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published in the 2002 State of the Nation’s Ecosystems Report as well as the 2003 and 2005 Web-Only Updates have been superseded by the 2008 Report and thus should be used with caution.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 analysis is based on changes between flow characteristics of a 20-year period beginning about 1930 and three 10-year periods (1970s, 1980s, and 1990s). All stream gauges used here had a 20- year record for the reference period and a 10-year record for the later comparison period. Some of these 20-year records began in 1930 and ended in 1949, while some began in 1931, 1932, and 1933, and ended correspondingly later. Twenty years was selected as a reasonable period that would allow characterization of hydrologic regimes, and 10 years as the minimum period to use to determine changes.

Data from the earlier period are being used here as a practical baseline for historical comparison, even though many dams and other waterworks had already been constructed by this time, and even though this period was characterized by low rainfall in some parts of the country. This decision means that it is more useful to focus on decade-to-decade changes in the number of streams with major changes in flow, rather than on the number or percentage of streams with such changes, compared to the baseline period.

The indicator assesses changes in magnitude and timing of low-flow events and high flows, extreme events that are ecologically important in riverine ecosystems. Four subindicators were included in the analysis:

• Average 7-day low flow (% change): assesses the degree of alteration in low-flow magnitude, a parameter of importance to aquatic life. Minimum flows determine habitat availability for aquatic organisms and can influence condition of riparian vegetation. Regulated streams are often required to maintain a minimum flow for aquatic life.
• Timing of the 7-day low flow (Julian day): describes how timing of low-flow conditions may have changed. A substantial change in seasonal timing of low flow can influence many ecological processes.
• Average 1-day high flow (% change): assesses the degree of alteration of the average annual peak flow. High flows are significant ecological and geomorphic events for streams and rivers, and a large change in the 1-day high flow is expected to have important ecological consequences.
• Timing of the 1-day high flow (Julian day): assesses the change in the timing of maximum annual high flow, an event of substantial ecological relevance. A substantial change in seasonal timing of peak flow can influence many ecological processes.

The Data

Data Source: Data reported here are from the U.S. Geological Survey (USGS) stream gauge network. USGS has placed stream gauges and maintained flow rate records throughout the United States since the end of the 19th century. These records are available on the Internet in the form of daily streamflow values reported as the average volume of water per second over a 24-hour period (http://waterdata.usgs.gov/nwis/discharge).

Data Collection Methodology: Stream gauging data are collected using standard USGS protocols.

Data Manipulation: Queries of the USGS Web site were used to identify 867 sites that had 20-year continuous records within 4 years of the target dates of 1930–1949 and 10-year continuous records for the decades of the 1970s, 1980s, and 1990s. The Data for these 867 sites were then put into a format compatible with the Indicators of Hydrologic Alteration (IHA) software package produced by The Nature Conservancy with Smythe Scientific Software (http://www.freshwaters.org/tools/), which was used to perform all subsequent analyses. The IHA software package compares the values for each subindicator (see list above) for the early 20-year period and the three later 10-year periods for each gauge. Each gauge is classified according to the degree of change of each of the four subindicators (see Table 4). Data analysis was conducted by David Raff, Department of Civil Engineering, Colorado State University, and N. LeRoy Poff, Department of Biology, Colorado State University.

Table 4. Values for Minimal, Moderate, and Large Change
	Minimal Change	Moderate Change	Large Change
Percent Change	<25%	25-75%	>75%
Time	<30 days	30-60 days	>60 days

Data Quality/Caveats: Although the sites analyzed here are spread widely throughout the United States, gauge placement by the USGS is not a random process. Gauges are generally placed on larger, perennial streams and rivers, and changes seen in these larger systems may differ from those seen in smaller streams and rivers. In addition, the USGS gauge network does not represent the full set of operating streamflow gauges in the United States. The U.S. Army Corps of Engineers, for example, operates gauges, and those data are not available through the USGS; they were not used in this analysis.

Data Access: Stream gauge data are available through the USGS Web site at http://waterdata.usgs.gov/nwis/discharge/. Analysis results are available through The Heinz Center.