Streamflow Alteration

Science Center Objects

Humans, just like aquatic organisms, need water.  Flood control, urban infrastructure, irrigation of agriculture, and myriad other ways we manage water affect the natural flow of streams and rivers.  How do the ways we manage land and water affect the natural patterns of streamflow that ecosystems depend on? 

Prehistoric fish weir

Prehistoric fish weir on Etowah River, Georgia.   (Credit: Alan Cressler, USGS.)

Civilizations sprang up where water was available. In modern times, humans have harnessed freshwater around the world for drinking, agriculture, industry, hydropower, and many other benefits. Human actions that can change streamflow patterns include dam building, stream diversion, pumping of shallow groundwater, and covering the landscape with impervious surfaces such as pavement and roofs.

Changes in climate also can alter flow by altering timing and amount of precipitation.  And in most areas, changes in streamflow pattern caused by climate are superimposed on streamflow modifications caused by land and water management.

Human activities and management of water, as well as climate, can affect many aspects of streamflow.  How high are the highest high flows, and how low are the lowest low flows?  When do those high flows and low flows occur, and how often? Changes to these naturally occurring patterns can have ecological consequences, causing species loss and the diversity of species present to change.

An assessment of streamflow at 2,888 sites across the United States found that natural streamflow patterns have been altered at more than 85 percent of the stream sites. There were marked differences in the occurrence, type, and severity of streamflow alteration depending on whether the climate was arid or wet.  The more that flow magnitude—how high the highest and lowest flows are—in a stream had diminished, the more likely that the ecological community of the stream was impaired.