Note that the data published in the 2002 State of the Nation’s Ecosystems Report as well as the 2003 and 2005 Web-Only Updates have been superseded by the 2008 Report and thus should be used with caution. For the most recent data, purchase the 2008 Report from Island Press.

The Indicator

This indicator would report the percentage of each of the major freshwater ecosystems (rivers and streams, riparian areas, wetlands, and lakes, ponds, and reservoirs) that are altered. “Altered” is defined differently for each of the following:

• Rivers and streams (all flowing surface waters) are altered if they are leveed, channelized, or impounded behind a dam. There are other types of alterations to streams that may be important; these include changes in sedimentation and temperature, and barriers to movement between stream reaches. Such changes can be caused by dams or other alterations to the river or its surroundings. As monitoring and reporting technology and understanding evolve, it may be possible to report on these and other alterations. At present, identifying such changes requires detailed sitespecific analyses, which have not been done on a widespread basis (see also The Heinz Center 2002). Both the stream habitat quality and changing stream flows indicators provide important complementary information on stream conditions.
• Riparian areas along rivers and streams are considered altered if they have a predominance of urban or agricultural land use.
• Lakes and ponds are considered altered if the area immediately adjacent to the shoreline has land cover that is predominantly urban or agricultural. Since there is no agreedupon proportion of shoreline that must be in these land use categories in order for individual lakes to be classified as “altered,” this indicator reports the overall percentage of lake shoreline in agricultural or urban use. This indicator focuses on “natural” waterbodies, that is, those that are not created by impoundment behind a dam. While reservoirs provide habitat, the prevalence of large and frequent fluctuations and associated poor development of the riparian/ littoral zone reduces this value. In this case, the number or percentage of natural lakes whose waterflow has been altered by damming would also be reported. Some impounded lakes are not subject to such fluctuations, but until it is possible to distinguish between different impoundment types, this indicator will be limited to natural waterbodies.
• Wetlands are considered altered if they are excavated, impounded, diked, partially drained, or farmed. These categories are used by the U.S. Fish and Wildlife Service’s National Wetlands Inventory; they are defined in Cowardin et al. (1979). Wetlands fragmentation (subdivision into smaller and more isolated patches by filling, roads, or other alterations) is also important, but measurement of this change requires detailed site-specific information.

The Data

The methods used to produce the data reported here for altered riparian areas are described in the technical note for the Extent of Freshwater Ecosystems, which immediately precedes this one. The extent indicator describes methods used to characterize riparian areas; the same method could be used to classify the shorelines of ponds and lakes, but the relevant database does not distinguish between natural and impounded lakes/reservoirs.

The Data Gap

There is no nationally aggregated database of the number of impounded river miles or the number of leveed river miles. There is also no method for calculating the extent of downstream effects of dams, other than by conducting site-specific investigations for each dam.

No nationally aggregated database distinguishes impounded waterbodies from natural ones, or identifies which natural lakes are dammed at their outlets. It is possible that existing databases on dam locations, such as those maintained by the U.S. Army Corps of Engineers, could be merged with other datasets, such as the National Hydrography Dataset, to derive this information.

Data on altered wetlands are available through the U.S. Fish and Wildlife Service’s National Wetlands Inventory (see http://www.nwi.fws.gov/). At present, these data are not available in electronic form for the entire United States. Further, these data are available only on a quad-sheet-by-quad-sheet basis. The Fish and Wildlife Service is in the process of integrating these data more fully, and it is likely that they will be available in the near future. However, they will be from different time periods in different states, and there is no plan for periodic updating. In addition, there are no plans to produce regional or national reports comparing any updates with past data.

References

Cowardin, L.M., V. Carter, F.C. Golet, and E.T. LaRoe. 1979. Classification of wetlands and deepwater habitats of the United States, FW/OBS-79/31. Washington, DC: U.S. Fish and Wildlife Service.

The H. John Heinz III Center for Science, Economics and the Environment. 2002. Dam removal: Science and decision making. Washington, DC: The Heinz Center. http://www.heinzctr.org/publications.htm.