What Are Environmental (Instream) Flows?

Transcription

1 1 What Are Environmental (Instream) Flows? Sustainable water management requires that both human needs and the needs of aquatic and riparian ecosystems be fulfilled. Dams and diversion of water for municipal and irrigation water supplies can drastically affect components of the natural flow regime of a stream by reducing the frequency and magnitude of peak flows, decreasing average low flows, changing the seasonality of flows and reducing or increasing the rate of rise and fall of flows. Concerns over the loss of fish due to the diversion of water from streams (Figure 1) in the western United States in the 1970 s gave rise to the concept of leaving a minimum amount of water in a stream to support fish during low flow periods, called a minimum instream flow (Postel and Richter, 2003). Today environmental flow prescriptions can go beyond simply leaving minimum flow in a stream or a minimum pool level in a lake, although these two items may be important goals of an overall water plan. Environmental flows are intended to prevent other water uses from impairing ecological functions of streams and surrounding riparian and terrestrial ecosystems that are dependent on water. Figure 1: Streamflow in a channel above (left) and below (right) a water diversion on a small stream in northern Colorado. Components of a stream s flow regime that support ecological functions Streamflow is complex and highly variable in streams, increasing and decreasing in response to short-term and seasonal patterns in precipitation and evapotranspiration in watersheds (Figure 2).

2 2 Figure 2: The pattern of mean daily streamflows for the Glover River, a 315 mi 2 basin in McCurtain County, OK. The Glover River is a free-flowing river with infrequent high flows and a large number of low or no flow days that are typical of the flow regime in this stream. Streams in Oklahoma that are fed by springs would exhibit higher low (or baseflows) flows. (Data from: USGS National Water Information System accessed July 22, 2011 at: Poff, et al. (1997) identified five components of a stream s flow regimen that are important for supporting ecologic functions: streamflow magnitude, frequency, duration, timing (daily and seasonal) and rates of change (how rapidly flow increases and decreases during storms). Richter, et al. (1996) created one method (of which there are many) for quantifying the flow regime of a stream for determining the degree of hydrologic alteration in streams based on the five components of flow identified by Poff, et al. (1997). The five groups of flow components and the ecological functions they support are: 1. Monthly Magnitudes: The magnitudes of mean monthly flows describe the seasonality of streamflow and determine the amount of habitat available for aquatic organisms in a stream as well as water supply for terrestrial organisms.

3 3 2. Magnitude and Duration of Annual Extreme Water Conditions: Flow magnitude may include quantifiable parameters such as the 1, 3,7,30 or 90 day mean maxima and minima and the number of zero flow days. Daily minima can determine the duration of stressful conditions such as low oxygen in a stream. Daily maxima can control the structure and availability of physical habitat in a stream (Figure 3). Figure 3: A typical stream cross-section showing a range of flow stages, an ecological function the different flows provide and the frequency at which the flows typically occur. 3. Timing and Seasonality of Annual Extremes: The life cycles of aquatic organisms in a stream are adapted to the seasonality of flow conditions in a stream. Seasonal high flows can trigger spawning and must occur at about the same time each year. Additionally, the development of some types of eggs and transport of larval fishes to appropriate rearing habits must occur in this period. Seasonal flow pulses help to flush (Figure 3) particulate organic matter and nutrients through streams. 4. Frequency and Duration: The frequency and duration of both low and high flow periods are descriptors of the variability of annual streamflow. High flow pulses (The highest quartile of mean daily flows) include floods that occur on average once every 5 or more years that provide water to riparian areas and wetlands (Figure 3). Elimination of floods and the stream s interaction with the flood plain can cause riparian vegetation to change, wetlands to dry up and reduce water in alluvial aquifers. High flow pulses are required for channel maintenance and move sediment that builds physical habitat through streams. High flows that occur on average once every 1-2 years transport the majority of sediment through time that forms aquatic habitat. Low flow pulses (the lowest quartile of mean daily flows) are a quantitative measure of conditions when anaerobic stress and water stress for plants may exist. Dams and water diversions can drastically alter natural patterns of high (Figure 4) and low flow pulses.

4 4 5. Rate and Frequency of Change: The rate at which streamflow increases and decreases during storms (events) can affect the ability of organisms to escape inundation if rates or rise are increased, or retreat back to water if rates of decline are increased. The rate of change of flow and the frequency at which events occur can be increased by watershed activities such as urbanization, severe fire or releases from dams for power generation. Figure 4: Annual peak flows on the Mountain Fork River at Eagletown before ( ) and after (1968) the construction of Beaver s Bend Dam. Construction on the dam started in 1961 and final impoundment began in October The graph shows a typical elimination of annual maximum peak flows (high flow pulse) that occur after a dam built for water storage is installed. Before construction of the dam, the river experienced peak flows of 20,000 ft 3 /second (cfs) or greater at least once in 28 out of 30 years. After the dam was operational (1970) no years had a flow >20,000 cfs. (Data from: USGS National Water Information System accessed July 22, 2011 at: ) Environmental Flow Recommendations Current approaches to environmental flows seek to balance human and social needs with those of aquatic and terrestrial ecosystems in a framework that mimics the natural flow regime. Environmental flow recommendations may be developed individually for a stream, a reach of a stream or tributaries. Recommendations may consider allocation of water for maintaining riparian vegetation and wetlands, aquatic biota and their habitats, fresh and saltwater balance in estuaries, sediment transport and channel maintenance, maintaining water quality, and groundwater recharge (Figure 3). Recommendations may be species specific, or apply to many species. Recommendations may seek to restore a semi-natural flow regime, or just parts. For

5 5 some rivers, restoring spring floods would be impracticable because flood plains below dams are developed and the flooding would cause millions of dollars of property damage. In wildland settings, however, restoring floods may be beneficial. Examples of environmental flow recommendations that have been made and implemented on streams in the United States are numerous (Annear, et al. 2004). One of the best known environmental flow recommendations was carried out on the Colorado River below the Glen Canyon Dam. The dam blocks sediment from the Colorado River downstream. As a result, sand and gravel bars in the Grand Canyon below the dam eroded away after the dam was built, robbing the river of valuable fish habitat and recreation sites. A series of flushing flow releases from the dam resulted in an expansion of sand and gravel bars (Collier et al. 1997). Rehabilitation of more natural monthly flows in the spring along with a reduction in the rate of change from low to high flows during hydropower cycles was prescribed to increase the reproduction of striped bass and restore bottomland forest communities in the Roanoke River in Virginia. In the Rio Grande River in New Mexico, the natural timing and duration of floodplain inundation was mimicked by water releases to maintain riparian ecosystem processes such as nutrient fluxes and vegetative litter decomposition (Annear et al. 2004). Additional examples are presented in Annear et al. (2004). In all cases, however, the environmental flow prescription addressed the needs of a specific species or ecological function. In many instances, minimum flows can ensure resources are available for aquatic and riparian ecosystems, municipalities and agricultural needs in the future. In several areas of the United States, extreme water usage has led to increased salinity levels in streams leading to the inability of farmers to water their fields without killing their crops. Portions of California have experienced loss of riparian vegetation due to reduced water availability and high salinity, resulting in a loss of arable land from stream bank erosion. Potential Effects of Environmental Flow Allocations on Other Uses in Oklahoma It is difficult to say whether or not allocating water for environmental flows will reduce the quantity of water available for municipal supply, irrigation or other uses. Environmental flows are specific to each site, stream conditions and goals as well as the annual variation in streamflow. In some cases water availability may be reduced, in others not. For example, regulating the rate of change from low to high flows during hydropower generation would use water already allocated for that purpose. Creating a flushing flow below a dam would likely occur in the spring when natural high flow pulses occur and water is normally available to replenish reservoir storage. The greatest conflicts will likely occur during dry years, when reservoir storage is low, irrigation and municipal and irrigation demand is high and a minimum baseflow is prescribed. Finding a balance between social, economic and ecologic needs and sustainability will benefit all water users.

6 6 For additional information about environmental flows, consult the following references: Instream Flow Council: The Nature Conservancy: References Annear, T., I. Chisholm, H. Beecher, A. Locke and 12 other authors Instream flows for riverine resource stewardship, revised edition. Instream Flow Council, Cheyenne, WY. 268 pp. Collier, M.P., R.H. Webb and E.D. Andrews Experimental flooding in the Grand Canyon. Scientific American 276: Poff, N.L., D. Allan, M.B. Bain, J.R. Karr, K.L. Prestegaard, B.D. Richter, R.E. Sparks and J.C. Stromberg The natural flow regime. A paradigm for river conservation and restoration. BioSciance (47)11: Postel, S. and B. D. Richter Rivers for life: Managing water for people and nature. Island Press, Washington, D.C. 253pp. Richter, B.D., J.F. Baumgartner, J. Powell and D. P. Braun A method for assessing hydrologic alteration within ecosystems. Conservation Biology 10(4):