Stream Hydrology Program

Division of Ecological and Water Resources

Instream flow methodology

A Description of the Instream Flow Incremental Methodology

by K.D. Bovee
from "A Guide to Stream Habitat Analysis Using the Instream Flow Incremental Methodology."
Instream Flow Information Paper 12
U.S. Fish and Wildlife Service. FWS/OBS-82-26, 1982

The Instream Flow Incremental Methodology (IFIM) adheres to the principle of incrementalism. In one sense, the IFIM can be thought of as a collection of computer models and analytical procedures designed to predict changes in fish habitat due to increments of flow change. Indeed, this methodology does enable such predictions. It can also be used to evaluate such diverse impacts as changes in channel structure or alterations in waste loading from a pollution source. In fact, it can be used to translate changes in land use to changes in the stream environment, if the user follows it that far. However, the IFIM is much more than a collection of computer models. It is, in fact, a thought process that begins with the structuring of the study design and carries through to the final negotiation of a problem solution. Several of the underlying principles of the methodology are presented and discussed below.

The first, and probably most important, principle is that implementation of an instream flow regime is inseparable from water management. Therefore, the IFIM should be thought of as a water management tool. It is not intended to be an ecosystem model. However, it is designed to have environmental and ecological applications. The IFIM will not ensure against ecological blunders, as is true with other decision systems, including ecosystem models. Ecological blunders can be prevented only insofar as we are able to foresee the consequences of our actions and adapt our management accordingly. The IFIM does allow a systematic evaluation of different management options, providing quantitative estimates of fish habitat are available under each option.

The second principle is that the method is not intended to generate a single solution, but to predict the impacts of different alternatives. Users seeking a mechanistic solution to a problem may find this methodology difficult to understand. The methodology has been specifically designed to provide multiple solutions. Therefore, the user must embrace the philosophy of incrementalism and iterative problem solving before the methodology can be used to its full advantage. Following this philosophy, the methodology best lends itself to a systems approach. Such an approach opens a wider variety of options and water management alternatives to an application of the methodology.

The third principle is that the objectives of any application must be rigorously defined. It is quite possible for two identical applications of the methodology to result in vastly different solutions, due solely to the objectives of the analysts. For example, two groups may have as their objective, "the design of a flow regime to maintain a fishery at a minimally acceptable level." To one group, this really means "to maximize fish habitat within the constraints of the available water supply." To the other group, the same objective means, "to maximize out-of-channel water use without eliminating the fishery."