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, . I. Ii 386 J. KORMAN. S. ~1. WIELE Ai\U M. TORIZZQ variation in discharge belo\\' Glen Canyon Darn results in 'destiJbilization' of nearshore habitats used by native fish that could have a negative effect on mainslem sur....ival. \Ve quantified the relative extent of this desmbilization between reaches by detennlning the area of suitable habitat that was stable across typica\ daily ranges in discharge. To do this. we first computed the average minimum, and maximum monthly discharges under specific operating regimes (Table Ill) as measures of the daily variation in discharge, Wc then computed the locations of suitable habitat in each study reach that were present at both the average minimum and maximum monthly discharges. The total area of this common habitat is referred to as the amount of 'persistent suitable habitat', To evaluate the effects of discharge-driven changes in suitable habitat area on CPE data, we queried the Grand Canyon Monitoring and Research Center fisheries database (GCMRC, Flagstaff, Arizona, unpublished data) to return the dates and times of all electro fishing sampling events that occurred in 1993 within the upstream and downstream range of our modelling sites (river km 99-107), We selected 1993 because it was a year of intensive study when many samples were collected during a period when hourly variation in discharge was relatively small (Table 111), Our analysis. therefore, provides a near-minimum estimate of the additional bias and variation in CPE data resulting from changes in discharge, To estimate the discharge during each of the 190 unique date-time sam- pling events that were returned from the database. we routed the relevant portions of the 1993 Lees Ferry discharge record to the upstream limit of sample sites (river km 99) using a one-dimensional model of diurnal discharge wave propagation (Wiele and Griffin, 1997), The one-dimensional model computed the length of time for a dis- charge wave measured at Ihe Lees Ferry gauge to travel downstream to the LCR confluence, a requirement to deter- mine the discharge at the time of each sampling event. We then generated a frequency histogram of the number of sampling events by discharge interval and compared it to the predicted changes in suitable shoreline habitat area over the same intervals, The potential for large, discharge.driven variation in CPE data was identified by the pre- sence of a significant number of CPE measurements collected across a discharge range where the predicted change in suitable shoreline area was large. Dispersal A particle-tracking algorithm was used to simulate the dispersal of /ish out of each study reach under a range of discharges, We began each simulation by randomly placing 500 particles within the wetted perimeter of each, reach, The movement of each particle. predicted on a 0,5 s timestep. was predicted from the cross-stream and downstream velocity vectors generated from the hydrodynamic model. A bilinear interpolation program (Press e/ al,. 1992) was used to compute the velocity vectors at the particles' positions at each timestep on the basis of their distance to the four nearest surrounding nodes of the hydrodynamic model grid, Particles were given a swimming behaviour and speed (Table IV) to evaluate the effects of swimming ability on downstream dispersal. The passive behaviour simulated the dispersal of larval/ish while the active behaviours simulated the movement of young-of-the-year and juvenile fish, The distance and direction of movement for each timestep were calculated as the vector sum of current velocities and swimming speeds in cross-stream and downstream directions. To quantify the ability of a reach to retain fish at a spedfic discharge, we computed the percentage of 500 par- ticles that remained in the study reach after 100 min of simulation time had elapsed, Retention rat.. were computed for all swimming speeds and behaviours (Table IV) in each of the seven study reaches across nine discharges (Table II), Table IV. Summary of swimming behaviours and speeds used for simula[ing dispersal of larval. young-or-year. and juvenile humback chub Behaviour Description Swimming speed (m/s) P<lssive Rheotactic Geotactic Drifting particle, no behaviour Particle allempts to mO\"e in cross-stream direction towards slower currenl Particle altemprs to move in cross-stream direction towards closest bank Particle attempts to mo~e in upstream direction o 0,\ and 0,2 , 0,2 Upstream 0,2 Copyright <<) 2004 John Wiley & Sons, Lid" R;~.t'r Rts. App/;c. 20: 379-400 (2004)