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August 1994 FLOW REGIME AND RIPARIAN VEGETATION 549 1.0 en 0.8 - 0 Q. - 0.6 0 C 0 0.4 U CO ...... LL 0.2 0.0 ..." "" .........., ...... """. "" '" ".... ...", ", "" '" "', ,...... ......" ", ,,,,, '" ",' ,...., "', "" "" ", "....... ..." ",' "", ,"" ", ......" ..." "" ""/ ",,, "", ....,..., ..." "" '" "",,, '" ...", ,....... ......" ", "",,, '" "" "... ..."... ", "", '" ,..." ....,... "" ", <,"" ", "" '" ,..." ", "" "" "" ..." ",' '"'' "" "" ",.... ......., ",' ,,,,, "" ", ......." '" ...", """ "'" ", ........, '" "" "" """ "" '" '" ...", ", ".." ", ........... "..., "" '" ...."... """ ,,"' "" ..""', "" ,...., "....... ............ '" "...... """ ,,"' '" ,,,,,, "" ......., ...,........ ...", ,,,... "...... ".." "" ,,,, -,"" ", ...,... ",... "" '" ......,... "", ", ", """ '" ..." "..., '" "... ",' "" ", "" """ ", ", ...,..., '" '" ...", ,,,,,.. ", "" "" "" ,..., ...,..., '" ,..., ,...." ~~~~ ~,,~ ~~~ ~~~~ ~~~ '" "" ",' ," "" ~~~~ ~~~ ~~~ ~~~~ ~~, ......, ...", "...... ,..., ...,...... ~,~~ ~~~ ~~~ ~~~~ ~~~ ......, ,," ............ '" ............ ~~~~ ~~, ~~~ ~~~~ ~~~ ...,... "" '" ,', "..., ~~~~ ,,~~ ~~~~ ~~~~ ~~~ '" ,......... '" "'" "" ~~,~ ~,,~ ~~, ~~~~ ~~~ '" ,......... "... ," "" ~~~~ ~,,~ ~~, ~~~~ ~~~ ", ............ "..., '" ",... ~,~~ ~~~ ~~~ ~~~~ ~,~ ,..., ",... ...,..., ,,' ",... ,~~~ ~~~ ~~~ ~~~~ ~~~ ..." "" ......" ,..., "..., ~~~~ ~~~ ~~~ ~~~~ ~~~ '" "" "'''' ..." ,..." ~~~, ~~~ ~~~ ,~~~ ~~~ '" "" ",... '" ,......, ~~~~ ,~~ ~~~ ~~,~ ~~~ "... ,......... '" '" '......, ~~~~ ~,,~ ~ ~~,~ ~~ o Eleocharis EJ Heterotheca I'2l Equisetum ~ Open water <.01 .01-.1 .1-.2 .2-.3 .3-.4 .4-.5 .5-.6 .6-.7 .7-.8 .8-.9.9-.99 >.99 Inundation duration FiG. 6. Normalized distribution of cover types along gradient of inundation duration. plots had from 50 to 75% cover. The substrate was also variable and consisted primarily of cobbles and boulders with large fractions of silt, sand, and organic matter. This was the wettest vegetated cover type and was found from the channel edge and the edge of off- channel pools, up to low and middle elevation gravel bars. This cover type was inundated frequently. Al- though the overall range of inundation duration was broad (Fig. 6), >80% of the plots had inundation du- rations between 50 and 89%. The low inundation du- rations calculated for some of the plots occupied by the Eleocharis cover type may reflect error in the hy- draulic model, which does not consider the effects of sideslope drainage or localized depressions. The Heterotheca cover type occurred on only 10.5% of the total bar area. Over half of the bar area (52.6%) was in the Equisetum cover type. The remaining 36.8% was occupied by the Eleocharis cover type. The Open- Water cover type did not occur in the set of sampled plots, but was defined based on the stream ward limit of vegetation in order to support prediction about plots that might become permanently inundated under an alternative hydrologic regime. Predicted vegetation changes The flow duration curves for the Reference regime and the hydrologic alternatives are shown in Fig. 7. The Diversion alternative decreases mean flow to 54% of the Reference mean, and Diversion-Increased-Min- imum decreases mean flow to 64%; the Moving-Av- erage alternative involves no change in mean flow. All the alternatives decrease flow variability. The model predictions of vegetation change under the alternative hydrologic regimes are shown in Table 2. Because the model is new and has not been field- tested, these predictions should be interpreted with caution. Equisetum remains the predominant cover type under all the alternatives (Table 2). Under the Diversion alternative some of the Eleocharis type is replaced byEquisetum. However, a larger area of Eq- uisetum is converted to Heterotheca, and as a result the total area of Equisetum declines. The Diversion-Increased-Minimum alternative in- creases mean flow by 19% relative to the Diversion alternative (Fig. 7); however, this additional water re- sults in a decrease in the wettest vegetated cover type, Eleocharis (Table 2). The reason is that permanent inundation under the Diversion-Increased-Minimum alternative transforms some areas occupied by the Eleocharis cover type into Open Water. The Moving-Average alternative decreases the area of the Equisetum and Eleocharis cover types and in- creases the area of both the driest cover type, Heter- otheca, and the wettest type, Open Water (Table 2). As the extreme flow events are moderated, inundation duration increases where it was already high and de- creases where it was already low. Thus a systematic reduction in the range of flow conditions toward the middle of the distribution actually increases the dis- persion of cover types by increasing the area of cover types at the edges of the gradient. DISCUSSION Model utility Our purpose in modeling vegetation dynamics is to incorporate a consideration of impacts on riparian veg- etation into water management decisionmaking. In or- der to be practical the approach should be consistent with the conceptual, dimensional, and computational framework for making these decisions. Representing the river through a series of hydraulic cross-sections and modeling water surface elevations using Manning's equation or a step-backwater model are standard prac- tices in water management. Likewise, summarizing hy- drologic time series in flow duration curves is a com- mon technique. Such curves would either be available as part of project engineering design or could be rea-