Hydropower Development on the Rio Usumacinta
F. N. Scatena

The Rio Usumacinta is the largest and longest river in Meso-America. Plans to develop its water resources have been considered since the 1940’s (see regional timeline). In the 1980’s a series of dams along the international boundary of Mexico and Guatemala were proposed but canceled because of civic resistance and environmental and economic concerns (Wilkerson 1991, McGahan 2003). Current plans (www.mexiccotm.com) focus on building an interconnected system of dams within Mexico that includes a large dam on the Rio Usumacinta at Boca del Cerro ( Fig 1.). According to the available information, these proposed dams will be some of the largest in the world (Figures 2, 3, 4).

While the benefits of developing the regions hydropower is widely acknowledged, the current proposal has generated considerable local and international debate. The majority of this controversy has centered on the potentially large, but generally unknown, impacts to the environmental and cultural resources of the area. The main points of controversy include:

Complexities of building large dams in this region

Recent experience with the construction of nearby dams indicates that earthquakes and complex subsurface geology greatly increase the cost and risks associated with building dams in the region. Unfortunately, publicly available discharge records for the Rio Usumacinta at Boca del Cerro end in 1983 (Fig. 5) and geotechnical studies done in the 1980’s have not been made publicly available. Nevertheless, since these initial studies were completed there have been significant changes within the headwaters of the basin that are known to alter the quality, quantity, and timing of stream flow. These changes include, the construction of hydroelectric dams and water diversions, deforestation and urbanization. The impact of these changes on the quantity and quality of stream flow at Boca del Cero must be assessed before any dam or large hydraulic structure can be adequately designed or evaluated. Likewise, similar analysis must be done for the entire, integrated project that is being proposed (Fig. 1).
The site proposed for the Boca del Cerro dam is underlain by complex karsts topography of marl and limestone. The associated sinkholes and solution features are well known to affect the impounding capability of reservoirs and the structural integrity of the dam foundation. Recent experience with the Chixoy dam in Guatemala and the Zimapan and La Amistad dams of Mexico indicate the needs to conduct detailed geophysical investigations and subsurface preparations prior to dam construction. Similar studies need to be conducted and adequately reviewed before the recently proposed dam projects can be designed and evaluated.

Figure 1: Map showing current or proposed dams. View a presentation of dams in the Sierra del Lacandón area, as compiled by Conservation International México.

The impact to known and unknown archeological sites

Analysis of reservoir water levels indicated that world-renowned archeological sites at Piedras Negras, Altar de Sacraficios, and Yaxchilan would have been flooded by the dams proposed in the 1980 (Wilkerson 1991). The impacts to known and unknown archeological sites of the current proposal have yet to be quantified, in part because of the lack of specific information on the design of the newly proposed dams. Nevertheless, a 90 meter dam at Boca del Cerro could inundate over 160 square kilometers and have direct impacts on parts of Piedras Negras and Yaxchilan. Moreover, over 700 km2 could eventually be flooded of all the proposed dams and diversions are built (Fig 4).

Impacts to downstream wetlands and fisheries

The Pantanos de Centla lagoon complex at the mouth of the Usumacinta was designated a biosphere reserve in 1992 and is considered to be one of the most important coastal wetlands in Mesoamerica. This wetland system contains both freshwater and saline water swamps and supports rice cultivation and local fishing. It is also a nursery for one of the regions most important commercial coastal fisheries. The Rio Usumacinta currently provided the majority of the systems freshwater inputs and during low flow periods (Fig 6) saltwater intrudes some 30 km upstream. How the proposed Boca del Cerro dam will affect freshwater inputs, the estuarine circulation, and biotic resources of the Pantanos de Cental lagoon are currently unknown. Nevertheless, they need to be evaluated before costs and benefits of the project can be completely assessed.

Impact to migratory aquatic species in area

Large dams, like those proposed for the Rio Usumacinta at Boca del Cerro, are known to alter upstream and downstream aquatic habitat and act as barriers to the migration of aquatic species. These impacts can be particularly acute where hydroelectric dams change the normal hydrologic regime by alternately storing and releasing water. Unfortunately, the abundance, ecology, and instream flow needs of aquatic species in the region are currently unknown. Therefore, neither the impacts of the dams or the potential to design and operate the dams to mitigate those impacts can be evaluated at this time.

Impacts of upstream land use on the proposed dams

The long-term economic success of the Boca del Cerro dam, and all the other dams in the recently proposed network depends on the quantity and quality of water flowing into the dams. Unfortunately, little public information exists at many of the proposed dam sites and the only publicly available information on stream flow at Boca del Cerro ends in 1983 (Fig 6). Since that time there have been considerable changes in the land use of the drainage above Boca del Cerro. A current assessment of the sediment and water yield at each of the proposed dams is needed to adequately evaluate the hydropower value and estimated design life of the dams. The type and cost of upland land management that will be needed to maintain the reservoir also needs to be determined to adequately evaluate the cost and expected useable lifetime of the dam