Las agujas

The increasing need to supply the growing world population and steadily expanding economy with water, food, and energy particularly determines political and economic decisions on constructing dam projects and hydropower engineering schemes not only in China but worldwide. According to the World Commission on Dams (WCD, 2000) and INTERNATIONAL RIVERS (2007) more than 800,000 dam projects (mostly small dams) exist worldwide. They provide approximately 19% of the global energy supply and provide water to 30-40% of worldwide irrigated land of 271 million hectares (WCD, 2000).

Estimated 12-16% of the global food production is directly linked to reservoirs of large dams (WCD, 2000). The number of large and major dams worldwide is estimated to range from more than 45,000 to more than 50,000 (WCD, 2000;

LEHNER ET AL., 2011). According to the criteria of

the International Commission of Large Dams (ICOLD), dams of a height of more than 15 m or a height of 5-15 m and a storage volume of more than 3 million m³ constitute large dams, while major dams are higher than 150 m (ICOLD). Both are mainly located in major river networks in North and South America and China (Figure 18). In China alone, approximately 45% of the world's large dams (~22,000) and more than 85,000 dams in total are located (WCD, 2000; PONSETI and LOPÉZ-PUJOL, 2006).

Large dam projects attract worldwide scientific and media interest due to their serious upstream and downstream environmental impacts and socio-economic consequences in space, time, and costs (e.g. WUET AL., 2004; NILSSON ET AL., 2005; STONE and JIA, 2006), inasmuch, as they strongly fragment more than 50% of large river systems worldwide (NILSSON ET AL., 2005).

Within the framework of the International Union for Conservation of Nature (IUCN), OUD and MUIR (1997) reported on a myriad of key potential environmental and social impacts caused by large dam projects differentiated into upstream and downstream effects mainly resulting from the loss of settlements and infrastructure, biomass, habitats of flora and fauna, mineral resources and numerous more (OUD and MUIR, 1997). The land

use changes mainly account for large-scale shifts in agriculture, land reclamation, and construction of new roads and settlements to compensate the inundation of valuable land.

By looking at the globally 400,000 km² of land that were lost by flooding and the estimated 40 to 80 million people that were resettled since 1958 (WCD, 2000; INTERNATIONAL RIVERS, 2007) it seems logical to perceive soil erosion in the surrounding of large dams and their reservoirs as a serious environmental threat. Caused dredging costs of annually US$100-150 billion (INTERNATIONAL

RIVERS, 2007) due to reservoir siltation also

illustrate the relevance of soil erosion and sediment production as economic threat to the lifespan of reservoirs (e.g., BENNETT and RHOTON, 2007).

One prominent European example is the multipurpose (i.e., hydropower generation, water storage and supply, tourist attraction) large Alqueva dam at the Guadiana River in the semiarid south of Portugal. Its reservoir presents the largest artificial lake (250 km²) in Europe. However, post- construction shifts in land use and human activities in combination with erosion-prone soils, high rainfall erosivity during periods of poor vegetation cover, and steep terrain enhanced the soil erosion risk potential and sediment production in the Alqueva dam watershed and lead to distinct decreases in storage capacity due to silting up

(SERAFIM ET AL., 2006; FERREIRA and

PANAGOPOULUS, 2010; 2012). Applying empirical

soil erosion modeling using RUSLE combined with Geographic Information Systems, FERREIRA and

PANAGOPOULUS (2012) revealed an average annual

soil loss by water erosion of approximately 29 t ha-1 which constitutes the threefold of what is considered as severe soil loss in entire Europe

(JONES ET AL., 2012). Dam-induced rapid

intensification of agriculture and irrigated area, biomass production, and development of tourism promoted by regional development plans as well as climate change are assumed to even raise the soil erosion risk in the affected watershed and to boost the reservoir siltation (ARVELA ET AL., 2012;

FERREIRA andPANAGOPOULOS, 2012).

A second prominent example is the binational Itaipú dam impounding Rio Paraná at the border of Brazil and Paraguay. The subtropical reservoir of Itaipú Lake has an area of 1,350 km² at the normal operation water level of 220 m a.s.l.

upstream the dam is 820,000 km² (NORTON ET AL., 2001). The Itaipú dam construction was completed in 1982 for the main purpose of hydropower generation. After the Three Gorges Dam at the Yangtze River in China, it is the second largest in world in terms of installed capacity. The Itaipú hydroelectric facility provides more than 24% of Brazil's and 95% of Paraguay's electricity needs

(COCHRANE ET AL., 2004). Since the start of its

construction in 1975 and as a consequence of poor environment-related planning and follow-up of the aggressive policies of agricultural expansion in Brazil since the 1950s, intense land use changes are attributed to the project (NORTON ET AL., 2001;

ALIXANDRINI, 2010). By conducting land cover

change analysis from remote sensing images for the period from 1973 to 2009, ALIXANDRINI (2010) reveals a progressive and drastic reduction of woodland area by approximately 74% from 4,666 km² to 1,206 km² in both, Brazil and Paraguay, mainly for the benefit of agricultural land that increased by approximately 76% from 3,062 km² to 5,393 km². Urban area increased more than fivefold from 30 km² to 152 km² within the same period. Those shifts dramatically increased soil losses by water erosion and enhanced sediment production from the major tributaries of Rio Paraná River by draining these areas of high erosion risk (NORTON

ET AL., 2001; COCHRANE ET AL., 2002).

High to extremely high rates of soil loss by water erosion in the Itaipú reservoir basin of annually more than 20, respectively, 50 t ha-1 refer to areas under conventional tillage directly adjacent to the impounded backwater area (COCHRANE ET AL., 2003). Initially, during the feasibility study of the dam, no risk of sedimentation was expected and the lifespan of the project was assumed to be more than 300 years (NORTON ET AL., 2001). However, the enhanced sedimentation loading into the reservoir is expected to dramatically shorten the dam's long-term safe operation by premature filling and to affect the water ecology (MORMUL ET AL., 2010).

THE THREE GORGES DAM AND ITS