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INTRODUCTION On March 11,! 1967, only 21 years after Miller (1946) described humpback chub (Gila cypha) as a species, it was listed as federally endangered (FR 32:4001). Presently, only six remnant populations still exist within the Colorado River or its major tributaries. The largest population resides in the Colorado River below Glen Canyon Dam within Grand Canyon National Park and the lower 14 river kilometers (RKM) of the Little Colorado River (LCR), Arizona (USFWS 1990; Valdez and Ryel 1995; Douglas and Marsh 1996). Currently, the perpetuation of the "Grand Canyon" population relies primarily on the lower LCR, which still maintains conducive abiotic and biotic conditions for their successful spawning, rearing, and recruitment to adulthood (Keeding and Zimmerman 1983; Gorman and Stone 1999). Historically, humpback chub may have been distributed much further upriver in the 'LCR (see Uyeno and Miller 1965 for progenitor fossil remains), but due to vegetation changes caused by livestock grazing, flow modifications related to irrigation and water impoundment by small earthen dams, and other anthropogenic disturbances, most of the river is no longer perennial (Colton 1937; Miller 1961). Although the LCR becomes perennial at Blue Springs (20.7 RKM above the LCR-Colorado River confluence), it is supersaturated with CO2 (566 mg/L as of July 2003) and exceeds the tolerance levels of-all fishes within this immediate vicinity (Mattes 1993). Humpback chub do reside just below Chute Falls (14 RKM) with higher densities arising further downstream below Lower Atomizer Falls (13.6 RKM, see Figure 1.; Keading and Zimmerman 11983; Mattes 1993; Douglas and Marsh 1996; Van Haverbeke and Coggins 2003). Robinson et al. (1996) conducted an experiment where caged humpback chub were held at three different sites above Chute Falls. They found that fish stress behaviors (e.g., gulping at surface, lethargy) primarily occurred at the uppermost 20 RKM location (19.2 RKM on more recent maps) where CO2 measured 348 mg/L. They postulated that at the 17.5 RKM site (16.2 RKM on more recent maps), CO2 levels (196 mg/L) were within humpback chub tolerance limits and that they could probably survive in this lower area if given the opportunity. They surmised that Chute Falls was an impassable physical barrier that prevented humpback chub from exploiting these resources. However, Mattes (1993), who previously quantified similar CO2 levels, hypothesized that humpback chub were restricted from this area by a CO2 chemical barrier rather than an impassable physical barrier. Strength (1997), who studied the geochemical formations of travertine dams, concurred with Mattes' hypothesis. Although CO2 levels below the range found in this LCR area have been used to anesthetize (Booke et al. 1978; Post 1979; Mishra et al. 1983; Yoshikawa et al. 1991) and kill fish (Fry et al. 1947; Black et al. 1954), there are numerous factors involved, including fish physiological responses and avoidance behaviors, acclimation times, dissolved oxygen, temperatures, etc. (Alabaster et al. 1957; Eddy et al. 1977; Ross et al. 2001). Historical and recent surveys have found a diversity of species existing in this upper LCR area, some of which died at lower CO2 levels during the above- mentioned studies. Therefore, potential exists that acclimated humpback chub could also, survive under: these elevated CO2 levels. A third hypothesis suggests that humpback chub'. are able to migrate above Chute Falls but are repeatedly flushed down river during flood events so that colonization of this area is constantly being pushed back in time. This is 4