Part 2
Understanding the system
This figure shows a 3D illustration of a river, from its source in the mountains to its outflow into the sea. It also shows a summary of human interventions in the world’s river systems as projected for 2050. This is done for five clusters of pressures on the river system: namely dams and reservoirs, agriculture, urbanisation and industrialisation, port development and shipping, and engineered deltas.
Select a scenario:
Projected trends: 2050
Anthropogenic stresses on river basins and deltas include flood risk, water stress and pollution. Under a business-as-usual scenario, these stresses will continue to grow.
A further increase in anthropogenic stresses will negatively affect human health, as well as the economic and ecological perspectives of river basins and deltas. Climate change, in addition, will alter the discharge regimes of the rivers across the world, and accelerate sea level rise.
More information on the impact of interventions and climate change on sediment transport and morphology:
The Rhine River caseIncreasing pressures of climate change
We present results of the business-as-usual scenario in which projected socio-economic developments and restricted mitigation efforts result in a moderate scenario of climate change with an average global temperature increase of 3.7 °C by 2100.
Precipitation patterns around the world are projected to change, weather extremes (heatwaves, drought, flooding, storm surges) will occur more often and will become more intense, and sea level rise will accelerate. In general, high river flows will increase, and low river flows will decrease.
Climate change affects water stress. More drought events (evaporation) and less precipitation in summers increase drought risk for agriculture. In addition, longer periods of low river discharge affect navigation and increase saltwater intrusion.
Along with the rise in air temperature, river water temperature will rise as well, up to about 4 °C in some world regions.
Projected changes in temperature and net precipitation, 2010–2050
Source: PBL IMAGE
Global average temperature is projected to increase by around 2 °C by 2050, with large regional differences. The northern regions face relatively high temperature increases. In general, the net result of changing temperature, precipitation patterns and evaporation is that most dry areas will become dryer and wet areas wetter.
Projected relative changes in low river flow (Q10) and high river flow (Q90), 2010–2050
Source: UU
Many rivers will see changes in discharge regime, and distribution of high and low flows throughout the year. The discharge of some rivers, for instance, may change from being mostly driven by snowmelt to being dominated by rainfall. As a result, the low flow will change, both in size and in time of occurrence within the year. Changes in high river flow, in general, reflect changes in annual precipitation volumes; an increase in total annual precipitation results in an increase in high river flow, and vice versa. There are exceptions, however. In small river basins, high flow may increase despite a decrease in annual precipitation volume because rainstorm intensity increases. Snowmelt-driven high flows may decrease because less snow accumulates in the winter.
Human interventions in rivers and deltas
River basins and their deltas are projected to remain attractive for human settlement and economic activities. By 2050, the projected increase in concentrations of people and economic value in river basins and deltas will lead to increased flood risks, water pollution and water stress, and an increase in dams affecting streamflow and sediment flow. This will jeopardise the functioning of river systems and long-term stability of deltas.
Water stress may result from population growth that increases water consumption and water yield gaps (through increased food production). Unsustainable abstraction of groundwater in coastal zones may induce soil subsidence and increase flood risk.
Change in global population in urban and rural areas, from 1950 to 2050
Source: United Nations Department of Economic and Social Affairs, Population Division (2018). World Urbanisation Prospects: The 2018 Revision, Online Edition
By 2050, 70% of the world population is projected to live in an urban environment, on 0.5% of the global land area. This urban area is expected to expand by more than 70%, not only in riparian and coastal areas and in deltas, but also in water-stressed regions, such as drylands.
Projected change in GDP per river basin, 2010 and 2050, for the world's 32 largest river basins
Source: PBL
Socio-economic pressures in river basins are particularly high in world regions with high population growth and little money to invest in the alleviation of these pressures.
impacts of interventions and climate change - global trends in dam building and sediment flows
Sediment flows from rivers to their deltas will change in future decades due to climate change, socio-economic developments and the building of more dams for hydropower and fresh water reservoirs.
3700 planned new dams (capacity in MW)
Source: Zarfl et al., 2015
Planned hydropower increase is especially large in the Amazon River Basin, the Congo River Basin, the Yangtze River Basin and the river basins in the Himalayas.
Changes in sediment flows in response to climate change and anthropogenic stress, for 23 of the world's largest river basins
Source: Dunn et al., 2019; Best, 2019
Sediment delivery to major deltas worldwide is changing, for a number of reasons. Climate change increases sediment runoff from the catchment to streams and the river. Dams, on the other hand, capture a large amount of sediment and reduce sediment flows downstream. Socio-economic developments affect land management and, thus, sediment delivery from the catchment, increasing or decreasing sediment flows, depending on land management. The combined impact of all these pressures varies from one river basin to another.
Impacts of interventions and climate change - global trends in water abstraction
Between now and 2050, global water consumption is expected to increase by about a quarter, due to the growing number of households and the growth in industrial and agricultural production. Irrigation will continue to dominate all water uses.
Projected changes in water consumption, per river basin, in 2010 and 2050, for the world's 32 largest river basins
in km3
Source: PBL IMAGE
The projected change in water consumption varies strongly from one river basin to another. The projected increase is strongest in tropical and subtropical regions.
Projected global water abstraction from rivers, in 2010 and 2050
Source: UU, PBL IMAGE
Projected water abstraction for agriculture, per river basin (km3), in 2010 and 2050
Source: PBL IMAGE
Water that is abstracted from river basins for irrigation may deplete rivers to such a degree that, in dry seasons, river flow hardly reaches the coast and salt water intrudes in the river outlets.
Projected gap in crop yields in rainfed and irrigated agriculture, in 2050
PBL IMAGE, WenR
Water shortages cause large yield gaps in many areas around the world. Improved water management may increase crop yields in rainfed agriculture by 40% to over 60%. Improved water management may also increase crop yields in irrigated agriculture.
Impacts of interventions and climate change - global trends in pollution
The business-as-usual scenario projects a total increase in nutrient emissions from agriculture and households of 10% and 100%, respectively, by 2050.
Projected increases in nutrient emissions to surface water, from agriculture and households, 2010–2050, in kg/km2.
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Source: PBL
If sewerage systems are not combined with adequate wastewater treatment systems, the emission of nutrients and polluting substances to surface waters will increase. The rapidly growing urban areas in the developing countries are projected to become major sources of nutrient emissions.
Impacts of interventions and climate change - global trends in flood risk
Population growth and urbanisation often mean more people and assets in flood-prone areas. This increases flood risk. Soil subsidence and increased riverbank erosion in populated areas and coastal wetlands will aggravate this situation.
River flood risk: Projected global change in annual economic damage and population exposed to river flooding, between 2010 and 2050
Population annually exposed to river flooding
x 1000 persons
Source: Deltares, IVM
River flood risk will strongly increase between 2010 and 2050, globally. Annual economic damage increases, in particular, due to socio-economic developments. Between now and 2050, the contribution of climate change will be relatively small. Between 2010 and 2050, the relative increase in the number of people exposed to river flooding, globally, will be smaller than the relative increase in annual economic damage.
Coastal flood risk: Projected global change in annual economic damage and population exposed to coastal flooding, between 2010 and 2050
Population annually exposed to coastal flooding, high emission scenario climate change (RCP 8.5)
x 1000 persons
Source: Deltares, IVM
The annual economic damage will increase, in particular, due to socio-economic developments. Between now and 2050, the contribution of climate change is very small. The contribution of sea level rise to coastal flood risk is projected to manifest itself not until after 2050. Between 2010 and 2050, the increase in the number of people exposed to coastal flooding, globally, will be relatively small, compared to those exposed to river flooding.
Projected contribution of population growth, sea level rise and subsidence to the number of people exposed to coastal flooding, in 2050
Source: Deltares, IVM
The contribution of population growth, sea level rise and subsidence to the increase in the number of people exposed to coastal flooding, between now and 2050, will vary per world region. Sea level rise and subsidence will dominate the contribution in Asia, where the deltas are almost at current sea level and even the slightest sea level rise or subsidence will have a huge effect. In most parts of the world, population growth is the main contributor to the increase in the population exposed to coastal flooding.
Impacts of interventions and climate change - global trends in ecological quality and biodiversity
Effluent from industries and households, and nutrient runoff from agricultural land both affect water quality. The increase in nutrient emissions will also result in an increase in nutrient loading to coastal waters, especially in the Asian region. This will increase the risk of toxic algal bloom and oxygen depletion in those waters and will negatively affect biodiversity.
Water stress increases the risk of saltwater intrusion in river outlets and groundwater.
Projected change in the quality of fresh water ecosystems, per river basin, in 2015 and 2050
Source: PBL
A decline in quality will continue, especially in Sub-Saharan Africa and parts of Latin America and Asia. In the sparsely populated northern regions, the quality of fresh water ecosystems will be least affected.
Impacts of interventions and climate change - global trends in migration and conflict
The relationship between climate change, conflict and migration is likely to follow the path of climate change exacerbating human conflict due to competition over scarce resources. For instance, recurring events of severe drought due to climate change may lead to conflict and instability in countries with poor natural-resource management. Such climate-induced conflict, in turn, drives displacement and outmigration.
Today, the quality of most river basins and deltas is under enormous pressure and this situation is unlikely to improve between now and 2050. The quality will dwindle further and sustainable development will not be achieved without an integrated approach, improved water management and adaptation to climate change. Using the set of SDGs as an inspiration may help to make a change towards more inclusive, sustainable and climate-robust development strategies, plans and projects.
Under a business-as-usual scenario, achievement of the SDGs is slipping further out of reach - both due to climate change and human interventions. Terrestrial and aquatic biodiversity (SDGs 14 and 15), in particular, is projected to deteriorate, due to drought, warmer water temperatures, disturbed sediment flows, unsustainable water and groundwater abstraction and further increases in nutrient emissions. A projected strong increase in hydropower improves the achievement score for SDG 7 (Affordable and Clean Energy).
Investments in the conditional SDGs Climate Action (13) and Partnerships for the Goals (17) are only limited, under the business-as-usual scenario. Climate change and sustainable development are fundamentally connected, as the former can undermine the latter.
How is the human impact on rivers and deltas affecting achievement of the Sustainable Development Goals?
This illustration shows the 17 Sustainable Development Goals. Colours indicate the extent to which the human impact on rivers and deltas has affected achievement of these SDGs. For each SDG the human impact and how it affects achievement of the SDG is elaborated in the text. Currently, the impact is negative for 8 of the SDGs and very negative for 5. For four of them, the impact is both positive and negative.