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Part 2

Understanding the system

This figure shows a 3D illustration of the landscape of any river, from its source in the mountains to its outflow into the sea. It also shows a summary of human interventions in river systems around the globe. This is done for the current situation and for the five clusters of pressures on the river system: dams and reservoirs, agriculture, urbanisation and industrialisation, port development and shipping, and engineered deltas.

Select a scenario:

Human interventions

Rivers and their deltas have always been attractive and productive regions for human settlements and economic development.

The water transport routes of rivers and the sea, the fertility of the floodplains and the fresh water supply provide ample opportunities for economic development. In turn, human settlements have altered the rivers and their deltas.

More information on the impact of interventions and climate change on sediment transport and morphology:

The Rhine River case

Human interventions in rivers

River systems and their deltas have always been attractive for human settlement because of the water, fertile soils and transport opportunities they provide. They are historically embedded in human cultures and economies. Today, these systems are under enormous pressure from human interventions in the system, such as water abstraction for agriculture, industries and households, deforestation, land reclamation along rivers, urbanisation, polluting emissions, dam construction and canalisation for transport.

These interventions reduce streamflow, sediment flow and water quality, and destroy natural processes and habitats. These trends jeopardise the future of human and ecological systems that depend on healthy and thriving rivers and deltas. In addition, the increased competition for water may increase tensions and conflict between countries sharing the same rivers. The concentrations of people, economic assets and activities in river and delta areas increase flood risk and health risks due to exposure to polluted water.

Mapping the world's free-flowing rivers: state of interventions

This map shows the state of human interventions for all of the world’s large rivers. An indicator is used to expresses the extent to which a river is still flowing freely or is disturbed by dams and other interventions. Most large rivers are disturbed by human interventions to some extent.

Source: Grill et al., 2019

Only 37% of rivers longer than 1,000 kilometres still flow freely over their entire length and 23% flow uninterruptedly towards the sea. Dams and reservoirs are the main contributors to the loss of river connectivity. Currently, there are an estimated 2.8 million dams around the world, and many more will follow. Dams are the dominant pressure factor in more than two-thirds of the river reaches that are no longer considered to be ‘free-flowing’.

At 2010, 52% of the world population lived in an urban environment. The rapidly growing urban population strongly increases the pressure on local resources, local environmental conditions, food availability, labour opportunities and public services.

Human interventions in deltas

Deltas face the impacts from human intervention, both upstream, across borders and in the delta and coastal zone itself, as well as the impacts from climate change affecting river flows, precipitation peaks, sea level rise and the intensity and frequency of storm surges. Deltas, thus, are an integrated part of both river basin and coastal zone.

In engineered deltas, urbanisation, industrial developments, land reclamation, agricultural and aquacultural developments, and groundwater exploitation for fresh water supply lead to additional water pollution, soil subsidence and saltwater intrusion, and reduce the autonomous delta-forming processes. The reduced natural processes supporting the delta affect biodiversity and require more and more infrastructural measures to protect the growing population and economic value against flooding and sea level rise.

Select a delta type:

Wave dominated deltas

In almost 80% of the world’s river deltas, waves are the dominant force. These deltas have the deltaic shape that has given deltas their name. The sediment that is carried into the delta is subsequently pushed back again by waves, creating a smooth coastline.

Most of these wave-dominated river deltas are relatively small, however. The traditional deltaic shape may be the most common, but the discharge from the rivers of these deltas is only 16% of all river discharges into the seas.

±8700 wave dominated deltas worldwide. In total, these deltas make:

Number of deltas
79%
Discharge
18%
Sediment flux
15%

Source: Utrecht University, Nienhuis et al., 2020

Wave dominated deltas

This figure shows three types of deltas
Those that are mainly formed by waves in coastal zones, those formed by tides, and those formed by river discharge and sediment outflow into the sea. By clicking on a delta type in one of the three circles on the right, the occurrence of this delta type is shown on the world map below. Wave-dominated deltas are the most common. They are relatively small, compared to the other types, since those have much larger sediment input from rivers.

Impacts of interventions - current dams and disturbed sediment flows

Floodplains along rivers and in deltas that are densely populated are often also threatened by the erosion of riverbanks and wetlands. High population density, generally, goes hand in hand with disturbed sediment dynamics and increased erosion. This is due to dams constructed for hydropower generation and reservoirs that capture river sediments upstream, people mining sand for construction, and intense shipping that erodes riverbanks.

Dams for hydropower generation and fresh water reservoirs in 2010

Density of current dams per waterprovince

Dams are one of the main human interventions in rivers. This world map shows the density of dams in all world regions. It is the highest in Asia, followed to a lesser extent by the United States and parts of Europe, Brazil and Southern Africa.

Source: Mulligan et al, 2020 (dams); Straatsma & Van Aalst, 2014 (waterprovinces)

Thousands of dams, sluices and canals have already been constructed, for hydropower generation, fresh water reservoirs and shipping. This has affected streamflow and sediment flow, and reduced the wetland areas in most river basins. Generally, natural river discharge regimes downstream from these dams are strongly disrupted. This affects water temperature, water quality and biodiversity. Also, less sediment is transported downstream, causing the river delta to erode.

The growing number of dams, from 1800 to 2017

Source: globaldamwatch.org/grand

The Global Reservoir and Dam Database contains more than 7000 records of large global dams. The related water storage capacity of each dam in this database is more than 0.1 km3. The largest growth in dam construction took place between 1950 and 1990. After 1990, globally the number of newly built dams decreased. However, relatively many new dams are still being built in Southeast Asia, Brazil and the Middle East.

Upstream sediment trapped by dams in 2010

In %

This illustration shows how much of the sediment that is being transported from the upper reaches of rivers becomes trapped by dams and settles in reservoirs.
Average percentages are shown for dams on all continents. With the exception of Oceania, dams on all continents, on average, trap more than half of the sediment coming from upstream.

Source: Ericson et al., 2006

A sample of 40 rivers and their deltas shows that reservoirs trap a large part of the sediment in the upstream reaches of rivers, around the globe. Loss of fluvial sediment input has already resulted in observed land loss and coastal erosion in many deltas, and these impacts will continue to occur as the demand for water resources increases. The loss of fluvial sediment due to it being trapped in reservoirs leads to subsidence of the delta and is the dominant factor in effective sea level rise for a majority of deltas included in this sample.

Impacts of interventions - current levels of water abstraction

Fresh water abstraction has strongly increased globally over the past decades. Especially groundwater reserves have been exploited unsustainably in many places around the world: more water was pumped in dry summers than could be recharged by rain in other seasons. As a result, groundwater levels have dropped, sometimes resulting in soil subsidence and damage to nature and biodiversity, and pumped water becoming too brackish for consumption.

Global water abstraction from rivers

About three quarters of the water used in agriculture, households, industry and electricity generation is abstracted from rivers (left). Agriculture uses the most water, by far, with more than 80% of total consumption (right).

Source: UU, PBL IMAGE

Impact of global sea level rise and average land subsidence, for several coastal cities

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A major cause of severe subsidence is the excessive groundwater extraction that accompanies rapid urbanisation and population growth. Subsidence increases the effective, relative sea level rise in deltas and, thus, also increases flood risk. This illustrates that different water issues can be strongly related; polluted surface water causes people to depend on groundwater for consumption. The resulting over-exploitation of groundwater reserves causes subsidence which, in turn, increases flood risk. Subsidence in Tokyo was brought to a halt when in 1975 excessive groundwater extraction was ceased.

Impacts of interventions - current levels of pollution

The emission of nutrients (nitrogen and phosphorus) to rivers, lakes and coastal seas has a negative impact on water quality and ecological quality, and may affect economic functions, such as aquaculture, fisheries and tourism.

Nutrient emissions to surface water in 2010

From agriculture and households, in kg/km2

This world map shows nutrient emissions from agriculture and households to surface water in 2010. Emission levels were the highest in Asia, and especially China, but were also high in Brazil and Europe, in 2010.

Source: PBL

Densely populated river basins and deltas, in particular in South and East Asia, are major sources of nutrient emissions. The strong increase in these emissions reflects the strong growth in urbanisation.

Impacts of interventions - current levels of flood risk

In the past, exposure to flooding has increased mainly due to socio-economic developments. Ever more people have settled in flood-prone areas. The population has grown there, and so have the investments in assets. So far, the increase in exposure is not reflected by an increase in vulnerability; the resilience of people in flood-prone areas has increased (although the poor are still very vulnerable).

River flood risk, 2010: Projected annual damage and population exposed to river flooding

x 1000 persons

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Source: Deltares, IVM

River flood risk, in terms of projected annual economic damage and annually exposed population, increased strongly, over the past decades. This is due to the increase in the exposure to river flooding due to ever more people settling in flood-prone areas. So far, no clear sign of climate change has been found in trend analyses of increased flood losses. Exposure to river flooding will increase, in future decades, due to population growth, climate change and more investments in flood-prone areas.

River flood risk, 2010: Projected annual damage and population exposed to river flooding

x USD million

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Source: Deltares, IVM

River flood risk, in terms of projected annual economic damage and annually exposed population, increased strongly, over the past decades. This is due to the increase in the exposure to river flooding due to ever more people settling in flood-prone areas. So far, no clear sign of climate change has been found in trend analyses of increased flood losses. Exposure to river flooding will increase, in future decades, due to population growth, climate change and more investments in flood-prone areas.

Coastal flood risk, 2010: Projected annual damage and population exposed to coastal flooding

x 1 person

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Source: Deltares, IVM

The frequency of storm surge flooding events with substantial loss of life has decreased since 1900. The share of human fatalities as a result of this type of flooding has decreased consistently in all global regions, except for South and East Asia. This downward trend probably reflects improvements in flood protection and forecasting, early warning systems and evacuation and the quality of residential buildings. This trend, however, could slow down or even be reversed. Storm surge events are very likely to become more severe due to sea level rise. Greater flood depths, subsidence and more population growth may affect potential loss of life, in future decades.

Coastal flood risk, 2010: Projected annual damage and population exposed to coastal flooding

x USD million

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Source: Deltares, IVM

The frequency of storm surge flooding events with substantial loss of life has decreased since 1900. The share of human fatalities as a result of this type of flooding has decreased consistently in all global regions, except for South and East Asia. This downward trend probably reflects improvements in flood protection and forecasting, early warning systems and evacuation and the quality of residential buildings. This trend, however, could slow down or even be reversed. Storm surge events are very likely to become more severe due to sea level rise. Greater flood depths, subsidence and more population growth may affect potential loss of life, in future decades.

Impacts of interventions - current levels of ecological quality and biodiversity

Socio-economic and climate-related pressures have deteriorated the water quality and biodiversity of rivers, deltas and coastal zones. The loss of wetlands that started in 1700 is still continuing.

Quality of fresh water ecosystems per riverbasin, 2015

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Source: PBL

High-quality river ecosystems in tropical regions are already severely affected over large areas; especially in Sub-Saharan Africa and parts of Central and South America, and South and East Asia. Also, in Europe and the United States, a strong decline has occurred over large areas. In the sparsely populated northern regions, the quality of fresh water ecosystems is affected the least.

Remaining natural wetland area

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Source: Davidson 2014

Since 1700, about 80% of the world’s wetlands, both inland and coastal, have been disappearing, and this decline is still continuing today. The main causes are wetland reclamation for agriculture and urban development, canalisation for shipping, and the construction of dams that reduce the inundation of wetlands and affect natural river flow. Loss of wetlands also means less carbon storage.

Impacts of interventions - current levels of migration and conflict

Conflict and migration result from complex social, economic and governance processes, which can differ on a local level. Depending on the circumstances, water issues can either lead to collaboration or contribute to migration and the risk of conflict.

Linking the sustainable development goals

Water is linked to most of the Sustainable Development Goals (SDGs) and this is particularly the case for river and delta systems. Today, the quality of most river basins and deltas is under enormous pressure and the SDGs are not being achieved.

Human interventions in, and effects on, rivers and deltas have an impact on all SDGs. Most impacts are negative, but some SDGs are affected in both a negative and positive way. Rivers and deltas provide various options for jobs, economic growth, industry, innovation and infrastructure. However, the increase in people and economic assets also increase flood risk, especially since investments are largely based on the 'old climate' (SDGs 8 and 9). Many more people are projected to have improved access to clean drinking water and sanitation, but in large parts of Africa and Asia, this improvement cannot keep up with the rapid population growth (SDG 6).

More information on the current situation with respect to the SDGs is presented in The Atlas of Sustainable Development Goals 2020.

How is the human impact on rivers and deltas affecting achievement of the Sustainable Development Goals?

SDG Wheel -->

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.