Modification of Streamflow Across the U.S.

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Human activities have caused flows in many of the Nation’s streams and rivers to be different from what they would be naturally. A new USGS study reports that, at a national scale, human management of land and water resources have modified natural patterns of streamflow along an estimated 1.2 million stream miles—more than one-third—of the Nation’s streams and rivers.

Some management actions, such as flood control, water storage, and water transfers, directly affect streamflow and in many cases modify natural streamflows in predictable ways. Other actions, such as groundwater pumping and the creation of impermeable surfaces in the watershed, indirectly and unintentionally affect streamflow. 

USGS scientists used machine-learning statistical models to analyze streamflow data for 1980–2014 compiled from more than 3,000 streamgaging sites across the United States. They found that low flows now are more frequent but occur for a shorter duration than they would naturally. High flows also are more frequent, but are lower in magnitude and shorter in duration than they would be naturally. Such changes have contributed to the deterioration of stream and river ecosystems.

Climate also has induced changes in streamflow, as assessed at 600 streamgaging sites where influences from land and water management are minimal. Climate factors that affect streamflows include air temperature, whether precipitation falls as rain or snow, and the timing of precipitation and snowmelt.  However, human management of land and water resources over the last 3 decades have modified streamflows more than variation in climate has over the same period.

In every region assessed, streamflow modification was associated with losses of native fish or invertebrates, such as larval mayflies and caddis flies. “Aquatic species have evolved strategies to live with the streamflow regime that has existed for thousands of years,” explains lead author Dr. Daren Carlisle. “When that streamflow regime changes it can reduce their ability to survive and reproduce.”

Damage to ecosystems from streamflow modification has been successfully mitigated in some cases. Efforts to address streamflow modification can focus on a key streamflow characteristic, such as low-flow magnitude, or can address the entire flow regime—streamflow magnitude, variability, duration, frequency, and timing. The choice depends on specific ecological needs and the flexibility of other goals, such as water supply or energy production.  All of these approaches require the scientific analysis and long-term monitoring of ecological and hydrological systems.

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