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

The method, called Salinity Zones Distribution (SZD) mapping, was created by T. S. Franklin of the Federal University of Bahia, together with P. C. C. Rosman of the Federal University of Rio de Janeiro and R. C. Carvalho of James Cook University. Rather than simply averaging salinity values across space and time, the technique counts how often each salinity class occurs at every point in the estuary over a full calendar year. The class that dominates most frequently is assigned to that location, producing a map of the most permanent ecological salinity conditions rather than a smoothed statistical blur.

To generate the underlying salinity fields, the team used TELEMAC-2D, a depth-averaged hydrodynamic model that solves the vertically averaged Navier-Stokes equations for free-surface flow and salinity transport. The model domain was discretized with a finite-element mesh of 34,699 triangular elements ranging from 16 to 700 meters, built from bathymetric data supplied by the Brazilian Navy and earlier field campaigns. Tidal forcing came from harmonic constituents measured at the Madre de Deus terminal, while salinity at the ocean boundary was set from moored sensor data collected between 2012 and 2014 near the estuary mouth in Todos os Santos Bay.

Validation was extensive and multi-metric. Water levels at stations near the estuary head and mouth achieved Model Prediction Skill values above 0.9, with root-mean-square errors of 0.12 and 0.29 meters respectively, well below the local tidal range. Depth-averaged currents at three mid-channel sections reached Skill values between 0.85 and 0.95. Salinity validation across five stations produced Skill values from 0.53 to 0.91, and the researchers were careful to explain that low Skill at the euhaline mouth station reflects the extremely narrow observed salinity range there rather than poor physical performance. Crucially, all salinity biases remained smaller than the width of any salinity class in the Venice System, the classification scheme adopted for the ecological zoning.

That classification, first consolidated at the 1958 Venice symposium, divides aquatic environments into five zones from limnetic fresh water below 0.5 practical salinity units to marine waters above 30 psu. It remains the most widely used framework for salinity-based ecological zones, and the SZD method can accommodate it or any alternative scheme, including multivariate classifications derived from local biological data. For each month of the simulated year, the researchers computed the dominant salinity class at every mesh node, resolving ties in favor of the class with the narrower salinity range to minimize classification uncertainty.

The team ran two contrasting scenarios for 2010, a year chosen because its wet and dry seasons were typical and because fish survey data were available. In the Natural scenario, freshwater inflow to the estuary equaled the river inflow entering the dam reservoir. In the Regulated scenario, inflow reflected the actual operation of Pedra do Cavalo, which releases water in daily pulses of roughly 45 cubic meters per second for four to eight hours, driven by electricity generation norms and a minimum sanitary discharge of about 10 cubic meters per second. The contrast between these two worlds proved dramatic.

Under regulated flows, polyhaline conditions between 18 and 30 psu dominated Iguape Bay for most of the year, whereas natural flows would have produced euhaline conditions above 30 psu much of the time. During the dry month of February 2010, the limnetic zone extended only 3 kilometers downstream of the dam under natural conditions but stretched 10 kilometers under regulation. Conversely, during the April 2010 flood, when natural discharges exceeded 1,000 cubic meters per second, the upper estuary shifted abruptly to limnetic conditions and the mesohaline zone expanded deep into Iguape Bay. The regulated regime, in short, holds the estuary in a persistently saltier state in the bay while pushing fresher, more variable conditions into the upstream channel.

The ecological consequences follow directly from the maps. Previous fish surveys in the Paraguaçu estuary identified three guilds: estuarine residents that complete their entire life cycle within the estuary, estuarine migrants with larval stages outside it, and marine stragglers that venture in from the sea. Estuarine residents concentrate in salinities of 18 to 26 psu, squarely within the polyhaline class that regulation now sustains in Iguape Bay, suggesting these fish may benefit. But the simulations also show that regulated discharges impose limnetic conditions along the channel upstream of the bay, implying significant stress for the oligohaline and mesohaline biota that historically inhabited those reaches, including larval and juvenile fishes that depend on low-salinity nursery habitats.

Benthic communities tell a parallel story. Local studies of the Paraguaçu system show a clear longitudinal replacement of taxa along the salinity gradient, with bivalves of the families Tellinidae and Veneridae, cirolanid isopods, cyclopoid copepods and nereidid polychaetes dominating low-salinity sectors, while nuculid bivalves, cirratulid polychaetes and amphiurid brittle stars characterize high-salinity, finer-sediment areas. The SZD maps can therefore be read as habitat-suitability predictors: expansion of polyhaline and euhaline zones under regulation should favor the marine-affiliated assemblages, while shrinking the spatial footprint available to freshwater-tolerant taxa. The framework also connects to broader phenomena, since the position of the low-salinity front relates to the estuarine turbidity maximum, where nutrients and phytoplankton concentrate and where algal blooms, including the red tide recorded in the bay in 2007, can originate.

Beyond its immediate findings, the study positions SZD mapping as a versatile management tool. The authors argue it can support environmental flow planning, help resolve conflicts between dam operators and artisanal fishing communities such as those in the Iguape Bay Extractive Reserve, guide aquaculture siting for species like the mangrove oyster whose larvae favor 25 to 30 psu, and even serve as a climate-change indicator by mapping how sea-level rise and altered rainfall shift salinity zones. Because the method works with any salinity classification and any estuary with hydrodynamic model output, the researchers believe it offers a practical, visually intuitive foundation for sustainable catchment management worldwide, from the lagoon of Venice to dam-removal sites on the Elwha River.

Subject of Research: A novel salinity zone mapping method for assessing how dam-regulated freshwater inflow alters ecological salinity zones in estuaries

Article Title: Evaluating salinity alterations in estuaries through ecological mapping

Article References: Franklin, T. S., Rosman, P. C. C., & Carvalho, R. C. (2026). Evaluating salinity alterations in estuaries through ecological mapping. Discover Oceans, 3(1), Article 46. https://doi.org/10.1007/s44289-026-00159-9

Image Credits: AI Generated

DOI: 10.1007/s44289-026-00159-9

Keywords: estuary, salinity, hydrodynamic modelling, Venice System, freshwater regulation, Paraguaçu River, Todos os Santos Bay, ecological zonation, environmental flows, benthic fauna, fish habitat, dam impacts

Freshwater ecosystems rank among the most biodiverse and most threatened environments on Earth, and headwater streams sit at the top of the conservation agenda because of their outsized influence on the biodiversity, functioning, and connectivity of entire river networks. In Mediterranean mountains, where irrigated agriculture and climate change have extensively reshaped flow regimes, these small streams face mounting pressure. Sierra Nevada, a designated Biosphere Reserve and a recognized Mediterranean biodiversity super hotspot, offers a striking case study. There, careo ditches have been documented since the eighth to tenth centuries, diverting snowmelt from headwater reaches and distributing it across upper slopes to artificially recharge aquifers in weathered fractured rock, extending water availability for lowland farms and towns through the dry season.

A research team led by scientists from the University of Almeria, the Andalusian Centre for Global Change, and the Geological and Mining Institute of Spain examined five first- and second-order headwater streams on the southern face of the park, in the Alpujarra region, at altitudes between 1800 and 2000 meters. In each stream, a small, rudimentary weir built of loose rocks and gravel diverts water into a careo ditch. The researchers compared upstream reference reaches with downstream flow-impacted reaches, sampling benthic macroinvertebrates in spring during snowmelt and in autumn after the prolonged dry period, while continuously monitoring discharge and water temperature from November 2022 onward.

The magnitude of diversion was severe. Proportional flow reduction downstream of the weirs most frequently ranged from 76 to 98 percent of upstream discharge, with mean reductions exceeding 0.85 for much of the year in the Cáñar, Mecina, and Bérchules streams. In absolute terms, downstream flows often fell below 10 liters per second. Yet despite these extreme withdrawals, the impacted reaches never dried. They remained perennial throughout the year, maintaining the longitudinal hydrological connectivity that ecologists consider critical for stream life. The team attributes this persistence to two concurrent processes: seepage returning from the leaky weirs and unlined or poorly sealed ditches, and groundwater effluents from the region’s weathered hard-rock aquifers.

The groundwater signal was unmistakable in the temperature records. Downstream reaches showed significantly higher winter temperatures despite their lower discharge, along with lower summer maxima and reduced spring daily fluctuations, a thermal fingerprint of groundwater-fed baseflow. Water chemistry, by contrast, remained remarkably stable across reach types, with only slightly lower oxygen and higher salinity downstream, differences that were not statistically significant. Total dissolved nitrogen was actually higher upstream, suggesting stronger surface runoff influence at reference sites. This chemical stability matters, because it means that any ecological differences between reaches could be traced primarily to physical habitat alteration, such as reduced flow diversity and increased sedimentation, rather than to water quality degradation.

The biological consequences were nuanced. Macroinvertebrate density was substantially reduced downstream, particularly in spring, when it was roughly 50 percent lower than at upstream sites, a pattern consistent across all functional feeding groups and supported by medium effect sizes. The prolonged duration of the reduction, which persisted for at least a full year, contracted the wetted channel width by 63 to 85 percent, slashing total benthic abundance per reach length to about one-tenth of reference values. The authors warn that this decline in benthic production, together with reduced insect drift and emergence, likely creates severe food shortages for trout and riparian insectivorous predators that depend on aquatic insects crossing the land-water interface.

Surprisingly, local alpha diversity and interlocal beta diversity, measured as taxa richness, Shannon diversity, and community turnover, did not differ significantly between upstream and downstream reaches, and community composition showed no detectable separation in ordination analyses. The researchers propose that the maintenance of hydrological connectivity prevented the impacted reaches from degrading into isolated pools, a fate that typically traps invertebrates, intensifies predation, and triggers prey population collapse. Groundwater subsidies may also have mitigated local diversity losses, since moderate upwelling is known to enhance macroinvertebrate abundance and richness in alpine streams by moderating the harsh abiotic stress imposed by meltwater.

However, the buffering was incomplete, and the damage emerged at larger scales. Regional gamma diversity, estimated from rarefaction curves, was significantly lower in the sets of downstream reaches in both seasons, indicating that even small, non-significant losses of local richness can accumulate into substantial regional declines. Sensitive components of the community also eroded: the taxonomic richness of collector-gatherers dropped significantly in spring, and several biotic indices based on the pollution- and disturbance-sensitive insect orders Ephemeroptera, Plecoptera, and Trichoptera, the EPT taxa, declined significantly at impacted sites. Notably, the IBMWP index, the regulatory standard for Iberian river biomonitoring, classified both reach types as being of good quality and failed to distinguish them, suggesting that current monitoring frameworks may be blind to the early functional erosion caused by flow diversion unless they incorporate EPT-driven metrics.

The study’s most consequential implication concerns the global push toward irrigation efficiency. Modern water policy has favored concrete-lined canals and sealed infrastructure that minimize seepage, yet the Sierra Nevada findings suggest that these very inefficiencies, the leaky weirs and porous ditches of the traditional system, are essential to sustaining downstream flow and biodiversity, particularly in reaches that lack groundwater inflow. The authors invoke the well-documented irrigation efficiency paradox, in which saving water at the infrastructure scale can paradoxically intensify overall water consumption and ecological harm, and they argue that reversing the efficiency-first trend is urgent to halt river degradation in biodiversity-rich mountain regions.

Ultimately, the researchers argue that preserving as many stream reaches as possible under natural or ecologically compatible flow regimes is mandatory in biodiversity hotspots, because moderate local losses compound into regional extirpation. As climate change intensifies aridity across the Mediterranean and agricultural water demands climb, the centuries-old careo systems, increasingly framed as nature-based solutions for aquifer recharge, offer a rare example of human water infrastructure coexisting with, and even underwriting, freshwater conservation. The lesson from Sierra Nevada is that sometimes the most valuable feature of an irrigation system is not how efficiently it moves water, but how much it lets slip away.

Subject of Research: Ecological effects of traditional irrigation water diversion on benthic macroinvertebrate biodiversity in Sierra Nevada headwater streams

Article Title: Leakage from Traditional Irrigation Systems and Groundwater Upwelling Buffer the Impact of Heavy Streamflow Diversion on Benthic Biodiversity

Article References: Casas, J. J., Fenoy, E., Rubio-Ríos, J., Villegas, J., Salinas-Bonillo, M. J., Zakaluk, T., Martos-Rosillo, S., & Cabello, J. (2026). Leakage from Traditional Irrigation Systems and Groundwater Upwelling Buffer the Impact of Heavy Streamflow Diversion on Benthic Biodiversity. Water Resources Management, 40(11), Article 517. https://doi.org/10.1007/s11269-026-04861-3

Image Credits: AI Generated

DOI: 10.1007/s11269-026-04861-3

Keywords: benthic macroinvertebrates, headwater streams, irrigation ditches, groundwater upwelling, hydrological connectivity, water abstraction, Sierra Nevada, EPT taxa, biotic indices, environmental flows, Mediterranean mountains, biodiversity conservation