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, . 384 J KORMAN, 5, ~1. WiELE AND M, TORIZZO The flow and sand transport model uses a simple turbulence closure to approximate the \'cnical structure of the velocity profile, This method does not account for regions with abruptly changing bed topography where the assumption about the now structure is violated. The regions where the flow patterns are more cDmp\kated than is represented by the use of a vertical velocity profile based on a turbulence closure tend to occupy a small part of the channel and have not had a significant e.frect on the calculations of erosion and deposition. For this study, we hu.....e ncg\e.cted the vertical strUcture of the flow and used only the vertically averaged velocity. \Vith the logarith- mic yelocity profile used in the model, the velocity in the upper 80% of the flow is nearl)' constant. The main compromise with our approach is in neglecting the velocity structure in the 100\'er 20% of (he flow. The qverall channel shape in our study reaches is fixed by bedrock, large talus blocks, and debris fans that are stabilized by cobbles and boulders that are rarely transported by dam-controlled flows, We modelled one channel shape in each of our study reaches. although channel shape, and consequently flow fields. can be altered by changes in the volume and location of sand deposits. Comparisons of nrea of favourabte habitat with differ~nces in discharge could be influenced by variations in sand deposition \vithin the study reaches, but relative changes between reaches are likely to be less sensitive to variations in sand deposition. Historical comparisons may be complicated by the variability in sand deposits in the low-velocity zones as a result of the decreased sand supply after the closure of Glen Canyon Dam, The difference between pre- and post-dam sand supply in our study reaches is partly mitigated, however, by their proximity to the Litlle Colorado River (Figure 1). one of two major sand-contributing tributaries, All study sites are located within 8 km below the confluence with the Little Colorado River, Mapping of shorelille types Field-surveyed bathymetric and shoreline topographic data (Hazel el a/,. 2000) were combined with photogram- metrically generated contour data (US Bureau of Reclamation, 1990), A triangulated irregular network (TIN) sur. face model was created using the Delauney method of triangulation. implemented by ARCfli'iFO (Environmental Systems Research Institute. Inc,. 1991), The TIN surface was then in'terpolated using a bivariate quintic interpola- (ion scheme in order to generate 6.25 m2 resolution grids. The in(erpolated surfaces \....ere then combined with ripar- ian vegetation maps (US Bureau of Reclamation, 1990) established on the basis of elevation. vegetation density, and surficial geology (Schmidt ef ai"~ 1999), The mapping units of these data sets were merged and reclassified to match the six shoreline types used by Converse ef ai, (1998) to describe the habitat preference of juvenile hump- back chub in Grand Canyon: bedrock, cobble, debris fan, sand. talus, and vegetation, Suitczble hllbirm We computed the amount of total suitable habitat by summing the total wetted area of each study reach where velocity was less than or equal to a critical value of 0,25 tnts. The amount of suitable shoreline habitat waS com- puted by placing an additional constraint that depth was less than or equal to I m, The amount of tOtal suitable habitat and suitable shoreline habitat was computed for each study reach for nine discharges (Table Il). . T<lble II. Significance of nine discharge levels used to develop habitat- discharge relationships Discharge {ml/s) Significance 84 141 226 425 566 907 1272 2123 2830 Annual average minimum pre-dam flow Nightime minimum under current operations Daytime minimum under current operations Approx:imatc average post-dam flow Typical high operating flows Approximate powerplllnt c<lpacity Glen Canyon Dam March-April 1996 experimental flood Exceeded in 28% of years between 1922 and 2000 Exceeded in 13% of years between 1922 and 2000 Copyright ~ 2004 John Wiley & Sons. Lid. River Re3. Applic. 20: 319-400 (2004)