The TWAP is an international project funded by the Global Environment Facility (GEF) with the goal of conducting a global-scale assessment of five major types of transboundary water systems; namely, lakes, rivers, groundwater aquifers, large marine ecosystems, and the open oceans. The objective is to rank these five major types of transboundary water systems in regard to their degree of vulnerability to human impacts. Being conducted in cooperation with the United Nations Environment Programme (UNEP), the results of this global assessment are two-fold, mainly to provide a baseline assessment of the status of these transboundary water systems, and a means of establishing priorities in regard to the most effective allocation of GEF funds within its International Waters portfolio.

The initial list of lakes identified for Lakes and Reservoirs component of the TWAP effort, totaling more than 600 000 lakes, was reduced to approximately 1 600, on the basis of national boundaries. A subsequent series of analysis, including a fine resolution GIS to identify　and separate transboundary lakes from the larger body of lakes, the use of Google Earth and related spatial analyses to facilitate confirmation of lake locations and surface areas, combined with visual inspection and the GEF-eligibility considerations, reduced the final number of transboundary lakes for analysis to 156. With the addition of 50 transboundary lakes located in developed countries for comparison purposes, the total number of transboundary lakes and reservoirs in the TWAP effort totaled 206.

To eliminate small lakes with sparse basin populations and/or lakes frozen over for significant portions of the year, a scenario analysis program was used to categorize the initial 206 transboundary lakes on the basis of areal (surface area>50 km2 ), population (density >5 persons/km2 ) and atmospheric temperature (mean >5 oC) criteria. The procedure reduced the number of priority transboundary lakes for analysis to a final list of 53 lakes considered to merit the most attention from the perspective of GEF goals. This list of 53 priority transboundary lakes comprised 23 African, 8 Asian, 9 European, 6 South American, and 7 North American lakes.