Streamflow Alteration

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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 analysis of streamflow data for 1980–2014 compiled from more than 3,000 streamgaging sites across the U.S. 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. That's because aquatic species have evolved strategies to live with the streamflow regime that has existed for thousands of years. When that streamflow regime changes it can reduce their ability to survive and reproduce.

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.

A previous 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.

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.