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published in the 2002 State of the Nation’s Ecosystems Report as well as the 2003 and 2005 Web-Only Updates have been superseded by the 2008 Report and thus should be used with caution.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.

View Data for Alteration of Key Flow Characteristics View Data for Major Changes in Low Flow View Data for Major Changes in High Flow

What Is This Measure, and Why Is It Important? This indicator describes changes in the amount and timing of river and stream flow by reporting the percentage of monitored streams or rivers with major, moderate, and minimal changes in low flow, high flow, and the timing of these two extreme events. The indicator also describes the nature of major flow changes. Flow characteristics were measured for three recent 10-year periods and compared against 1930–1949 as a reference period.

How a stream flows––the volume of its high and low flows, and when these extreme flows occur—is critical in determining what plants and animals live in the stream or river. For example, low flows define the smallest area the stream or rivers will occupy and thus the amount of fish habitat that will be available year-round; high flows shape the river channel and clear silt and debris; and some species require certain flows at specific periods, such as spawning season.

Changes in flow can be caused by dams; by pumping water for drinking, irrigation, or other uses; by groundwater pumping (which reduces flows into the stream); by changes in the type and amount of development and other land cover in the watershed; or by changes in long-term weather patterns, such as droughts or wet periods.

What Do the Data Show? The percentage of streams or rivers with major changes in the size of their highest or lowest flow, or in the timing of these flows, increased slightly from the 1970s to the 1990s. In addition, the number of streams or rivers whose high flows were well above those in the 1930–1949 reference period rose markedly from the 1980s to the 1990s.

The reference period used here included periods of relatively low rainfall, but it also predated much development activity (dam building, irrigation, etc.) that might affect flows. Therefore, it is more useful to focus this indicator on increases or decreases in the number of streams or rivers with major changes in flow, rather than on the actual number of streams or rivers with such changes. Finally, it is not possible to use these data to identify the cause of flow changes.