toward the lower end of contamination among the major tributaries of the Mississippi system during the study period.

Nutrient Pollution and Algal Bloom Risk

Agricultural runoff is one of the less dramatic but most widespread water quality problems in the Tennessee River basin. Fertilizers applied to crops wash into streams during rainstorms, carrying nitrogen and phosphorus into the river system. When these nutrients build up, they fuel algal blooms, including blooms of cyanobacteria (sometimes called blue-green algae) that can produce toxins dangerous to people and animals.

An analysis of nutrient ratios across the Mississippi-Atchafalaya River basin found that the Ohio-Tennessee subbasin, along with the Upper Mississippi subbasin, showed conditions favorable for cyanobacterial blooms. Agriculture is the dominant land use in both regions, and these subbasins controlled the overall nutrient balance of everything delivered to the Gulf of Mexico. Between the 1980–1994 and 2001–2015 periods, phosphorus loading increased significantly in the Ohio-Tennessee subbasin.¹¹ Rising phosphorus is the kind of trend that makes water quality scientists nervous, because phosphorus is often the nutrient that tips the balance from clear water to green, scummy water in reservoirs and slow-moving river reaches.

There is some hope on the nitrogen side. Modeling work has shown that switching portions of the Tennessee River basin’s agricultural land to perennial bioenergy crops, which require less fertilizer, could greatly reduce nitrate and total nitrogen loadings in the river. The effects on sediment and phosphorus were smaller, but nitrogen reductions alone would be a meaningful improvement.¹²

What Dams Do to River Health

The Tennessee River is one of the most dammed major rivers in the country. TVA operates a system of 49 dams, and the mainstem Tennessee is essentially a chain of reservoirs rather than a free-flowing river. Dams provide flood control, navigation, and electricity, but they also alter water temperature, dissolved oxygen, and flow patterns in ways that affect everything living in and around the water.

Below many dams, water released from deep in the reservoir is cold and low in dissolved oxygen. Fish and mussels that evolved in warm, oxygen-rich rivers can struggle in these tailwater zones. TVA has invested in dam modifications over the years, including weirs, turbine venting, and other aeration technologies, and those changes have produced real results. At modified dams, yearly average dissolved oxygen levels and minimum water velocities increased following the changes.¹³ The improvements are genuine, though they are working against the fundamental reality that turning a river into a chain of lakes changes its ecology in ways that cannot be fully reversed without removing the dams themselves.

Freshwater Mussels Tell the Story

If you want a single group of organisms that reflects the Tennessee River’s water quality over time, freshwater mussels are it. The Clinch River watershed in the upper Tennessee River basin supports one of North America’s greatest concentrations of freshwater biodiversity, including 46 surviving species of native freshwater mussels, 20 of which are federally listed as endangered.¹⁴ Mussels are filter feeders that spend their entire adult lives in one spot, pulling water through their bodies. That makes them exceptionally sensitive to pollution: whatever is in the water ends up in their tissues.

Despite the global significance of this mussel diversity, populations are declining in some reaches of the Clinch, both in the number of species present and the total number of individuals. Researchers found that polycyclic aromatic hydrocarbons, a class of chemicals produced by burning fossil fuels, were present in mussel tissue at concentrations that correlated negatively with mussel density. The higher the PAH levels in the mussels, the fewer mussels were found. The contamination was traced largely to a tributary called the Guest River.¹⁴ Losing mussels is not just an ecological concern in the abstract. Mussels filter enormous quantities of water, and their decline removes a natural cleaning mechanism from the river.

Thermal Pollution From Power Plants

The Tennessee River cools a lot of machinery. TVA operates coal, gas, and nuclear plants along the river, and all of them discharge heated water back into reservoirs. At the Sequoyah Nuclear Plant on Chickamauga Reservoir, for instance, the permitted thermal discharge limit was a maximum temperature rise of 3°C. TVA sought to raise that limit to 5°C during winter months, arguing the existing standard was more stringent than necessary to protect the local fish population.¹⁵ Thermal discharge can lower dissolved oxygen, speed up metabolic rates in fish, and shift the species composition toward heat-tolerant varieties. The cumulative effect of multiple plants warming the same reservoir system is harder to study than any single plant’s discharge, but the concern is real in a river system where many power plants cluster.

What Climate Change May Add

Overlaying all of these existing pressures is climate change. Modeling of the Ohio-Tennessee River basin under various future climate scenarios found that water quality changes could go either direction depending on the specific climate model used, but crop yields for corn and soybeans were projected to drop by up to 20% on a mean annual basis.¹⁶ That matters for water quality because changes in precipitation patterns affect how much fertilizer runs off fields, how frequently rivers flood, and how warm the water gets during summer low-flow periods. A hotter, more erratic climate does not create new pollutants, but it can make every existing problem harder to manage: warmer water holds less oxygen, heavier storms flush more contaminants off land surfaces, and longer droughts concentrate whatever is already in the river.

The Tennessee River is measurably healthier than it was in the 1960s and 1970s, when untreated industrial waste poured directly into its channels. Dam modifications have improved dissolved oxygen. Regulatory pressure has cut point-source pollution. But legacy contaminants like PCBs and coal ash metals persist in sediments, emerging chemicals like PFAS are being detected across the basin, and nutrient loading from agriculture continues to climb. For anyone swimming, fishing, or drawing drinking water from the Tennessee, the practical picture is a river that is generally safe for contact and treated drinking water, but one where fish advisories still apply in certain stretches and where the list of contaminants scientists are tracking keeps getting longer rather than shorter.