Propagation of hydro-geomorphic disturbances through continental-scale river basins: Future evolution of the Amazon River and its floodplain

Lead Research Organisation: UNIVERSITY OF EXETER
Department Name: Geography

Abstract

Hundreds of millions of people live close to, and depend upon, the world's large rivers for water, food, transport and the maintenance of a thriving ecosystem. However, these rivers are increasingly vulnerable to the effects of a wide range of natural and human-induced disturbances, including climate change, construction of large dams, river engineering works, deforestation, agricultural intensification, and mining activity. Over the past 20 years, climate change and deforestation have impacted on the hydrology and sediment fluxes within the Amazon River Basin. However, the Amazon has remained one of the few large river systems that has been largely unaffected by dams. This situation is changing rapidly, because widespread hydropower dam construction in Brazil, Bolivia, Peru and Ecuador now threatens the basin, with >300 dams planned or under construction. These dams are expected to trigger severe hydro-physical and ecological disturbances throughout the basin, including massive reductions in sediment and nutrient delivery to the lowland Amazon and its floodplains, substantial degradation of river beds and banks, significant changes in river water levels and flooding, and adverse impacts on river and floodplain ecosystems, on which the human population depends.

Recent high profile studies highlight the need for international action to assess and mitigate these impacts, both in the Amazon and elsewhere. However, our capacity to do this is severely restricted by an absence of quantitative models that can predict how environmental disturbances propagate through large rivers and floodplains, over continental distances, and decadal to centennial time periods. Critically, environmental disturbances driven by dams, climate and land cover change promote dynamic river responses (e.g., changes in river width, depth, slope, sediment size, degree of branching and rate of floodplain reworking), which in turn control changes in flood conveyance and downstream sediment delivery. Despite advances in modelling of river dynamics over short distances (<100 km), hydrological models that are applied to continental-scale drainage basins treat rivers and floodplains as static conduits. Consequently, such models are unable to represent or predict the future impacts of environmental change on flooding, sediment fluxes or river and floodplain functioning.

This project will deliver a step-change in our ability to model, predict and understand how the world's large rivers are impacted by, and respond to, environmental change. We will achieve this by implementing a research strategy that involves six elements: First, we will develop a new multi-scale numerical modelling approach that enables the effects of river dynamics on environmental disturbance propagation through continental-scale drainage basins to be simulated. Second, we will develop a suite of environmental scenarios representing climate and land cover changes and dam construction throughout the Amazon Basin for the recent past (1985-2015) and future (up to 2200). Third, we will collect new field datasets at sites on the Amazon River that are required to test key components of the model. Fourth, we will work with an international team of project partners to assemble high-resolution field, satellite and model datasets that quantify channel and floodplain processes, and river morphology and dynamics throughout the Amazon Basin. Fifth, we will use these data to carry out rigorous testing of our new model. Sixth, we will apply the model to predict the future evolution of the Amazon River and its tributaries for a wide range of environmental change scenarios, and quantify the controls on hydro-geomorphic disturbance propagation within large drainage basins. We will work with our project partners to disseminate our model code, datasets and project outcomes to non-academic stakeholders, both nationally and internationally.

Planned Impact

The scientific advances that will be delivered by this project have direct relevance to a wide range of agencies concerned with economic, social, management and policy issues. Key beneficiaries of this research will include:

1. National and international engineering organisations and environmental consultancies that use numerical models to assess flood risk, infrastructure stability, sediment management problems, or aquatic habitat suitability.

2. Government agencies with responsibility for river management and policy planning in the context of flood mitigation, disruption to bankside communities and infrastructure, navigation, sediment management, and future climate change. Potential benefits of our research to these agencies are diverse, and include an ability to understand and quantify the impacts on riparian and floodplain communities of policy decisions related to dam construction, land cover change, flow regulation, and river engineering works.

3. International, national and regional NGOs that work to support local communities and seek to: (a) understand the potential environmental, social and economic impacts in river and floodplain landscapes resulting from the effects of climate change and anthropogenic activity; (b) support local communities affected by these impacts; (c) raise public awareness of impacts and their causes; and (d) influence and inform government policy designed to adapt to, and mitigate, these impacts.

4. GCSE and A Level geography teachers and students who study flood processes and flood vulnerable environments as part of the curricula and have to demonstrate quantitative data skills as part of their fieldwork and formative assessment.

These groups will benefit from the research undertaken here in multiple ways:

1. The numerical models developed in this project will be directly applicable to the river management problems addressed by these beneficiaries. Our models (and open source code) will be freely available to these agencies. We will also deliver workshop activities in the UK and USA to support training in the use of our models and their uptake by a broad group of beneficiaries.

2. To deliver benefits to these groups rapidly, we will work with two such agencies (the US Army Corps of Civil Engineers and their partners the Brazilian National Department of Transportation Infrastructure) to facilitate their use of our models and model datasets. This will include the co-design of our model simulations to address questions of specific concern to these Agencies on the Madeira River in Brazil, and support to enable the application of our models in other river basins of particular interest to these groups.

3. To maximise the longer-term benefits of our research we will disseminate our project outcomes in policy relevant formats to NGOs and government agencies with interests throughout the Amazon Basin. We will also run a widely publicised forum event, hosted by our partner the Royal Geographical Society, to debate the outcomes of our work with national and international engineering and environmental consulting companies, charities, government advisors and environmental lobbyists.

4. To deliver benefits from our research to teachers and students we will work with the RGS to produce educational resources targeted at GCSE and A level students, and undertake educational outreach activities at national events.

Publications

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Description Two main aspects of the project are noteworthy at present:

The project has carried out computer modelling work that has shown what the impact might be of the construction of dams on the Andean tributaries of the Amazon River. Our results show how dam construction could change the evolution of these tributaries, leading to the erosion of an alluvial trench within the floodplain. A key outcome of the formation of this trench would be a change in the water levels during floods, and the disconnection of large floodplain wetlands from the main river. This would have significant implications for the wetland ecosystems. The magnitude of this impact would depend on the rate of sediment transport within the river, prior to dam construction and on the rate of river bank erosion (channel migration). The process of channel migration is responsible for floodplain reworking and determines the width of the trench that is formed once the dam is built. This floodplain reworking also mobilises floodplain sediment and compensates for the trapping of sediment within behind the dam. Consequently, for rivers with high rates of lateral migration, the drop in the sediment load of the river that would normally be anticipated to be an outcome of dam construction would be much less pronounced.

The project has also conducted field work on the lowland section of the Amazon River and shown, for the first time, that the stability of the river is strongly controlled by differences in the erodibility of the river banks. By combining these field measurements with the analysis of satellite imagery we have shown that the form and evolution of the whole of the lower Amazon (over 1600 km) is closely linked to the strength of sediments in the river banks. Our analysis suggests that the bank sediments provide a mechanism that keeps the river pinned against the valley sides and has done so in many places for thousands of years.
Exploitation Route The work on river and floodplain responses to dam construction could be used by engineers, NGOs or in environmental impact assessments to evaluate the potential geomorphic, hydrological and ecological consequences of dam construction over periods of years to centuries and distances of hundreds of km.

The work on the role of heterogeneity in bank strength as a control on river behaviour could be used by a wide range of environmental scientists to understand and interpret how floodplain sediment age and composition controls the dynamics of lowland rivers (e.g., to better understand rates and patterns of river bank migration and floodplain reworking).
Sectors Environment