9410 - Laserfiche WebLink

Home Browse Search

I 1996) requires "restoring peak flows needed to reconnect and periodically reconfigure channel and floodplain habitats". I I Large-river ecologists recommend mimicking the natural hydrograph as a first step in habitat enhancement/restoration (Bain et al. 1988; Poff et al. 1997; Stanford 1994; Stanford et al. 1996; Ward 1989). OVerbank flooding is required to reconnect floodplains with rivers. However, overbank flooding is a controversial issue with the public because of potential economic loss in agricultural crops and private property as well as sociological issues such as (1) increases in mosquitoes and potential for encephalitis outbreaks and (2) spread of noxious introduced weeds (U.S. Bureau of Reclamation and U.S. Fish and Wildlife Service 1998). I 1 I If overbank flooding does not occur, the lateral floodplain productivity is curtailed with the result that the productivity of Upper Basin rivers are adversely affected. As streamflows increase and subside during the spring runoff, various aquatic habitats are produced that are used by the different life stages of native fishes (Schlosser 1990, 1991). Low velocity habitats in streams and floodplains can be maintained only by preserving fluvial geomorphological processes of watersheds (Kellerhals and Miles 1996). Kellerhals and Miles also stated that efforts to restore fish habitat in river channels and floodplains have resulted in low success in rivers that have been extensively altered. Regulation of streamflows through dam releases for irrigated agriculture and peak power generation reduces the diversity of aquatic habitats -- some of which may be required by various life stages of the razorback sucker for successful recruitment (Wick 1997). Stanford et al. (1996) stated that reregulation of most rivers can be accomplished without substantially compromising storage or power generation. I I I If economic, political, and sociological pressures prevent increasing streamflows to restore the river-floodplain integrity, then it will be necessary to excavate floodplain habitats to aid in the recovery of the endangered fishes. Many of the existing floodplain terraces in broad alluvial valleys of Upper Basin rivers were deposited during much higher peak streamflows than presently occur. Excavation of floodplain habitats would allow inundation at lower river elevations so that private property along Upper Basin floodplains that are either occupied by humans or used for agriculture will not be adversely affected. Modde (1997) reported that some recruitment of razorback suckers occurred in floodplain depressions along,the middle Green River. I I Most known historic spawning aggregations of razorback suckers in the Upper Colorado River Basin were located upstream from river reaches with broad floodplains where rivers meandered without restriction. The best example is "Razorback Bar" on the middle Green River that is located just downstream of Dinosaur National Monument. A broad floodplain extends about 90 km (56 mil downstream from "Razorback Bar" to pariette Draw. The razorback sucker population using this bar was estimated to be about 500 adult razorback suckers (Modde et al. 1995). About half of the adult razorback suckers captured by electrofishing at Razorback Bar in 1998 were marked fish, suggesting that the population is declining (L. Shanks, 1998. personal communication) . I I I I The timing of streamflow was found to be important in maintaining a cobble bar (Razorback Bar) that is a primary spawning site for razorback suckers in the middle Green River, upstream from Jensen, Utah. Twelve cross- sections at "Razorback Bar" during 1993 and again in 1996 indicated that this site was subject to a backwater effect and significant sedimentation occurred at discharges exceeding 340 m3/s (12,000 cfs). Some sedimentation of the site began at a discharge of 200 m3/s (- 7,000 cfs) and resulted in 0.6 m (- 2 ft) of sand deposition as streamflows I I 22 I I