Dams, Deforestation, and Diversions Are Starving Northern Iran’s Rivers of the Flow They Need

Rivers are the quiet workhorses of civilization, delivering water for farms, cities, and ecosystems while quietly purifying pollutants along the way. But a new study of seven river basins in Mazandaran Province, northern Iran, shows just how quickly that service can unravel when land is converted, dams rise, and water is piped away. The research, published in the open-access journal Heliyon, tracked nearly three decades of land use change, water quality, and river flow across roughly 23,842 square kilometers of territory stretching from the Caspian coast to the high Alborz mountains. Its central finding is stark: in the most heavily developed basins, the water flows needed to keep river ecosystems alive went unmet for a third or more of the study period, and the situation worsened with each passing decade.

The concept at the heart of the study is environmental flow requirement, or EFR, the minimum flow a river must carry to sustain freshwater ecosystems and the functions they provide. Measuring EFR is deceptively complex, with more than 240 methods described in the scientific literature. The researchers selected four widely used hydrological approaches: the Variable Monthly Flow method, which scales required flows to seasonal patterns of mean monthly and mean annual flow; the Tessmann method, which distinguishes low-flow and high-flow months; and the Smakhtin and Tennant methods, which rely on flow exceedance statistics and fixed fractions of mean annual flow, respectively. Applying all four to discharge records from 1989 to 2018 gave the team a robust picture of when and how often each river fell short of its ecological minimum.

To understand what was driving those shortfalls, the team turned to satellite eyes in orbit. Using the MODIS Land Cover Type product, which has mapped the planet annually at 500-meter resolution since 2001 aboard NASA’s Terra and Aqua satellites, they classified land cover across all seven basins for the years 2001 through 2019. After atmospheric and radiometric corrections, the imagery was classified into seventeen land categories and consolidated into four: agriculture, pasture, forest, and residential. The results revealed a landscape under pressure from multiple directions at once, with forests shrinking, farmland contracting in most basins, pastures expanding, and residential areas steadily creeping outward.

The forest losses were the most alarming signal. Between 2002 and 2019, forest cover declined in every single basin, with the largest relative reductions in the Chalus, Babolroud, and Haraz watersheds, where forest area fell by roughly 21 percent in each case. At the same time, agricultural land contracted substantially in several basins, dropping by more than a quarter in the Talar watershed and over a fifth in the Tajan. Pasture land expanded everywhere, and residential areas grew by as much as nearly 19 percent in the Haraz basin. The simultaneous retreat of both forests and farmland is a pattern the authors highlight as underappreciated, reflecting a region where population growth, tourism-driven demand for second homes, and migration from nearby Tehran are reshaping the land faster than national forest protection plans can respond.

Water quality data told a parallel story. Electrical conductivity and total dissolved solids declined in most rivers over the thirty-year record, but the Tajan River moved against the trend, with average conductivity of 841.5 microsiemens per centimeter, the highest of the seven rivers and a peak reading of 1369.5. The explanation lies in intensive irrigated agriculture. After the Tajan Irrigation and Drainage Network came online, the river’s average conductivity rose from 764.4 to 872.8 microsiemens per centimeter, and nitrate concentrations climbed steadily over the last decade to an average of 10.5 milligrams per liter, the highest in the province. The likely culprit, the researchers suggest, is heavy nitrogen fertilizer use in flood-irrigated rice paddies within the network, where enhanced drainage carries nutrients directly into the river.

The flow records themselves were sobering. Every river except the Haraz showed declining mean monthly flow, with the steepest drops concentrated in the third decade of the record, from 2010 to 2019. The Tajan River’s average flow swung from 35.7 percent above its long-term average in the 1990s to 31.4 percent below it in the 2010s. But the study’s most dramatic findings concern infrastructure. The Shahid Rajaei Dam, completed on the Tajan in 1997 with a capacity of 162.5 million cubic meters, was followed by a collapse in monthly yields, with reductions ranging from 27 percent in March to nearly 72 percent in May, averaging 44.9 percent. The river’s average annual yield fell from 485.7 million cubic meters before the dam to 337.7 million cubic meters afterward.

The pattern repeated across the province. On the Babolroud River, the Shiadeh and Alborz dams, built in 1999 and 2010 with combined reservoir capacities of 155 million cubic meters, pushed the river’s mean monthly flow down from 19.9 to 11.9 cubic meters per second, and the river spent 70 percent of the time below its long-term average after the Alborz Dam began operating, up from 52 percent before. A water transfer scheme completed in 2007, which pipes 13.2 million cubic meters annually from the Rouzieh Spring to the neighboring province of Semnan, cut the Talar River’s annual yield by 14.5 percent. And on the Nekaroud, the newly constructed Gelevard Dam, though not yet operational, has a storage capacity equal to nearly 90 percent of the river’s long-term average annual yield, a looming threat the authors flag as a critical factor for future risk assessment.

When the team connected these threads, the relationship between human pressure and ecological shortfall became unmistakable. In the least disturbed basins, the Haraz, Chalus, and Cheshme-Kileh, average EFR violations stayed between 1.4 and 2.5 percent. In the developed basins, they soared: 31.1 percent in the Tajan, 21.8 percent in the Nekaroud, 18.5 percent in the Talar, and 12.8 percent in the Babolroud. The trajectory over time was equally telling. Tajan’s average violation climbed from 9.6 percent in the first decade to 39.3 percent in the third, and before the Shahid Rajaei Dam the river failed to meet its environmental flow just 5.6 percent of the time, compared with 36 percent afterward. The Talar’s violations jumped from 12.2 to 22.2 percent after the water transfer. Groundwater adds another layer of stress, with water tables in the Tajan irrigation district falling about 7.5 centimeters per year, undermining the base flows that sustain rivers between rains.

The consequences of chronic EFR failure are not abstract. When rivers run below their ecological minimums, habitat conditions degrade, oxygen-dependent organisms suffer, pollutants concentrate rather than dilute, and the water cycle itself is disrupted. Elevated nitrate raises public health concerns and can fuel algal blooms and downstream hypoxia. The findings echo global research, including studies on the Yangtze and Lancang rivers in China, where human activities, not climate variability, accounted for the overwhelming majority of flow regime changes. Iran’s national development plans have attempted to counter deforestation through reforestation targets, livestock regulation, and harvest bans, yet the pace of land conversion in Mazandaran has outstripped these efforts.

The authors argue that restoring the Nekaroud, Tajan, Talar, and Babolroud rivers will require reconciling development policies with ecological limits: enforcing land use regulations, promoting low-water-use cropping patterns, guaranteeing environmental flow releases from dams, and establishing formal regional EFR rules backed by monitoring. They also call for future work incorporating climate change scenarios and water demand projections. For a province that produces more horticultural output than any other in Iran, the stakes extend well beyond ecology, touching food security and rural livelihoods. The study offers a scalable framework for any developing region facing the same trade-off between growth and the quiet, essential services that rivers provide, and it delivers a clear warning that those services, once eroded, are far harder to restore than they were to lose.

Subject of Research: Impacts of land use change, dams, and water transfers on environmental flow requirements and water quality in northern Iranian river basins

Article Title: Assessing the impact of land use changes, dams, and water transfers on river sustainability

Article References: Darzi-Naftchali, A., Mashhadi-Kholerdi, F., Shabani, M., & Abdi-Moftikolaei, M. (2026). Assessing the impact of land use changes, dams, and water transfers on river sustainability. Heliyon, 12(15), Article e45445. https://doi.org/10.1016/j.heliyon.2026.e45445

Image Credits: AI Generated

DOI: 10.1016/j.heliyon.2026.e45445

Keywords: environmental flows, land use change, dams, water transfers, river sustainability, water quality, deforestation, Iran, MODIS, nitrate pollution, irrigated agriculture, Casptian basins