Rivers are one of the fundamental elements that have shaped both ecological diversity and human society. Acting as natural barriers, they have enabled diversification and speciation among living organisms, while also providing fertile grounds for the emergence of early societies. Across many regions of the world, settlements emerged along river systems, including the Nile, the Ganges, and the Tigris–Euphrates of the Fertile Crescent.

Today, more than half of the world’s surface freshwater is appropriated for human use. This includes dam construction, inter-basin water transfers, and extraction from freshwater bodies, primarily for agricultural irrigation, hydroelectric power generation, industrial production, and domestic water supply. Such interventions have profoundly altered natural hydrological regimes, resulting in river fragmentation, reduced flows, and severe drought conditions. A study of 225 river basins found that 83 (37%) were highly fragmented and 54 (24%) were moderately fragmented, pointing to the scale of human impact on river systems.1

Throughout history, rivers have not only structured settlements and agricultural practices but have also held deep cultural significance. In ancient societies, rivers were perceived as sacred entities or, at times, sources of fear.2 They feature prominently in mythology, religion, music, art, and poetry, embodying flowing waters as forces that bring both fertility to soils and destruction through floods, erosion, and disasters. Rivers are, therefore, not merely physical resources but also cultural and symbolic elements embedded within social systems.

With the increasing need for water, rivers became sites of multiple uses and interventions.3 The earliest examples of large-scale river diversion for irrigation date back approximately 6,000 years. Similarly, hydraulic infrastructure—such as dams, canals, culverts, conduit systems, and channel modifications—has a history spanning millennia. Some of the world’s oldest dams in the Eastern Mediterranean were built over 4,500 years ago, while the regulation of the Yellow River in China has continued for more than 2,000 years. These practices intensified drastically during the Industrial Revolution, beginning in the late 18th century, when the use of steam power, turbine technologies, and fossil fuels enabled an unprecedented transformation of water supply systems and large-scale engineering interventions in running waters.4

Over time, excessive freshwater use has resulted in widespread and often irreversible ecological degradation. This, in turn, led to increased interest in the notions of ecosystem services and natural capital.5 Here, natural capital refers to the flows of material, energy, and information generated by ecosystems that contribute directly to human well-being, while capital is commonly defined as a stock of material or knowledge existing at a given time. Attempts to artificially recreate natural capital—most notably in the “Biosphere 2” experiment—revealed the immense complexity of ecological processes.6 Designed as a self-sustaining environment for human life without any external inputs, the project ultimately failed to resolve key systemic issues. Meanwhile, “Biosphere 1”—Earth’s own ecosystem—functions with far greater efficiency, productivity, and resilience.

Estimates of the annual economic value of global ecosystem services range from 16 to 54 trillion USD, with an approximate mean of 33 trillion USD.7 Even within this margin, the actual value is likely to be significantly greater. If ecosystem services were truly accounted for within the global economic systems, prevailing models of pricing would look dramatically different.

Freshwater ecosystems are central to sustaining natural capital. Rivers, lakes, and wetlands provide essential ecosystem services that support biodiversity and ecological balance. The continuity of these services depends on maintaining natural flow regimes within river systems. Changes in hydraulic and hydrological characteristics reduce ecosystem service production, degrading habitats and diminishing natural capital. It is within this context that the concept of environmental flow has emerged.8

Environmental flow refers to the quantity and quality of water flows that sustain freshwater ecosystems and the livelihoods depending on them. Studies to determine environmental flow requirements began in the late 1940s, initially focusing on protecting fish populations in cold-water rivers of the western United States.9 Over the following decades, the field evolved from species-specific models to ecosystem-based conservation approaches. Large-scale hydrological alterations driven by dam operations, water abstraction, and inter-basin transfers have since caused widespread degradation in riparian ecosystems, loss of biodiversity, and disruption of natural capital continuity.

In Turkey, studies on environmental water requirements began in 2007 in parallel with the rapid expansion of hydroelectric power plant projects.10 While environmental impact assessment reports for projects prior to 2007 merely contained the phrase “the minimum amount to be released for the survival of living organisms,” after 2007, the water quantities required to be released downstream to ensure ecosystem continuity began to be calculated using the Tennant method, one of the hydrological estimation techniques for environmental water requirements.11 In rapidly developing regions like Turkey, the tension between hydraulic engineering and ecological integrity is palpable. A pertinent example is the Darlık Basin in Istanbul. Providing approximately 11% of the city’s water demand, the basin has historically remained relatively protected from intense urbanization.12 However, population growth and climate change are placing increasing pressure on its resources, exposing the limitations of treating the basin solely as a static “water warehouse.”

Mehmet Kalfazade’s research13 on the Darlık Basin marks a pivotal shift in this perspective. By integrating hydrological process models (WEAP) with watershed network models (MODSIM), the study moves beyond traditional management approaches focusing on reservoir levels. This integrated modeling incorporates flow-routing methods to simulate the timing and magnitude of water movement throughout the river network.

Such an approach reveals that the river is not a passive conduit feeding a reservoir, but a dynamic system with a specific pulse and rhythm. By simulating these flow regimes, it becomes possible to identify not just how much water is needed for human consumption, but also the specific discharge patterns required to sustain riparian corridors, aquatic habitats, and biodiversity.

Today, the role of environmental flows extends beyond ecological integrity, forming a cornerstone of sustainable water governance, legal river protection, and climate-resilient basin management. Emerging frameworks emphasize adaptive environmental flow allocations, participatory decision-making, and policy mechanisms that recognize rivers as living systems requiring ecologically meaningful discharge regimes to safeguard ecosystem services essential to planetary well-being.14