All five Great Lakes are freshwater, and none of them has ever been salt water during the roughly 14,000 years they have existed in their current form. Their combined volume represents about a fifth of the planet’s surface fresh water, and the dissolved salt content in any of them is a tiny fraction of what you would find in the ocean. But the relationship between the Great Lakes and salt is more complicated than that clean answer suggests. Massive ancient salt deposits sit deep beneath the lake beds, road salt is steadily pushing chloride levels upward in several of the lakes and their tributaries, and the ecological consequences of that creeping salinization are already measurable.
How Fresh the Great Lakes Actually Are
Ocean water contains roughly 35,000 milligrams of dissolved salts per liter. By contrast, the Great Lakes hover around 60 to 200 milligrams per liter of total dissolved solids, depending on the lake and the decade of measurement. That puts them comfortably in the “fresh” category by any standard definition. Lake Superior, the largest by surface area and volume, tends to have the lowest dissolved-solid concentrations because it receives relatively little runoff from developed land. Lakes Erie and Ontario sit at the higher end, partly because they are downstream in the system and partly because the watersheds draining into them are more heavily urbanized and industrialized.
Researchers have tracked the concentrations of major ions in the Great Lakes since the mid-twentieth century. One important thread in that work, starting with the limnologist Alfred Beeton in the 1960s, documented a progressive increase in total dissolved solids and several major ions over time. Beeton recognized that salts could serve as a general indicator of human impact on the system, even though the concentrations did not themselves pose an immediate water-quality threat at that time.1ScienceDirect. Long-term trends of Great Lakes major ion chemistry The trend has continued. The lakes are still overwhelmingly fresh, but the direction is toward saltier, not fresher.
Why the Great Lakes Are Freshwater in the First Place
The Great Lakes owe their existence to glaciers. The basins were carved by repeated advances and retreats of continental ice sheets. The lakes were partially or totally covered by glacier ice at least six times since about 780,000 years ago, and the version of the lakes we see today took shape during the retreat of the last ice sheet, roughly 10,000 to 14,000 years ago.2Journal of Great Lakes Research. Origin and Evolution of the Great Lakes As ice melted, enormous proglacial lakes pooled in the scoured-out basins. Their levels fluctuated dramatically as the retreating ice margin opened and closed outlets, and as the crust slowly rebounded from the weight of the ice. What filled those basins was meltwater, rain, and snowmelt draining from the surrounding land. No connection to the ocean meant no infusion of seawater.
This is the fundamental reason the Great Lakes are fresh. Salt water bodies typically get that way either by having a direct connection to the ocean or by losing water to evaporation faster than fresh inflows can dilute the minerals washing in. The Great Lakes have neither condition. They are connected to the Atlantic through the St. Lawrence River, but water flows out, not in, so ocean water never backs up into the system. And while evaporation is significant in the lakes, it is more than offset by precipitation and river inflows, keeping the water cycling and dilute.
The Salt That Lies Beneath
Ironically, even though the Great Lakes themselves are fresh, they sit above one of the world’s major repositories of rock salt. The Michigan Basin, centered beneath Lake Huron and Lake Michigan, contains enormous quantities of evaporite minerals. These deposits formed hundreds of millions of years ago when the region was covered by shallow tropical seas. As those ancient seas evaporated, they left behind thick layers of halite, gypsum, and other evaporite minerals. Most of the thick evaporite accumulations occur in rocks from the Silurian and Devonian periods, and the maximum aggregate thickness of halite in the Silurian Salina Group exceeds 650 meters.3GeoScienceWorld. Evaporite facies of the Michigan Basin
Study of drill cores from the Michigan Basin and from active salt mines has identified pyramidal hopper crystals of halite in the Salina salt, comparable to crystals produced in solar salt manufacturing, confirming that these deposits formed through evaporation of ancient brines.4Journal of Sedimentary Research. Origin of the Salina salt of Michigan Salt mining continues today in the region, including a vast mine that extends under the city of Detroit and beneath part of the lake bed. So while the water above is fresh, the rock below is saturated with remnants of a much saltier past.
In certain spots, that ancient brine finds its way upward. In Lake Huron’s Middle Island Sinkhole, for instance, groundwater seeping through the bedrock creates a pocket of low-oxygen, sulfur-rich water on the lake floor. A microbial mat dominated by cyanobacteria thrives in that unusual chemistry, capable of both oxygen-producing and non-oxygen-producing photosynthesis as well as chemosynthesis.5PubMed Central. Groundwater shapes sediment biogeochemistry and microbial diversity in a submerged Great Lake sinkhole These sinkholes are tiny, hyper-local features. They do not meaningfully change the salinity of the lake, but they are a vivid reminder that salty groundwater is always pushing against the fresh system from below.
Road Salt and the Creeping Rise in Chloride
The bigger threat to the lakes’ freshness is not geological. It is the millions of tons of road salt spread across the Great Lakes watershed every winter. Use of deicing salts in the United States has tripled over the past 45 years, and the Great Lakes region, with its long winters and dense road networks, bears a disproportionate share of that load.6Frontiers in Ecology and the Environment. Road salts, human safety, and the rising salinity of our fresh waters When snow melts, the dissolved sodium chloride washes into storm drains, streams, and rivers that ultimately feed the lakes.
Lake Ontario’s watershed provides a stark illustration. Chloride loading to Lake Ontario has increased significantly in recent years, and a growing share of that chloride enters through groundwater rather than surface runoff. Significant chloride accumulation is occurring in watersheds and aquifers within the basin, and concentrations are expected to keep rising until the system reaches a new equilibrium. That means sustained, year-round elevated chloride in the tributaries feeding the lake.7Journal of Great Lakes Research. Groundwater as a source and pathway for road salt contamination of surface water in the Lake Ontario Basin: A review The groundwater pathway matters because it introduces a long time lag. Even if road salt use stopped tomorrow, the chloride already soaked into aquifers would continue draining into streams and lakes for years or decades.
The Cuyahoga River, which drains into Lake Erie at Cleveland, offers a detailed timeline. Seasonal data show that winter chloride levels in the Cuyahoga have exceeded the U.S. EPA’s chronic water quality guideline of 230 milligrams per liter in more than half the years of the twenty-first century, compared to just one year exceeding that value in the late twentieth century.8Journal of Great Lakes Research. When it snows it pours: Increased chloride concentrations in the Cuyahoga River during the last half century Those pulses of salty water hit the river hardest in winter and spring, during and right after the de-icing season. But the background levels in summer and fall have been climbing too, driven by that slow groundwater contribution.
To be clear, none of this is turning the Great Lakes into salt water. Even the most chloride-enriched tributary is nowhere close to ocean salinity. But “freshwater” is not a single threshold below which everything is fine and above which everything is ruined. Aquatic organisms have evolved in water with very low dissolved salt, and for many of them, even modest increases in chloride are harmful.
What Rising Salt Does to Freshwater Life
The ecological effects show up well before chloride reaches anything close to ocean levels. Research on Daphnia, the small crustaceans sometimes called water fleas that form a crucial part of the freshwater food web, found decreased reproduction and increased mortality at chloride concentrations between just 5 and 40 milligrams per liter. That is far below the EPA’s chronic guideline of 230 milligrams per liter and far below what many urban streams now carry. Analysis of preserved remains in lake sediments revealed shifts in the composition of cladoceran communities that lined up with the introduction of road salt in the region. Roughly a quarter of recreational lakes in Ontario have chloride concentrations in that 5-to-40 range, meaning the zooplankton in those lakes may already be affected.9PubMed. Road Salt Impacts Freshwater Zooplankton at Concentrations below Current Water Quality Guidelines
When zooplankton decline, the effects cascade through the food web. In experimental wetland studies, high sodium chloride concentrations caused a strong decrease in zooplankton abundance, which led to a trophic cascade: with fewer grazers, phytoplankton bloomed.10PubMed. Investigation of road salts and biotic stressors on freshwater wetland communities Algal blooms are already a recurring problem in parts of the Great Lakes, especially western Lake Erie, and anything that weakens the grazer community can make those blooms worse.
Some municipalities have turned to alternative deicers such as magnesium chloride and calcium chloride, hoping they are gentler on ecosystems. Research in outdoor experimental ponds suggests the opposite. All three salts reduced algal biomass and amphipod abundance. Magnesium chloride had the largest and longest-lasting effects on zooplankton, particularly cladocerans and copepods, and it caused significant increases in phytoplankton and rotifers as a result. Calcium chloride, meanwhile, increased ostracod populations and decreased snail abundance, while sodium chloride increased snail abundance. At similar chloride concentrations, the alternatives were not safer for aquatic ecosystems.11PubMed. The combined effects of macrophytes and three road salts on aquatic communities in outdoor mesocosms This finding complicates the search for solutions, because the problem is not just sodium chloride specifically but chloride in general, regardless of what cation it is paired with.
Why the Lakes Are Slow to Change and Slow to Recover
The Great Lakes respond sluggishly to any new input because of their enormous volumes and long water-residence times. Lake Superior holds water for close to 200 years on average before it flows out. Lake Michigan’s residence time is about 62 years. Even the shallowest and smallest of the five, Lake Erie, holds its water for a little over two years.12ScienceDirect. Hydraulic Residence Times for the Laurentian Great Lakes Those long residence times mean that dissolved salts entering the lakes accumulate slowly but also persist for a very long time. A pollutant pulse that enters Lake Superior today will, on average, still be partially present two centuries from now.
This is a double-edged dynamic. On one hand, the lakes’ sheer volume buffers them against rapid change; a single bad winter of heavy salting does not suddenly spike the lake’s salinity. On the other hand, decades of steady chloride input have a cumulative effect that is extremely difficult to reverse. Even aggressive reductions in road salt use would take years to show up in the lakes’ chemistry, especially in the deeper, longer-residence lakes.
Can Anything Be Done About Rising Chloride?
Some municipalities in the Great Lakes watershed have begun testing whether reducing road salt in targeted areas actually lowers chloride in drinking water wells. In one well-documented case in the Regional Municipality of Waterloo in Ontario, a plan was implemented to cut salt application by 25 percent on roads within the capture zone of an affected well field. About six years after the reduction measures were put in place, average chloride concentrations in shallow soil water had dropped by roughly 60 percent, and stored chloride mass in the soil had declined by about 40 percent. Shallow groundwater monitoring wells showed about a 35 percent decrease in chloride. However, the deeper public supply wells showed a much slower, time-lagged response, reflecting the years it takes for reduced surface inputs to percolate down through the ground.13Science of The Total Environment. Efficacy of urban road salt reduction strategies on public supply well quality
This is cautiously encouraging. It confirms that controlled reductions in road salt within vulnerable areas can reduce the impact on groundwater over time. But the emphasis on “over time” is important. Decades of heavy salt application have loaded the soil and groundwater with chloride, and flushing that out is a slow process. Communities that wait until chloride levels are already high in their drinking water wells face a long recovery period even after taking action.
The broader challenge is that road salt saves lives. Salted roads are significantly safer in winter driving conditions, and communities are understandably reluctant to cut back. The evidence suggests that best management practices like pre-wetting salt, calibrating application rates, and protecting the most sensitive recharge areas offer a middle path between public safety and freshwater protection.6Frontiers in Ecology and the Environment. Road salts, human safety, and the rising salinity of our fresh waters But as deicing salt use continues to grow nationally, those practices need to be adopted at much larger scales to meaningfully bend the chloride curve in Great Lakes tributaries.
Marine Invaders in a Freshwater System
One of the stranger footnotes in the Great Lakes’ relationship with salt involves the organisms that have crossed from salt water into this freshwater system. The sea lamprey, an ocean-going parasite that devastated Great Lakes fisheries after gaining access through shipping canals in the early twentieth century, is a prime example. Lamprey manage to survive in fresh water partly because they have specialized cells in their gills that actively pump ions inward, compensating for the constant loss of salts that all freshwater fish and fish-like animals face. Feeding on the blood of host fish also helps, since blood has salt concentrations roughly comparable to the lamprey’s own internal fluids.14Frontiers in Fish Science. How the resilient ecophysiology of the sea lamprey allowed them to invade the Laurentian Great Lakes and could protect them from climate change
Sculpin fish of the genus Triglopsis tell a similar story of adaptation. Genetic analysis of these bottom-dwelling fish has revealed signs of increased positive selection in mitochondrial genes among species that successfully colonized low-salinity environments, suggesting that the transition from salt to fresh water imposed significant evolutionary pressure on their energy metabolism.15PubMed. Osteological, Biological, and Genetic Consequences of Colonization of Fresh Waters by Marine Cottoid Fishes (Cottidae) These invasions and adaptations underscore a basic truth about the Great Lakes: they are fresh enough that marine organisms need substantial physiological retooling to survive there. The lakes are not some brackish halfway point between salt and fresh. They are genuinely, deeply freshwater, and anything that evolved for the ocean has to change dramatically to make them home.