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- The simplest, least costly method that would give an approximation of ecosystem units would be to use existing HEC-RAS modeling efforts. Stream cross-sections and HEC- RAS modeling developed either by the Corps for flood damage reduction studies or by the In-stream Issues GrouplUDFCD (see pages 33 and 37 of South Platte River Corridor Project, In-stream Issues Report, November 14, 1996) would be used. The HEC-RAS modeling would enable a calculation of channel top width surface area for any given flow, which is a quantification of stream habitat. For a more detailed and accurate approach, and also because of the age of the cross sections, stream geometry data would be collected at low flow by means of digital topographic surveys, and then entered into a geo-referenced HEC-GeoRAS model and ArcGIS extension. This method would then translate the information into a depth grid of inundated area. For either method, an ecosystem function model (HEC-EFM) would be used that would input flow and stage time series and eco-hydro relationships data. An example of eco- hydro relationships data would be wintertime target flows of 60 to 85 cfs. Relationships regarding summertime target flows should also be developed by a fish and wildlife stakeholder workgroup. For the more detailed approach, relationships regarding depths and ecosystem indicator species should also be developed. The more detailed and accurate modeling effort would be used to visualize existing ecologic conditions, highlight promising restoration sites, and assess and rank alternatives according to the relative enhancement (or decline) of ecosystem aspects. Either approach will also involve outputs in quality. Quality indexes will need to be developed that relates existing and forecasted in-stream flows and stream bank conditions to the stream habitat/riparian habitat river ecosystem quality. The eco-hydro relationships would be used in the development of the quality index. A GIS vegetation survey completed in 1996-1998 by the Denver Audubon Society would also be used. The resulting assessment will likely result in acres of a habitat type multiplied by a quality index ranging from 0.25 (poor) to 1.00 (excellent) in order to obtain habitat or ecosystem units. The inventory and forecast will address fish and wildlife, Federal and State-listed threatened and endangered species, other species of special concern, and migratory birds recognizing 17 species of fish-eating birds. Information from the 1996 South Platte River Corridor Project In-stream Issues Report, and other readily available information will be researched, extracted, and consolidated for this step of planning. No plant or animal surveys will be required. 4