The many varied landscapes of the earth are characterized by their watersheds, which in turn are defined by the rivers that nourish and sustain them. During the past century, humans have intensively developed these river systems for irrigation, flood control and more recently hydropower by building dams, diversions, and levees. In the last sixty years, the pace of development has increased: some 49,000 major dams are now in operation, the vast majority built since the Second World War.
Human alterations have profoundly affected river systems and the ecosystem services they provide. Development often transforms highly dynamic river systems into static impoundments and water delivery channels. Natural sediment processes are disrupted, resulting in incised, unproductive stream channels and increased flood risk. Water diversions and groundwater extractions deplete even the minimum flows necessary to maintain fisheries. Cut off from interacting with their floodplains, wetlands, estuaries and deltas, river systems are unable to support the habitats, species diversity and human livelihoods that thrived under natural conditions.
Especially in arid regions, the consequences of development for the downstream river basin can be profound. Many great rivers run dry though formerly productive reaches. Others no longer reach the sea or have so altered their estuaries that these engines of biological productivity no longer function. Those who depend on the river for their livelihood suffer.
Scientists and resource managers have recognized the importance of mimicking natural flow patterns to improve the overall watershed health of developed river systems. Much effort has been devoted to understanding the environmental flow requirements of various components of aquatic ecosystems, especially the seasonal variability. This work defines the environmental water demands for the system.
NHIís unique contribution is to illuminate the environmental water supply side of the equation, which has thus far been comparatively neglected in defining restoration strategies. We acknowledge the legitimacy of water development aspirations, especially in developing countries, and the economic benefits conferred by hydraulic infrastructure. Thus, the tools and techniques we are exploring and demonstrating for restoring natural functions to developed rivers seek to preserve these economic benefits by optimizing existing infrastructure for environmental performance. NHIís approach enables watershed development facilities to make environmental flow releases a permanent operational feature without reallocating water from economic to environmental uses.
Our strategies include:
- Changing water storage and release patterns in hydropower facilities by linking irrigation reservoirs to groundwater banks (a bold new concept for conjunctive water management);
- Redesigning flood management systems and floodplain land uses to restore periodic connections between rivers and their floodplains; and
- Converting hydropower facilities to run-of-the-river operations by changing the role that these facilities play in the array of generators feeding the electrical grid.