YCT Multi-state Assessment February 1 0, 2003 



be found at http://nris.state.mt.us/scripts/esrimap. dll?name=MFISH&Cmd=INST) for organizing 

 and displaying the data. 



A Latitude- Longitude Identifier (LLID) 1 : 100,000 hydrography layer that was edge-matched 

 across state boundaries was used as the primary base-layer. The Idaho portion of the study area 

 was obtained from the Pacific Northwest River Reach Files. The U.S. Geological Survey 

 (USGS) in Portland, in cooperation with Bonneville Power Administration, the Northwest Power 

 Planning Council, and other Federal and state agencies and NW Indian Tribes produced a 

 1 : 100,000-scale River Reach data layer for the Pacific Northwest in the early 1990s. The Pacific 

 Northwest (PNW) River Reach Files are a geo-referenced river reach data layer that 

 encompasses the Columbia River Basin within the conterminous United States, the coasts of 

 Oregon and Washington, Puget Sound in Washington, the Klamath and Goose Lake Basins in 

 southern Oregon and the Bear Lake Basin in southeastern Idaho (PNW Reach File, Gladstone, 

 Oregon: Stream Net, August 2002. Imp: wa'.w .strL'amnct.oi!.! pnwr nnwrlionichtmn . A 

 Latitude-Longitude Identifier (LLID) 1 : 100,000 hydrography for the Montana portion of the 

 study area was obtained from Montana Fish Wildlife and Parks (Streams. Helena, MT: Montana 

 Fish Wildlife and Parks, March 2001 and are available at 



imp. Iw p. stale. ml, LIN iiiskiclwp b\[ilibrar\ m.s ). A Lafitude-Longitude Identifier (LLID) 

 1 :100,000 hydrography layer was not available for Wyoming; thus we utilized the USGS 

 National Hydrography Dataset (IMational Hydrography Dataset, Reston, Virginia: United States 

 Geological Survey, March 2001; available at hup: nhd.ub^s.LioN ). The National Hydrography 

 Dataset (NHD) uses different stream routing methods and was not immediately compatible with 

 the LLID stream layer. Steve Carson and Jeff Hutten, Montana Fish Wildlife and Parks, assisted 

 in the rerouting and editing of Wyoming NHD data to create a Wyoming LLID routed stream 

 layer. This LLID hydrography layer routes stream segments by uniquely identifying each stream. 

 Streams missing from the LLID hydrography layer with known YCT populations were added 

 from the unrouted NHD data. Delineating lower and upper segment boundaries as distances 

 above each stream's mouth identified each stream segment occupied by YCT. All known fish 

 barriers were located as points, also using distance upstream from a stream's mouth. 



Scale issues 



Using a standard 1 : 100,000 base-layer allowed for consistent summaries among states and other 

 entities. However, summaries based on this scale will underestimate "true" field lengths of 

 stream habitats due to scale-based error. There are several potential sources of bias associated 

 with using 1:100,000 scale LLID hydrography. First, map-derived stream lengths under-estimate 

 actual stream lengths. Firman and Jacobs (2002) found that while hip-chained measurements of 

 Oregon coastal streams were significantly correlated to stream lengths computed using 

 MapTech® Terrain Navigator software and 1 :24,000-scale maps, map lengths needed to be 

 multiplied by about 1 . 14 to estimate measured stream lengths. 



Secondly, there are scale-differences between 1 : 1 00,000 and 1 :24,000-scale hydrography. We 

 evaluated the magnitude of these scale-differences during the Westslope Cutthroat Trout 

 assessment. We compared lengths of 30 streams from three different 4'^ code HUC's (10 per 

 HUC) and found that lengths of streams derived from 1 : 100,000-scale hydrography were only 

 about 1% shorter than estimates of that same stream using 1 :24,000-scale hydrography (Shepard 

 et al. 2003). However, these were small unnamed streams that are mapped at 1 :24,000 scale that 



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