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... ------..->".>.~c-'h,~""'".".C<o.,..,',..'>.,d;'".=;'''''<C.,','"" '""'."","~'>""-"'f'-n/'''''';'''''''''''''V''H''fL \1W'!'!._tla'i"'~. ...Vll. .IIIIR~ < - - ..~, "", --'~""""",-","""" -U_..-......~Wti'j.'iI-~~;Wi;p~j_~~,f'"il;i~~~J.;;.-i-$cF'Ji*-~~ - _ --;;) '--~''" . ",' . . .. methodology does not consider chemical or water quality changes. 10. What important assumptions are part of the IFG incremental methodology? As with any analytical approach using the scientific method, certain assumptions are made which define those conditions under which the approach is valid. The assumptions in the IFG incremental methodology must be explicit, so that the limitations of the methodology can be recognized. All present methods assume that if habitat is maintained at some given level, the fish population will be maintained. The IFG incremental methodology is based on the following primary assumptions: A. The physical variable of depth, velocity and temperature as associated with substrate are assumed to be the most important fish habitat variables when considering changes in stream flow regimes or channel geometry. Water chemistry, light and other variables known to be extremely important in determining the level of fish production, standing crop, and yields ar:~ qssl1p1eci not to change significantly, and are thus considered as constants in the instream flow analysis. Of course, there are many cases in which significant changes in water chemistry may occur with a proposed water development. In such cases, conventional water quality models must be used in addition to the IFG incremental method to adequately display the environmental impacts. B. The structure of the stream channel is not altered by changes in flow regime. Of course, periodic flooding as well as long periods of low flow will mod ify -the shape of any stream channel. To a lesser extent, incremental modifications of flow may change the size and shape of the stream channel. Nevertheless, this methodology is based on the assumption that the shape of the stream channel and associated boulders, trees, and other features will remain the same when assessing different dis- charges. The user should exercise judgement in assessing the range of flows under which this assumption is valid in any particular stream. Known or predicted changes in channel shape must be treated as new input to the model. ~i . ~~1!~~"'~~~~D11' ~ \]--- a~ / ~ Hr11ft.I_i!'''''' ~___<_''''' _,,",__,,'Y~;" ~L~}{.~.--.. _ ,;:;__,-_>"",~~~ r-::i: 8