Instream flow methods have been developed by biologists and hydrologists working for agencies having regulatory responsibility related to water development. Such efforts since the late 1960's have provided the impetus for ecological studies leading to a growth in the understanding of the relations between stream flow and aquatic habitats. Most of the evidence gathered to date has focused on fish and macro-invertebrate habitat requirements, with recent emphasis on the relation between stream flow and woody riparian vegetation and recreation. Water management problem solving has matured from setting fixed minimum flows with no linkage to a specific aquatic habitat benefit to incremental methods in which aquatic habitats are quantified as a function of discharge.
Following the reservoir and water development era of the mid-twentieth century, resource agencies became concerned over the loss of fisheries in the west. Consequently, several states began issuing rules for protecting existing stream resources from future depletions. Many assessment methods appeared during the 60's and early 70's based on hydrologic analysis of the water supply coupled with empirical observations of habitat quality and an understanding of riverine fish ecology. These efforts led to a class of instream flow techniques meant to help reserve water within the channel for the benefit of fish and other aquatic life. Application of these methods usually resulted in a single 'minimum' flow value for a stream reach, below which water may not be withdrawn for consumptive use. The minimum flow is almost always less than the optimal habitat condition. These 'reservations' of water form the basis for issuing water permits in many states.
Following enactment of the National Environmental Policy Act (NEPA) in 1970, attention shifted from minimum flows to evaluation of federally funded water projects. Methods capable of quantifying the effect of incremental changes in streamflow to evaluate alternative development schemes were needed. This led to the development of habitat versus discharge functions developed from life-stage-specific relations for selected species, that is, fish passage, spawning, and rearing habitat versus flow.
Research took the form of analyses correlating the well-being of fish populations with physical and chemical attributes of the flow regime. A set of these variables was shown to contribute significantly to the variation in fish production. These were
- water velocity
- minimal water depths
- instream objects such as cover
- bottom substrate materials
- water temperature
- dissolved oxygen
- total alkalinity
- turbidity
- light penetration through the water column
These elements were integrated into methodologies for analyzing proposed water withdrawal or storage-release activities and were applied to federal water projects.
During the late 1970's and early 1980's, small hydropower development began. Hydropower sites came under intensive examination by state and federal fishery management interests. During this transition from evaluating large federal reservoirs to evaluating small hydropower, the Instream Flow Incremental Methodology (IFIM) was developed under the guidance of the U.S. Fish and Wildlife Service. This methodology attempted to integrate the planning concepts of water supply, analytical models from hydraulic and water quality engineering, and empirically derived habitat versus flow functions. This methodology produced simulations of the quantity and quality of 'potential habitat' resulting from proposed water development. Such efforts involving incremental methods and analyses of alternatives through time were enhanced during the next 10 years, driven by relicensing applications submitted to the Federal Energy Regulatory Commission.
Opportunities were seen by the natural resource agencies to restore riverine aquatic resources that had long been impacted. Simultaneously however, hydropower companies wanted to shift their operations toward hydropeaking and pump-storage to enhance the revenues. Many peaking projects operated by private power companies or utilities were accessible for recreational use. The recreational interests seized on the relicensing opportunity to enhance river recreational use. NEPA guidelines for examining alternatives forced decision makers to balance conflicts among users of the river. Incremental methods became the tools of choice for describing alternative ways of managing flowing waters, setting the stage for negotiation and better informing the decisionmakers for conflict resolution.
As the multiple-use ethic emerged, it became clear that allocating part of the water supply to various uses is not sufficient to resolve conflicts. The same water can be used many times if it is managed so that the timing of release serves instream purposes while still being delivered to downstream consumptive users. This multiple-use management philosophy allowed fishery biologists to identify management prescriptions for restoration and enhancement of salmon runs in the Columbia River basin. A minimum flow did not provide sufficient protection for stream resources during droughts, nor did it provide the opportunity for optimal fish production during wet years. Water budgets allocating a portion of water stored in upstream reservoirs for fishery benefits reserve flows that could be released when needed. When downstream water users are not calling for delivery through critical spawning or rearing reaches, the 'fish water' can be released to relieve any habitat constraints.
The shift from minimum flow constraints to a water budget changed the role of the fisheries manager to include the roles of water and habitat managers. Thus, resource agencies need to acquire a more interdisciplinary mix of expertise. These agencies must be prepared to decide the deliveries of water daily during particular seasons and to decide what portion of the river basin fishery resource will be favored during droughts. By adopting water budgets, a mutual 'sharing' of storage in federal water projects across all user groups during droughts is facilitated. Under the traditional diversionary allocation philosophy of 'first-in-time-first-in-right' practiced under Western water appropriation doctrine, the fishery may have first or last priority during a water-short year depending on the circumstances. By gaining a 'seat at the management table' the instream-fishery interests get involved in decisions regarding water stored during high-flow periods and its release when the most critical conditions occur downstream. Sharing the storage allows for delivery to relieve critical conditions.
IFIM unfolded against the backdrop of minimum flow standards, quantitative impact analyses, water budgets, and interdisciplinary analyses. IFIM was developed by an interdisciplinary team approach and was founded on a basic understanding and description of the water supply and habitats within stream reaches of concern. Historical analysis of the flow regime using a monthly, weekly, or other appropriate timestep to describe the baseline hydrologic conditions was considered essential because this type of analysis was normal practice within the water resource profession. Looking at streamflow through time allows one to compare the frequency and duration of wet and dry periods, to examine the difference between snow-melt and rain-driven systems, and to determine the intensity and duration of short-term events such as cloud bursts and peaking cycles. To influence operating decisions within large-scale water development settings, a tool was needed that illuminated conflicts and complementary water uses, considered and evaluated each user's needs, and was understandable, acceptable, and easy to use by a broad clientele. Such decision arenas involve a diversity of disciplines, including engineers, hydrologists, biologists, recreation planners, lawyers, and political scientists.
This interdisciplinary effort led to the conclusion that an analytical methodology should handle a variety of instream flow problems, from simple diversions to complex storage and release schemes involving hydropeaking schedules, and a network of interconnected reservoirs. For such a methodology to be suitable for evaluating alternatives, it had to be useful in identifying, evaluating, and comparing potential solutions, be capable of being tailored to a specific stream reach, and be expandable such that reach information could be applied throughout a river basin. With this general charter, IFIM was developed over a period of 15 years into a river network and decision arena analysis that incorporates fish habitat, recreational opportunity, and woody vegetation response to alternative water management schemes. Information is presented as a time series of flow and habitat at selected points within a river system for various existing and proposed water system operation alternatives.
U.S. Department of the Interior | U.S. Geological Survey
URL: http://www.fort.usgs.gov/Products/Software/ifim/history.asp
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