A Gigaton-Scale Carbon Removal Opportunity Is Flowing Through Rivers

By Sarah Willson, Luke Connell, Alessia Priolo

By 2050, the world is expected to need 5 to 10 gigatonnes of carbon dioxide removal each year to avoid the worst impacts of climate change. Rivers already play a quiet but important role in the global carbon cycle, naturally transporting roughly one gigatonne of carbon to the ocean annually. With restoration and enhancement of river alkalinity processes, that capacity could potentially increase to around two gigatonnes per year, making rivers a meaningful contributor to the scale of carbon removal the world will require.

River chemistry across North America and Europe has been gradually changing, degrading ecosystems and weakening fisheries. The impact on river ecosystems is visible, but what is less understood is the impact of this degradation on the planet’s carbon cycle. In healthy rivers, naturally occurring alkaline minerals convert dissolved CO₂ into bicarbonate. This bicarbonate is carried downstream and stored in the ocean for tens of thousands of years, permanently removing carbon from the atmosphere. When rivers become acidic, this process slows or stops, and carbon that would have been safely stored instead re-enters the atmosphere.

This dynamic is the focus of CarbonRun, a Halifax-based climate technology company developing River Alkalinity Enhancement (RAE) as a pathway for durable carbon removal and freshwater restoration. CarbonRun’s work starts from a simple premise: rivers already play a critical role in transporting and storing carbon, but decades of acidification have impaired that function. Restoring alkalinity can allow rivers to resume that role, measurably and at scale.

CarbonRun was co-founded by Dr. Shannon Sterling, Associate Professor of hydrology at Dalhousie University, and the company's Chief Science Officer, alongside Chief Technology Officer Dr. Edmund Halfyard and CEO Luke Connell. The company's scientific foundation originates in Dr. Sterling's work restoring Nova Scotia's acidified salmon rivers in collaboration with the Nova Scotia Salmon Association, which had been applying Scandinavian liming techniques to counteract acid rain damage. 

The critical insight came in 2018, when Dr. Sterling recognized that the same alkalinity restoration process being used for habitat recovery was also generating measurable atmospheric CO₂ drawdown, a connection that had been entirely overlooked by the carbon removal research community. That observation established RAE as a distinct carbon removal pathway, one with no direct precedent in the existing literature on ocean or land-based alkalinity enhancement. Field research conducted at West River, Pictou, beginning in 2023, demonstrated that carbonate weathering reactions occur within minutes of limestone introduction, enabling direct, real-time measurement of carbon flux at the river mouth. The team's monitoring protocol, developed in partnership with Dalhousie University and supported by Scotiabank's Climate Action Research Fund, draws on more than five decades of Scandinavian river liming data confirming ecological safety, while introducing the carbon accounting and third-party verification frameworks necessary to generate credible removal credits.

Rivers as Distributed Carbon Delivery Networks

The process of RAE involves applying prescribed doses of finely milled limestone (CaCO₃) to acidified rivers, increasing alkalinity and enhancing carbonation weathering. This process converts atmospheric CO₂ into dissolved bicarbonate while also reducing CO₂ evasion by shifting river chemistry toward greater stability. The resulting bicarbonate flows downstream to the ocean, where it can be stored for tens of thousands of years; CarbonRun cites marine residence times of up to approximately 90,000 years.

Unlike centralized industrial carbon capture systems, this approach operates within distributed watersheds. Each project integrates dosing infrastructure and monitoring stations positioned upstream and downstream to track changes in water chemistry. Rather than enclosing carbon within a facility, the model works within existing hydrological systems and seeks to make the impacts measurable.

This process restores river alkalinity, improving water quality, supporting fish populations in historically acidified rivers, and helping mitigate downstream coastal acidification. These ecological co-benefits are embedded in the operating logic rather than incidental. Beyond these co-benefits to local ecosystems, the permanence of RAE distinguishes the pathway from temporary sequestration approaches. Limestone feedstock is widely produced globally and familiar to regulators due to its longstanding use in environmental remediation, which reduces the novelty risk associated with more experimental interventions.

The scientific foundation of this intervention is not new. Limestone dosing has long been used to restore acidified rivers and support fisheries. What differentiates the current model is the integration of carbon accounting, formalized MRV protocols, and registry-backed, 3rd party verification. The central challenge lies in quantification, particularly given the variability of river systems. Flow rates fluctuate seasonally, tributaries alter chemistry, and water moves through both surface and subsurface pathways, making defensible carbon accounting foundational to credibility. 

A Multi-Pronged Approach to Offsetting 

Carbon removal will not be solved via a single pathway, and cutting emissions alone will not allow us to meet climate targets. Even within carbon removal, RAE is unlikely to replace approaches like OAE or DAC, but instead to serve as a durable complement within a diversified portfolio.

While the science behind carbon removal continues to progress, the harder problem is operational: translating that science into systems that can be measured, scaled, financed, and trusted. Many existing approaches to carbon removal struggle to scale due to capital-intensive infrastructure, limited scientific backing, and public resistance. 

River-based carbon removal is gaining traction at a moment when several dynamics are shifting simultaneously. The underlying science is well established: alkalinity addition has been practiced in Norway and Sweden since the 1970s to mitigate acidification and restore salmon habitats (see early research on freshwater liming). What has evolved is the recognition that restoring alkalinity can also generate measurable, permanent carbon drawdown.

At the same time, demand for high-integrity carbon removal is maturing. Registry-backed standards and structured offtake agreements are creating clearer pathways for durable removal, and CarbonRun has issued credits under Isometric’s River Alkalinity Enhancement protocol, the first standardized framework for river-based removal. Independent validation does not eliminate risk, but it indicates that the pathway has moved from conceptual theory toward measurable deployment.

Building a Measurable River Model

Scaling RAE projects at scale requires vigilance across site selection, standardized monitoring, local community engagement, and conservative carbon accounting. Because rivers are open and variable, quantifying incremental alkalinity enhancement against fluctuating baselines demands continuous measurement and defensible modeling. A central challenge is quantification, given the inherent variability of river systems. Flow rates shift seasonally, tributaries influence chemistry, and water moves through both surface and subsurface pathways, making defensible carbon accounting critical to credibility. To address this, CarbonRun’s RAE projects rely on continuous monitoring across key parameters, including carbonate alkalinity, pH, salinity, and temperature within defined watersheds, etc. 

Early deployments are higher cost as infrastructure and monitoring systems are refined, but larger rivers with favorable chemistry and higher flow rates offer stronger removal potential per site. Scale will depend less on singular facilities and more on identifying and optimizing high-quality watersheds. Not all rivers are suitable; removal potential varies based on hydrology, baseline acidity, access, and regulatory context. Where chemistry and hydrology align, however, rivers represent an existing environmental network capable of delivering durable carbon removal without requiring entirely new industrial stacks. In practice, restoration projects that historically focused on ecosystem recovery can, under the right conditions, also function as measurable carbon infrastructure, altering how such interventions are valued and financed.

Why Rivers, Why Now

For years, degraded rivers have been treated as symptoms of industrial progress, costs to be managed rather than assets to be restored. CarbonRun's work reframes this challenge into an opportunity for rivers to become part of the solution. The same watersheds acidified by decades of emissions can, with targeted intervention, resume their natural role as carbon infrastructure, provided restoration is measurable, verifiable, and ecologically beneficial.

Carbon removal will require a portfolio of approaches operating in parallel across geographies and regulatory contexts, and no single pathway carries the full weight alone. RAE's position in that portfolio is unusual: it operates within existing natural systems, generates tangible ecological co-benefits, and draws on decades of established science. Rivers also occupy a unique physical position, connecting terrestrial and marine systems while intersecting directly with the communities and regional economies that any durable climate solution must bring along.

What remains is the harder work of scale: identifying the right rivers, earning the trust of the communities around them, and building the operational systems to make each new site as credible as the last. 

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CarbonRun is a Halifax-based climate technology company developing River Alkalinity Enhancement (RAE) as a pathway for durable carbon removal and freshwater restoration. By applying prescribed doses of limestone to acidified rivers and integrating continuous monitoring systems, CarbonRun enhances natural carbon transport processes while generating registry-backed carbon removal credits. The company operates pilot deployments in Nova Scotia and has issued credits under Isometric’s River Alkalinity Enhancement protocol.