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011 .. 9~ . HUMPBACK CHUB HABITAT MODELLING 387 RESULTS Respollse of !wbirat availability to discharge TOlal suirable shoreline habitat typically declined with increasing discharge; however, the response varied among reaches and depended on local morphology (Figure 3), Reaches ALC, RI. and R4 showed the grealest declines between 226 and 425 m'/s. while R2 and R5 showed the greatest declines at a lower discharge range of 84-141 m)/s. Some reaches were relatively insensitive to changes in flow (e.g. R3, R5), whereas others (ALC, R I. and R4) were very sensitive, The morphology of the insensitive reaches is dominated by large fans creating large eddies downstream. These fans have a Sleep profile that maintains the eddies, and therefore the avail- ability of low-velocity habitat, as discharge is increased, At very high discharges (e,g, 2830 m'/s). however. the fans eventually are overtopped leading to the loss of habitat with suitable velocities. In contrast, a very sensitive reach such as R4 has a relatively low profile fan that is overtopped at discharges of 425 m'/s and higher, ALC has a morphology that is not dominated by debris fans. Low discharges expose a large mid-channel cobble bar creating low-velocity habitat upstream and downstream from the bar and in the side channel created by the bar, As dis- charge increases. this low~velocity area is progressively reduced until all that remains is a thin ribbon of suitable habitat adjacent to the banks. As a result. ALC had a consistent decline in suitable habitat availability with increas- ing discharge, The Palisades (PAL) reach showed a unique bimodal response where total suitable shoreline habitat was highest when discharges were either low or high, with the lowest habitat values occurring at intermediate discharges of 907-1272 m'/s. This bimodal response is a result of the channel shape. which differs from the shape of other study reaches in several key attributes, Unlike the banks in the other reaches that tend to be steep on both sides of Ihe channel. the bank along river left in the PAL reach lIallens to a low angle above the stage associated with 1272 m'/s, As a result. the flow at discharges above 1272 m'/s expands into Ihis area creating a large amount of suitable habitat. Below 1272 m',s. the lIow is relatively confined resulting in a smaller area of suitable habitat than at higher discharges down to about 907 m'/s, Below 907 m'/s. discharge is low enough to create a considerable area of suitable habilat along the banks of the main channel. The effect of discharge on the availability of suitable shoreline habitat, when stratified by substrate type, also was highly variable among reaches (Figure 3), In the ALC reach. the availability of suitable shoreline habitat decreased with increasing discharge, yet debris fan and lalus habitat slayed relntive]y constant, and Ihe amount of vegetated shoreline habitat increased considerably, reaching a maximum at 1272 m'/s. R2 and R5 showed rela- tively large changes in total suitable shoreline habitat between 84 and 141 m'/s, yet the amount of suitable habitat over debris fans or talus increased or was constant across these discharges. The relative change in total suitable habitat due to increasing discharge was remarkably similar across reaches, with near-minimum values reached at 425 m'/s (Figure 4a), The large gain in habitat at the lowest discharges reflects a typical cross-sectional response where the largest decreases in velocity occur at the low end of the dis- ch3rge range. In contrast, change in the area of suitable shoreline habitat to variations in discharge was much more variable across reaches (Figure 4b), suggesting it was much more sensitive to local differences in morphology, The shape of the response of habitat availability to discharge was insensitive to the value of the velocity criterion used to quantify suitable habitat. Reducing the value of the velocity criterion from the base value of 0.25 mfs reduced the amount of suitable habitat predicted at a given discharge, but the relative pattern across discharges remained unchanged, On average. the trend in habitat availability with discharge based on one velocity criterion could explain 95% of the habitat trend based on a different criterion, Effects of Glell CallyolI Dam 011 discharge alld habiwl availability Construction and operation of GCD caused considerable changes in the magnitude and shape of the hydrograph of the Colomdo River in Grand Canyon (Figure Sa), Impoundment has reduced average flows in the months of May and lune from about 1500 to 400 m'/s and has increased the annual minimum flows (based on monthly averages) from 226 to 300 m'/s, Seasonal variation in discharge since impoundment has been greatly reduced, Impoundment has resulted in a large increase in b3se flows; in the pre.dam era, flows in excess of 226 m3ts occurred 50% of the time. whereas in the post-dam era, they occur at a frequency of greater than 70% (Figure 5b). Copyright fJ 2004 John Wiley & Sons. Ltd. Riyer Res. Applic. 20: 379-400 (2004)