Kyle Sandera

Throughout the world, river water is commonly diverted and used by humans for various purposes. This has traditionally been viewed as an engineering task; take water from the river and supply it to the nearby communities and farmland. However, it has recently become clear that these anthropogenic impacts have become a visible environmental issue that require the input of many disciplines. It is now known that this precarious balancing act involves many complex issues that require a deeply involved study to determine the viability of the project before it is implemented.

River alignments support a diverse array of organisms that depend on the water and nutrients supplied by the river for survival. Changes to the flow regime must be considered from a broad array of disciplines, since the changes affect not only the hydraulics, but also the aquatic and terrestrial biota as well. Many tools have been developed to determine the point at which hydrologic alteration adversely affects the ecology of a riverine system. Most of these tools are considered inadequate because they focus on specific portions of the ecosystem, such as aquatic biology or riparian functionality, instead of the system as a whole.

Another limitation of these tools involves the fundamental differences in language and analytical methods that exist between hydrologists and ecologists or biologists (Swanson, 2002). There is a growing need to predict the biological impacts (or recovery) associated with water management activities, and to set water management targets that maintain riverine biota and socially valuable goods and services associated with riverine ecosystems. This need has spawned what amounts to a new scientific discipline of "instream flow" modeling and design.

According to Richter et al. (1997), there exist numerous methods for setting streamflow-based river management targets, none of which sufficiently addresses the full natural range of variability in hydrologic regimes. Many instream flow models or methodologies are extremely simplistic, such as the "Montana Method," wherein environmental flow regimes are prescribed on the basis of the average daily discharge or the mean annual flow (MAF). In general, 10% of the MAF is recommended as a minimum instantaneous flow to enable most aquatic life to survive; 30% MAF is recommended to sustain good habitat; 60-100% MAF provides excellent habitat; and 200% MAF is recommended for "flushing flows." Such approaches have obvious shortcomings, the most serious being the elimination of ecologically important flow extremes and a lack of attention to flow timing.

The Indicator of Hydrologic Alteration (IHA) is a fairly new tool that is supposed to bridge the gap between disciplines and view the system as whole. The IHA is a software program that provides information to aid in the understanding of the hydrologic impacts of human activities and to develop environmental flow recommendations. The program was developed by scientists at the Nature Conservancy to facilitate hydrologic analysis in an ecologically-meaningful manner (, 2008). The IHA uses the Range of Variability Approach (RVA) that begins with a comprehensive characterization of ecologically-relevant attributes of a flow regime and then translates these attributes into more simple, flow-based management targets (Richter et al., 1997).

Another method uses the Flow Duration Curve (FDC) and Annual Flow Duration Curve (AFDC) to determine an appropriate flow regime to maintain ecological integrity. This method is based on ecological conditions over the entire flow regime. The FDC/AFDC method can estimate allowable river withdrawal based on ecological control points representing riverine ecology. The result is a tool that allows users to identify faults in proposed water allocation schemes and enhance sound multi-discipline decision-making (Ripo, 2003).

As stated above, many tools exist to help determine the minimum flow rate to maintain ecological integrity. The more comprehensive tools are the IHA and the FDC/AFDC methods that take into account several indicators related to multiple disciplines. While these two methods are currently the preferred tools used in determining minimum flow requirements, each site requires its own comprehensive analysis. The IHA and FDC/AFDC tools should be used to determine a rule of thumb flow alteration for a site. When the project is critical or loss of a certain habitat would be severe, a detailed study must be undertaken.


"Indicators of Hydrologic Alteration (IHA): Software for Understanding Hydrologic Changes in Ecologically-Relevant Terms." October 5, 2008.

Richter, B. D. et. al. 1997. How Much Water Does a River Need? Freshwater Biology, Volume 37, No.1, pp. 231-249.

Ripo, Charles G. 2003. Framework to Assess Streamflow Withdrawal Availability and Impacts of Ecological Conditions. University of Florida.

Swanson, S., 2002. Indicators of Hydrologic Alteration. Resource Notes No. 58, September 09, Bureau of Land Management.