7922

• UCREFRP Catalog Number: 7922
• Author: Bovee, K. D.
• Title: A Guide To Stream Habitat Analysis Using The Instream Flow Incremental Methodology, Instream Flow Information Paper No. 12.
• USFW Year: 1982.
• USFW - Doc Type: \
• Copyright Material: NO

sources, they can be treated as dispersed point sources because their effects
can be reduced by water management.
Watershed disturbances that result in removal of vegetation or exposure
of easily weathered geologic formations may cause accelerated solution of a
variety of anions and cations. Bormann and Likens (1967) found that removal
of vegetation can upset the nitrogen cycle in forest ecosystems. The primary
result of such an imbalance is a preponderance of highly soluble nitrate salts
at the expense of ammonium salts. Following denudation of the watershed, the
net output of dissolved inorganic substances increased to nearly 15 times the
predisturbance yield, and the pH was reduced from 5.1 to 4.3 (Bormann and
Likens 1967). Manipulation of the water supply during base flow periods has
little effect on the water quality in streams draining such watersheds. The
only way that instream flow management could be effective in reducing this
type of impact would be to introduce water from an unaffected watershed. This
alternative is often feasible only at major confluences, where the affected
stream is essentially treated as a point source.
1.3.2 Step 2: Determine Flow Regime With and Without Project
It is almost a foregone conclusion that an application of the IFIM will
involve a change in flow regime. The principal exceptions are channelization
and stream rehabilitation projects. This step is illustrated in the top half
of Figure 3. The determination of the flow regime with and without the project
is frequently more complicated than it appears in Figure 3. Many of the
streams which will be analyzed will not be gaged, so hydrographs must be
synthesized. A project that alters the runoff pattern in the watershed will
change the flow regime in the stream. Hydrographs for these stream must be
synthesized from a runoff model. The most straightforward impact analyses
would probably be flow modifications caused by reservoirs and diversions.
However, project sponsors often do not know exactly what the operating
schedules will be, especially early in the development process. These
problems, and some of their solutions, are discussed in detail in Chapter 6.
The output from Step 2 is an anticipated flow regime with and without a
project. The project might simply be a recommended flow regime in the case of
an instream flow study, but the recommended flow will quite likely be different
from the existing flow regime. Note that the output from Step 2 is used in
each of of the next three steps: (1) to ensure that a change in flow regime
does not result in a change in channel structure; (2) to determine the effect
that altered flows might have on water quality; and (3) to determine the range
of flows over which microhabitat availability will be calculated.
1.3.3 Step 3: Determine Channel Structure With and Without Project
Channel structure is considered first at a macrohabitat level to determine
the longevity of the existing structure. This factor is separated from water-
shed considerations because not all channel disequilibria are caused by
watershed disturbances. Streams in unaltered watersheds may be in disequili-
brium due to manipulations of the stream itself. Three types of alterations
are notorious for creating channel changes: (1) construction of dams;
(2) channelization; and (3) diversion of peak flows.
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