Threats - Altered Stream Flows

Definition:  Streamflow has been described as the "master variable" that limits the distribution and abundance of riverine fish species.  Naturally, although streamflow exhibits variation on many time scales (hourly, daily, seasonally, annually, and longer), flows in a given river typically follow a general flow pattern.  This characteristic pattern of the magnitude, timing, and variability of streamflow defines a river's flow regime.  Alternation of any facet of this natural flow regime can result in significant ecological consequences for the river in question.

Actual Ecosystem Stress:  Streamflow quantity and timing are critical components that help to maintain the ecological integrity of rivers and streams.  As the "master variable", the flow regime in a given stream defines the physical, chemical, and biological dynamics of that ecosystem, and so changes in the natural flow regime can lead to ecological responses.  For example, the elimination or reduction of seasonal peak flows can cause significant life-cycle disruptions for fish, can eliminate wetland and backwater habitats, and can lead to an invasion of exotic riparian species.  Further, a prolonged low flow condition can cause stream water temperatures to climb above lethal tolerance thresholds for resident fish populations, can lead to the spread of disease through the concentration of aquatic organisms into reduced habitats, and can facilitate a shift in species diversity through the creation of an artificially constant environment.  While many ecological stressors can result from an human altered flow regime, the ecological response is dependent on how the specific alteration changes the natural regime for that particular stream or river.        

Sources:  The alteration of natural flow regimes by humans occurs in many different ways. Dams and diversions are the most conspicuous disrupters of the natural flow regime.  People regulate river flows with dams and diversions to meet diverse needs, including water supply, power generation, flood control, navigation, and recreation.  By their ability to store large volumes of water in the reservoirs that they create, dams often reduce the magnitude and frequency of high flows, which are important in maintaining the physical, chemical, and biological characteristics of a river or stream.  In many cases, particularly in the Western US, water is often removed from its basin of origin to meet demand for waters in another drainage, resulting in a permanent dewatering of some flow from the original stream.  Hydroelectric dams can create significant flow problems for downstream riverine ecosystems by rapidly "ramping" flows to meet peak power demands.  Another major source of alteration to natural flow regimes is caused by significant changes in land-use resulting in an increase in the impermeability of a drainage.  An increase in paved surface area can cause runoff to occur quicker, leading to a "flashier" receiving stream.

These hydrographs show how dam operations can impact stream flows.
Left: An upstream diversion ditch has depleted stream flow so that much of the streambed is dry. Fish and aquatic macroinvertebrates suffer from reduced habitat, increased water temperature or stranding. Credit: Andrew Todd
Right: A river in Connecticut reduced to puddles from excessive groundwater pumping. Credit: Kirt Mayland

Measures to stop or mitigate threat:  The primary way to mitigate the threat of altered stream flow regimes is to try to mimic, as well as possible, the natural flow regime of that stream.  In the case of dams and water diversions, water providers may be able to time the diversion of water or the release of water to coincide with natural patterns.  Additionally, ramping rates at hydropower facilities may be flexible, leading the way to the operation of the dam in a way that is less damaging to the downstream ecosystem.  Another potential method to preserve stream ecosystem in flow altered systems is to attempt to preserve some of the natural streamflow as "instream flows".  Unfortunately, the "protection" of instream flows historically has emphasized only one aspect of water quantity: minimum stream flow.  To maintain the integrity of stream ecosystems, this type of "protection" must be expanded to account for timing, magnitude, and duration components of the natural flow regime.

Links:
Water Encyclopedia Definition
TU's Western Water Project
EPA - How much water does a river need?
Instream Flow Council
TNC software for flow analysis

Recommended Reading:

The Natural Flow Regime: A paradigm for river conservation and restoration. N. Leroy Poff et. al. (1997) Bioscience 47(11): pg 769-784.

Applied River Morphology, 1996.  David Rosgen.  Pagosa Springs, Colorado: Wildland Hydrology.

A Dry Legacy: The Challenge for Colorado's Rivers.  Trout Unlimited Colorado Water Project (2002).