The Empresa Nacional de Energía Eléctrica (ENEE) of Honduras requested that The Nature Conservancy (TNC) conduct an environmental flow assessment for the Patuca River below a proposed hydroelectric project, referred to as Patuca III. The Patuca River is the longest river in Honduras, third longest in Central America, and is currently undammed. The river supports globally important aquatic biodiversity and flows through a reserve for indigenous communities and other protected areas. Communities within these reserves rely heavily on the river for water and transport and as an important source of fish protein. Additionally, the river fertilizes agricultural fields by depositing nutrient-rich sediments on the fields during floods. Due to the Patuca River’s important biological and cultural values, ENEE sought information on how the proposed dam may affect the river and its resources and asked TNC for guidance for a managed flow regime that will minimize the impacts of Patuca III on the river’s ecological integrity and resources.

An environmental flow assessment was conducted based on a variety of information sources, including field trips along the river, scientific data and expertise derived from similar river systems, analysis of hydrological data, and traditional ecological knowledge. These information sources were synthesized and presented during two workshops. During the workshops, participants from the communities, ENEE, government agencies, non-governmental organizations and academia worked to define collaboratively ecologically and socially important river processes and associated environmental flow recommendations.

This report describes the environmental flow assessment for the Patuca River. Following five background sections, the details of the flow assessment are presented in Section 6, with three sub-sections. Section 6.1 describes the recommended Environmental Flow Components (EFCs)—which include low flows, high-flow pulses, and floods — for normal, wet, and dry years. The EFCs are described in terms of magnitude, frequency, duration, and season (for more detail on the EFCs see “Overview of Environmental Flow Assessment” within this Executive Summary and the main report). The EFCs and hydrological year types (wet, dry, normal) reflect the natural intra-and inter-annual hydrological variability of the Patuca River. In addition, this section describes the linkages between EFCs and important physical and biological processes in the river, such as sediment transport and fish migration. These linkages are framed as hypotheses to be tested and refined through further monitoring and research on the Patuca River. Future research and monitoring should target the research questions and uncertainties identified through this process to optimize and refine the flow recommendations.