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Rural populations in river deltas experience multiple water risks, emerging from intersecting anthropogenic and hydroclimatic drivers of change. For more than 20 million inhabitants of coastal Bangladesh—living on the lower reaches of the Ganges–Brahmaputra–Meghna mega-delta—these water risks relate to access to safe drinking water, management of water resources for farm-based livelihoods, and protection from water-related hazards. To address these risks, water policies in the 20th century emphasized infrastructure development, ranging from embankments for flood protection to handpumps for rural water supply. However, interventions designed to promote aggregate economic growth often resulted in sociospatial inequalities in risk distribution, particularly when policy-makers and practitioners failed to recognize the complex dynamics of human–environment interactions in the world’s most hydromorphologically active delta. In Bangladesh’s southwestern region, construction of the polder system (embanked islands interlaced with tidal rivers) since the late 1960s has augmented agricultural production by protecting low-lying land from diurnal tidal action and frequent storm surges. However, anthropogenic modification of the natural hydrology, emulating the Dutch dyke system, has altered the sedimentation patterns and resulted in severe waterlogging since the 1980s. Contrary to their intended purpose of keeping saline water out, the polders also facilitated growth of export-oriented brackish water shrimp aquaculture, resulting in widespread environmental degradation and social inequalities from shifting power dynamics between large and small landholding farmers. Throughout the 1990s, there were several incidences of violent conflicts between the local communities and government authorities, as well as between different farmer groups. Waterlogged communities demanded to revert to indigenous practices of controlled flooding. Despite being formally adopted as a policy response, the implementation of tidal river management by the government has only been partially successful owing to bureaucratic delays, unfair compensation, and design flaws. Similarly, antishrimp movements gained momentum in several polders to ban the deliberate flooding of cropland with saline water. These narratives of conflict and cooperation demonstrate the complexities of policy outcomes, the unequal distribution of water risks, and the need to integrate local knowledge in decision-making. Social and spatial inequalities are also prevalent in access to safe drinking water owing to heterogeneity in groundwater salinity and infrastructure investments. Public investments are skewed toward low-salinity areas where tubewells are feasible, while high-salinity areas are often served by uncoordinated donor investments in alternative technologies, such as small piped schemes, reverse osmosis plants, and pond sand filters, and household self-supply through shallow tubewells and rainwater harvesting. These struggles to meet daily water needs from multiple sources pose uncertain and unequal water quality and affordability risks to coastal populations. The path-dependent sequences of infrastructure and institutional interventions that shaped the development trajectory of coastal Bangladesh exemplify the complexities of managing water risks and varied responses by public and private actors. While structural solutions still dominate the global water policy discourse, there is increased recognition of the nonlinearity of risks and responses, as well as the need to incorporate adaptive decision-making processes with room for social learning and uncertainties.

Deltas have played a significant role in the growth of human civilization because of their unique economic and ecological importance. However, deltas are becoming increasingly vulnerable because of the impact of intensive human developmental activities, high population and urban growth, subsidence, climate change, and the associated rise in sea level. The trapping of sediments by dams is another major threat to the long-term stability and sustainability of deltas. The emergence and global acceptance of the concept of sustainable development in the 1980s led to the advent of several multidisciplinary and applied fields of research, including environmental science, environmental geology, and sustainability science. Environmental geology focuses on the application of geologic knowledge and principles to broad-ranging environmental and socioeconomic issues, including the specific problems confronting deltas. The key environmental geologic challenges in deltas (especially urban delta areas) are: increasing exposure and vulnerability to geologic hazards (flooding, cyclones, etc.), rise in sea level, decreasing sediment load supply, contamination of soil and water resources, provision of adequate drinking water, and safe waste disposal. The application of geologic knowledge and principles to these challenges requires consideration of the critical geologic controls, such as the geological history, stratigraphy, depositional environment, and the properties of the alluvial sediments. Until recently, most of the traditional engineered solutions in the management of deltas were designed to keep out water (fighting nature), typically without adequate geological/hydrological input, rather than building with nature. Recent innovative approaches to delta management involve a paradigm shift from the traditional approach to a more integrated, holistic, adaptive, and ecologically based philosophy that incorporates some critical geological and hydrological perspectives, for instance, widening and deepening rivers and flood plains as well as constructing secondary channels (i.e., making more room for water). A key challenge, however, is the establishment of a close and functional communication between environmental geologists and all other stakeholders involved in delta management. In addition, there is growing global consensus regarding the need for international cooperation that cuts across disciplines, sectors, and regions in addressing the challenges facing deltas. Integrating good policy and governance is also essential.

The Mississippi River, the longest in North America, is really two rivers geophysically. The volume is less, the slope steeper, the velocity greater, and the channel straighter in its upper portion than in its lower portion. Below the mouth of the Ohio River, the Mississippi meanders through a continental depression that it has slowly filled with sediment over many millennia. Some limnologists and hydrologists consider the transitional middle portion of the Mississippi, where the waters of its two greatest tributaries, the Missouri and Ohio rivers, join it, to comprise a third river, in terms of its behavioral patterns and stream and floodplain ecologies. The Mississippi River humans have known, with its two or three distinct sections, is a relatively recent formation. The lower Mississippi only settled into its current formation following the last ice age and the dissipation of water released by receding glaciers. Much of the current river delta is newer still, having taken shape over the last three to five hundred years. Within the lower section of the Mississippi are two subsections, the meander zone and the delta. Below Cape Girardeau, Missouri, the river passes through Crowley’s Ridge and enters the wide and flat alluvial plain. Here the river meanders in great loops, often doubling back on itself, forming cut offs that, if abandoned by the river, forming lakes. Until modern times, most of the plain, approximately 35,000 square miles, comprised a vast and rich—rich in terms of biomass production—ecological wetland sustained by annual Mississippi River floods that brought not just water, but fertile sediment—topsoil—gathered from across much of the continent. People thrived in the Mississippi River meander zone. Some of the most sophisticated indigenous cultures of North America emerged here. Between Natchez, Mississippi, and Baton Rouge, Louisiana, at Old River Control, the Mississippi begins to fork into distributary channels, the largest of which is the Atchafalaya River. The Mississippi River delta begins here, formed of river sediment accrued upon the continental shelf. In the delta the land is wetter, the ground water table is shallower. Closer to the sea, the water becomes brackish and patterns of river sediment distribution are shaped by ocean tides and waves. The delta is frequently buffeted by hurricanes. Over the last century and a half people have transformed the lower Mississippi River, principally through the construction of levees and drainage canals that have effectively disconnected the river from the floodplain. The intention has been to dry the land adjacent to the river, to make it useful for agriculture and urban development. However, an unintended effect of flood control and wetland drainage has been to interfere with the flood-pulse process that sustained the lower valley ecology, and with the process of sediment distribution that built the delta and much of the Louisiana coastline. The seriousness of the delta’s deterioration has become especially apparent since Hurricane Katrina, and has moved conservation groups to action. They are pushing politicians and engineers to reconsider their approach to Mississippi River management.