Lake Salto Grande is a reservoir constructed on the Uruguay River between Argentina and Uruguay to produce hydroelectric power for the two countries. Much of the energy is used in Uruguay, which often surpasses the consumption of Montevideo. It also is an important recreational center. Its construction resulted in the relocation of about 22,000 people. The vast flat plains area downstream of the reservoir, experiences results in accelerated soil erosion in the rainy season. Siltation is a resulting problem, in spite of afforestation efforts undertaken around the waterbody. The reservoir is facing a wide range of environmental problems, including eutrophication and trace organic chemical contamination. The suitability of this lake for possible GEF-catalyzed management interventions depends on many factors, including the potential economic and social development gains to be realized for the region. It also requires an assessment of the lake’s current scientific status.

TWAP Regional Designation Southern America   Lake Basin Population (2010) 5,001,392
River Basin La Plata Lake Basin Population Density
(2010; # km-2)
Riparian Countries Argentina, Uruguay Average Basin Precipitation
(mm yr-1)
Basin Area (km2) 216,544 Shoreline Length (km) 683.5
Lake Area (km2) 533 Human Development Index (HDI) 0.74
Lake Area:Lake Basin Ratio 0.002 International Treaties/Agreements Identifying Lake

Salto de Grande Basin Characteristics

(a)Salto de Grande basin and associated  transboundary water systems

(a)Salto de Grande basin and associated transboundary water systems

(a)Salto de Grande basin and associated  transboundary water systems

(b)Salto de Grande basin land use

Salto de Grande Threat Ranking

A serious lack of global-scale uniform data on the TWAP transboundary in-lake conditions required their potential threat risks be estimated on the basis of the characteristics of their drainage basins, rather than in-lake conditions. Using basin characteristics to rank transboundary lake threats precludes consideration of the unique features that can buffer their in-lake responses to basin-derived disturbances, including an integrating nature for all inputs, long water retention times, and complex, non-linear response dynamics.

The lake threat ranks were calculated with a spreadsheet-based interactive scenario analysis program, incorporating data and information about the nature and magnitude of their basin-derived stresses, and their possible impacts on the sustainability of their ecosystem services. These descriptive data for Salto de Grande and the other transboundary lakes included lake and basin areas, population numbers and densities, areal extent of basin stressors on the lake, data grid size, and other components considered important from the perspective of the user of the data results. The scenario analysis program also provides a means to define the appropriate context and preconditions for interpreting the ranking results.

The Salto de Grande threat ranks are expressed in terms of the Adjusted Human Water Security (Adj-HWS) threats, Reverse Biodiversity (RvBD) threats, and the Human Development Index (HDI) score, as well as combinations of these indices. However, it is emphasized that, being based on specific characteristics and assumptions regarding Salto de Grande and its basin characteristics, the calculated threat scores represent only one possible set of lake threat rankings. Defining the appropriate context and preconditions for interpreting the lake rankings remains an important responsibility of those using the threat ranking results, including lake managers and decision-makers.

Table 1. Salto de Grande Relative Threat Ranks, Based on Adjusted Human Water Security (Adj-HWS) and Reverse Biodiversity Threats,
and Human Development Index (HDI) Score

(Estimated risks: red – highest; orange – moderately high; yellow – medium;
green – moderately low; blue – low)

Adjusted Human Water Security
(Adj-HWS) ThreatScore
Relative Adj-HWS Threat Rank   Reverse Biodiversity (RvBD) Threat Score Relative RvBD Threat Rank   Human Development Index (HDI) Score Relative HDI Rank
0.67 40 0.70 10 0.74 38

It is emphasized that the Salto de Grande rankings above are discussed here within the context of the management and decision-making process, rather than as strict numerical ranks. Based on its geographic, population and socioeconomic assumptions used in the scenario analysis program, the calculated Adj-HWS score for Salto de Grande indicates a moderately low threat rank compared to other priority transboundary lakes.

The Reverse Biodiversity (RvBD) for Salto de Grande, which is meant to describe its biodiversity sensitivity to basin-derived degradation, places the lake in a high threat rank, compared to the other transboundary lakes. Management interventions directed to improving the biodiversity status must be viewed with caution, however, since we lack sufficient knowledge and experience to accurately predict the ultimate impacts of biodiversity manipulations and preservation efforts. Further, the RvBD scores indicate the relative sensitivity of a lake basin to human activities, and high threat scores per se do not necessarily justify management interventions. Such interventions may actually increase biodiversity degradation, noting that many developed countries have already fundamentally degraded their biodiversity because of economic development activities. Thus, activities undertaken to address the Adj-HWS threats may actually degrade the biodiversity status and resources, even if the health and socioeconomic conditions of the lake basin stakeholders are improved as a result of better conditions, thereby increasing stakeholder resource consumption.

The relative Human Development Index (HDI) places the Salto de Grande basin in a moderately low threat rank in regard to its health, educational and economic conditions.

Table 2. Salto de Grande Threat Ranks, Based on Multiple Ranking Criteria

(Scores for Adj-HWS, RvBD and HDI ranks are presented in Table 1; the ranks may differ in some cases because of rounding of figures; Estimated risks: red – highest; orange – moderately high; yellow – medium;
green – moderately low; blue – low)

Adj-HWS Rank HDI Rank RvBD Rank   Sum Adj-HWS + RvBD Relative
Threat Rank
  Sum Adj-HWS + HDI Relative Threat Rank   Sum Adj-HWS + RvBD + HDI Overall Threat Rank
40 38 11 51 28 78 39 89 32

When multiple ranking criteria are considered together in the threat rank calculations, the Adj-HWS and HDI scores considered together place Salto de Grande in the lower third of the threat ranks. The relative threat is somewhat increased when the Adj-HWS and RvBD threats are considered together. Considering all three ranking criteria together, Salto de Grande exhibits a medium threat ranking.

Further, a series of parametric sensitivity analyses of the ranking results also was performed to determine the effects of changing the importance of specific criteria on the relative transboundary lake rankings. This analysis involved increasing or decreasing the weights applied to the threat ranks derived from multiple ranking criteria to reassess the relative impacts of the weight combinations on the threat ranks. For example, in determining the sensitivity of the Adjusted Human Water Security (Adj-HWS) and Biodiversity (BD) ranking criteria, the threat rank associated with the first was assumed to be of complete (100%) importance (i.e., rank weight of 1.0), while the other was assumed to be of no (0%) importance (i.e., rank weight of 0.0). The relative importance of the two ranking criteria was then successively changed, with weight combinations of 0.9 and 0.1, 0.8 and 0.2, etc., until the first ranking criteria (Adj-HWS) was assumed to be of no importance (rank weight of 0.0) and the second (BD) was of complete importance (rank weight of 1.0). In the case of Salto de Grande, the 0.5 and 0.5 weight combinations for three cases of parametric analysis for Salto de Grande resulted in respective threat rankings of 6th, 6th and 5th, respectively, among the total of 6 South American transboundary lakes in the TWAP study (see Technical Report, Section 4.3.3, pp44-51).

In essence, therefore, identifying potential management intervention needs for Salto de Grande must be considered on the basis of both educated judgement and accurate representations of its situation. A fundamental question to be addressed, therefore, is how can one decide that a given management intervention will produce the greatest benefit(s) for the greatest number of people in the Salto de Grande basin? Accurate answers to such questions for Salto de Grande, and other transboundary lakes, will require a case-by-case assessment approach that considers the specific lake situation and context, the anticipated improvements from specific management interventions, and its interactions with water systems to which the lake is linked.