9545 - Laserfiche WebLink

Home Browse Search

likely drift less than roundtails. Humpback chub are not common outside of Westwater Canyon and the Black Rocks (Valdez et al. 1982) area of the Colorado River. Although YOY chubs were not identified to species, we assumed YOY collected above the canyon were roundtail and those collected in Westwater Canyon were of both species. Juvenile and adult life stages were sampled within Westwater Canyon to determine the stability of both the humpback and roundtail chub populations there. Basic life history and ecology information for Colorado River chubs (Gila spp.) is relatively scarce in the literature when compared to the body of information available for Colorado pikeminnow. We have tried to add to that general database where possible. STUDY AREA The study area encompassed the 28.3 km stretch of the Colorado river from'the Bureau of Land Management boat launch at RK 204.3 to the Cisco Takeout at RK 176 (Figure 1). Westwater Canyon (RK 199.2 - 183.2) represents a geologic anomaly of upthrust Precambrian material. The exposed metamorphic gneiss is harder than the overlying sandstones resulting in a deep (maximum depth of 21.8 m recorded at RK 198.6), confined channel. The habitat in the upper portion of the canyon consists of runs, eddies, and pools interspersed between riffles and rapids. Average gradient through Westwater Canyon is 11.8 ft /mile; for comparison, gradient through Desolation Canyon averages 6.2 ft /mi. The steepest part of Westwater Canyon, RK 190.4 - 186.4, was not sampled due to the turbulent flows and Class III rapids. This stretch likely supports native chubs. The lower portion of Westwater Canyon, RK 186.4 - 183.2, was a confined canyon reach, but gradient diminishes and habitat consists primarily of a homogeneous run and chubs are scarce. The canyon is bracketed both upstream and downstream by broad alluvial areas with complex channels flowing over cobble substrates. Colorado River Hydrology For the period of record (1951-1996) at the State Line USGS gage (09163500), mean daily Colorado River flows have ranged from a low of 960 cubic feet per second (cfs) in 1956 to 68,300 cfs in 1984. Throughout the period of study flows ranged from a low of 2020 cfs recorded in late July, 1994, to an instantaneous peak of 49,300 cfs recorded June 19, 1995 (Figure 2). The effect of the Aspinall project on Gunnison and Colorado River flows dates back to 1965, when Blue Mesa Reservoir began storing water. Colorado River flows as measured at the Near Cisco, Utah, gage (09180500) were used for pre- and post-Aspinall comparisons due to the longer period of record (1923 -present) at that gage. The Cisco gage is located approx 24 km below Westwater Canyon; immediately downstream of the confluence with the Dolores River. Both pre- and post-Aspinall exceedence flow curves were calculated for the following flow metrics: annual peak (cfs), annual flow volume (total acre-feet) and runoff volume (April-July acre-feet totals) (Figure 3). Aspinall project operations have decreased the average (50% exceedence) annual Colorado River peak flow and average runoff volumes by 31.2% and 23.3%, respectively. Water years in this study (1992-1996) were characterized as wet or dry by relating their runoff flow volumes (April -July) to the historical exceedence values. Wet years were defined as having runoff volumes that were exceeded < 30% of the time, i.e., having flow volumes in excess of 3.77 million acre feet (maf): 1993 (4.94 maf) and 1995 (5.53 maf). Dry years exhibited runoff volumes that have been exceeded during at least 70% of the years, which corresponds to a value of 1.99 ma£ 1992 (1.83 maf) and 1994 (1.68 maf). Moderate year's runoff fell between the 30% and 70% exceedence flows: 1996 (3.23 maf). 2