Measures of the Health of Rivers and Streams - Hydro-Environmental Analysis: Freshwater Environments - page 190

7.4 HABITAT REQUIREMENTS

7.4.1 I nStreaM f Low I ncreMentaL M etHodoLoGy
The instream low incremental methodology (IFIM) (Stalnaker et al. 1995) is really a broad con-
ceptual and analytical framework for addressing streamlow management issues, consisting of ive
phases:

• Phase I—problem identiication

• Phase II—study planning

• Phase III—study implementation

• Phase IV—alternatives analysis

• Phase V—problem resolution

The overall incremental approach is illustrated in Figure 7.3.
One component of the IFIM that may be used if habitat is an issue is the physical habitat
simulation (PHABSIM) model previously discussed in Chapter 4. While the IFIM provides a
problem-solving outline, the PHABSIM is a modeling component of the IFIM, designed to cal-
culate the amount of microhabitat available for different life stages at different low levels. That
is, the method is based on an assessment of available habitats, as an indicator of stream integrity.
This could be considered a “if you build it they will come” method, in that it addresses habitats
and is not a direct measurement of water quality or biological integrity. This differs from methods
discussed in the following sections based on low (such as the RVA), or methods discussed later
based on biota.
The basic PHABSIM methodology is illustrated in Figure 7.4. First, using a combination of
surveys and hydraulic models, variations in habitat indicators, such as velocity, depth, and cover
(bottom type or substrate), are determined for lateral sections of a reach of river as a function of low
conditions. Each section is represented by an area. Habitat suitability indices or criteria are devel-
oped or obtained for each of these habitat indicators (e.g., depth, velocity, and cover) for the species

Negative
solution
achieved?

No

Ye s

Alternative

Start

Ye s

No

Institutional
analysis model

Stop

Formulate
alternatives

Microhabitat
model

Strategy
design

Te chnical
scoping

To tal habitat
model

Macrohabitat
model

Network
habitat model

FIGURE 7.3 Overview of incremental methodology. (Redrawn from USGS, Available at http://www.fort.
usgs.gov/Products/Software/iim/5phases.asp.)