Each population density map shows people per square kilometer by basin, as well as the location of the main large cities.

Each population density map shows people per square kilometer by basin, as well as the location of the main large cities. The maps were created from the Gridded Population of the World (GPW) database overlaid with basin boundaries (Center for International Earth Science Information Network, International Food Policy Research Institute, and World Resources Institute. 2000. Gridded Population of the World, Version 2. Palisades, New York: CIESIN and Columbia University. Available on-line at: http://sedac.ciesin.org/plue/gpw/). The GPW dataset was compiled from the latest available census data for over 120,000 administrative units worldwide, at a variety of subnational district levels. The population figures were standardized to 1995. The polygon data were converted to 2.5' grid with an assumption that the population is evenly distributed within a census unit. The average population density for each basin was calculated by clipping the population grid to each basin’s extent in an equal-area projection and then using a GIS to calculate the mean of all population density grid cell values in each basin.

Basin area was estimated by summing the number of 1 square kilometer cells within each basin using GIS. Basins were modeled based on elevation data, therefore these values only reflect horizontal extent (slopes are not accounted for) and may underestimate total land surface in the drainage area. Intermittent tributaries are included in most cases; for example the northern part of the Kalahari Desert in Bostwana is included in the Okavango basin, as well as many of the intermittent tributaries within the Lake Chad basin. Water surface of rivers and lakes (i.e., Great Lakes in St. Lawrence River basin) are included in the total basin area; however the tidal portions of rivers, such as the St. Lawrence and Ob, are not included in the drainage area calculation.

Number of large cities is from the World Cities dataset (Environmental Systems Research Institute, Inc. (ESRI), 2000. World Cities. Redlands, California, USA: ESRI). Large cities are defined as cities with greater than 100,000 people in an urban area. The World Cities dataset provides a base map layer of the cities for the world, as well as corresponding population categories. Only those cities in the population category greater than 100,000 were counted and displayed on the map. The base data used to create the World Cities database comes from Geographic and Global Issues Quarterly, Volume 3, Number 4 (United States Department of State, Bureau of Intelligence and Research, 1993/1994), The World Factbook (United States Central Intelligence Agency, 2000), Missions Database (Global Mapping International, 2000), and the World Port Index (U.S. National Imagery and Mapping Agency (NIMA), 2000).

Water supply per person (1995) indicates the average annual renewable water supply per person for individual river basins as of 1995. Estimates are in cubic meters per person per year. Water experts define areas where per capita water supply drops below 1,700 m3/year as experiencing “water stress”—a situation in which disruptive water shortages can frequently occur. In areas where annual water supplies drop below 1,000 m3 per person per year, the consequences can be more severe and lead to problems with food production and economic development unless the region is wealthy enough to apply new technologies for water use, conservation, or reuse. This map is based on the analysis carried out by WRI for the Pilot Analysis of Global Ecosystems: Freshwater Systems (PAGE) (Revenga, C., J. Brunner, N. Henninger, K. Kassem, and R. Payne. 2000. Pilot Analysis of Global Ecosystems: Freshwater Systems. Washington DC: World Resources Institute). These estimates were developed by combining a global population database for 1995 and a global runoff database developed by the University of New Hampshire and the WMO/Global Runoff Data Centre (Fekete et al. 1999). The runoff database combines observed discharge data from monitoring stations with a water balance model driven by climate variables such as temperature and precipitation combined with variables on land cover, and soil information. For those regions where discharged data were available, the modeled runoff was adjusted to match the observed values; for regions with no observed data, the modeled estimates of runoff were used.

Degree of river fragmentation is an indicator of the level of modification of a river system due to dams, reservoirs, canals, interbasin transfers and irrigation consumption. Irrigation consumption refers to the water that is evaporated or used by crops through transpiration, but excludes the amount of water returned to the river after irrigation. For the regions analyzed, rivers with a historical virgin mean annual discharge equal to or above 350 m3 per second were selected. River systems are classified into three levels of fragmentation: high, medium, and low. These categories are based on the number of dams in the main river channel and tributaries, the level of flow regulation, and the length of the main channel segment without dams in relation to the entire length of the river. Generally rivers with low fragmentation do not have dams in the main channel, and if present, dams on tributaries do not change the river’s discharge by more than 2 percent. Highly fragmented river systems include those with less than one quarter of their main channel left without dams, where the largest tributary has at least one dam, as well as rivers whose annual flow patterns have changed substantially. This indicator is from analyses carried out by Dynesius, M. and C. Nilsson. 1994. “Fragmentation and Flow Regulation of River Systems in the Northern Third of the World.” Science 266: 753–762 and Revenga, C., J. Brunner, N. Henninger, K. Kassem, and R. Payne. 2000. Pilot Analysis of Global Ecosystems: Freshwater Systems. Washington DC: World Resources Institute.

Number of dams in basin and in main stem of river. A large dam is defined by the industry as one higher than 15 meters high and a major dam as higher than 150 meters. There are more than 45,000 large dams in the world; however, information on their location is not readily available for most of the world. For the basin profiles in the United States, a complete dataset of 75,187 dams from the National Inventory of Dams (Army Corps of Engineers, 1995-96) was used. A subset of this dataset categorized by height of dam (approximately 5,055 dams), was used to calculate the number of dams (>15m high) and (>150m high) by basin for the United States basins. For all other basin profiles, as well as to identify all large and major dams in the main stem of a river for all profiles (including the United States) the World Register of Dams (1998) from the International Commission on Large Dams (ICOLD), and their 2002 update for Chinese dams were used. Because ICOLD’s on-line and CD-Rom register can only be queried by river name or by dam name, list of main tributaries for each main river were compiled by basin using several world atlases. Once the tributary names were identified, the register was queried to calculate how many large and major dams were in each basin. This same process was used to query the database for dams located on the main stem of rivers. Some countries do not report their dams to ICOLD, others, like Russia, only report a fraction of their dams or do not include the river name in the database. Finally, many rivers in China, Brazil, and other countries have identical names; therefore it was hard to identify all dams in river basins. In China, for example, only the main tributaries of major rivers were queried for number of dams. Because of these data limitations, these figures should be interpreted with caution and are probably underestimate the number of dams in each basin and rivers.

Number of dams (>60m high) under construction includes dams at least 60 meters high that were under construction in 1998 and reported in the “1998 World Atlas and Industry Guide” of the International Journal of Hydropower and Dams. The approximate location of the dams was referenced based on continental-scale maps. The number of dams was later aggregated by river basin.