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01193 HU:i!PBACK CHUB HABITAT MODELLING 385 The maximum \'elocity criterion we used to define suitable habitat is consistent with numerous field observa- tions and laboratory studies on humpback chub. Av~rage and "maximum velocities used by YOY chub (21-74 mm TL) in the upper Colorado River were 0.06, and 0,30 mls. respectively (Voldez el al.. 1990), Laboratory studies of 30-100 mill YOY chub at 14'C, a near.maximum summertime temperature in the mainstcm downstream of the LCR (Valdez and Ryd, 1995). measured sustained swimming speeds of 0,2-0.4 mfs and cruising speeds of 0,1- 0,2 mls (Bulkley el al,. 1982), YOY and juvenile chub in Grand Canyon have been sampled in velocities of up to 0,1 mfs in talus h,1bilJts, 0,2 mls in debris fan, sand beach, and vegetated bank habitats, and 0,6 and 0,3 mfs in cobble bar and bedrock habitats, respectively (Converse el al,. 1998), The 0,25 mfs criterion we used is slightly above the maximum juvenile humpback chub cruising speed measured in the laboratory and the typical velocity from which fish were sampled in their preferred habila!s. We examined the sensitivity of the habitat availability- discharge curves predicted by the model to changes in the velocity criterion by recomputing the habitat indices under a series of velocity criteria from 0.05 to 0.25 m1s. \Vithin-sitc correln(ions in habitat availability based on the different criteria were then examined. To compute historical changes in suitable shoreline habitat. we used the 1921-2000 continuous discharge record computed from stage records and stage-discharge relations from the US Geological Survey (USGS) Colorado River at Lees Ferry gauging station (09380000) located 99 km upstream from Ihe LCR confluence (digitized 1922-1986 records supplied by D, Topping, USGS, wrillen communication, 2000: the 1921-1986 data are avail- able from ftp,gcmrc,gov; the 1987-2000 data were provided by the USGS Arizona District office), The continuous Lees Ferry record is ideal for assessing habitat change in a system where power. load following causes large var- iation in discharge over the course of a day. A corresponding habitat value for each discharge observation was computed by linear imerpolalion using lhe sile-specific suitable shoreline habitJI-discharge relations, A daily mean habitat value was then computed as a time.weighted average of the instantaneous values. Mean monthly habitat values and 95% confidence limits. based on the variation in monthly averages among years. were computed for the pre-dam (1922-February 1963) and post-dam (:Vlarch 1963-2000) periods. One-way analysis of variance (ANOVA) was used to test for significant differences among monthly means between pre- and post.impoundment periods. In addition, we computed mean monthly habitat values for particular periods of the post-dam record to assess specific operating regimes (Table n\), Historical changes in GCD operations have resulted in dramatic differences in the e"tent of daily variation in discharge. There is evidence that the persistence of low-velocity habitats over short time scales affects survival rales of young flsb (Bowen et al,. 1998: Freeman el 01., 2001). Valdez and Ryel (1995) hypothesized that hourly Table Ill. Summary or Glen Canyon Dam (GCD) operating regime characteristics compared in our anillysis. The pre-dam per- iod of record used was from 1922 to February 1963. Note thal the maximum daily flow fluctuation under "Interim' and MLFF operating regimes is dependent on lhe monthly release volume from GCD Operating Period Years or Lees regime Ferry record used for analysis No action 1963-\990 \987-1989 Interim flows 1991-\995 1993 Modified low 1996-present 1997-\999 flucluation flows (MLFF) , May-Sepl, 2000 Minimum flow (m'/s) Maximum daily flow fluctuation, m:'/s (GCD monthly release volume. mJ x 106) Ramping rate (m'/sth) Maximum flow (m'/s) 25 (winter) 892 28 (summer) 141 (day) 566 226 (night) \41 (day) 708 226 (night) <864 Unlimited 141 (<740) t 70 (740...987) 226 (>987) 141 (<740) 42 (dOlvnromp) 7\ (upramp) 42 (downramp) \13 (upramp) 226 \70 (74o...987) 226 (>987) o o M.y-Sept. 2000 226 Low summer steady flow experimenl (LSSF) Copyright lD 2004 John Wiley & Sons. Ltd. Rh'er Res. Applic. 20: 379-400 (2004)