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Backwater and Gravel Pit Investigations Earlier CRFP investigations suggested that river backwaters -- naturally indented areas with little or no water current -- are a particularly important habitat for young Colorado squawfish and humpback chub, and that the limited availability of such habitat could limit the survival of these young endangered fishes. If the relative scarcity of river backwaters is limiting, then an increase in backwater availability might appreciably increase the rate of survival for young endangered fishes. Gravel pits are defined in this study as man-made habitats constructed within the river floodplain. They communicate with the river during periods of high river flow but may or may not communicate when river discharge is low. Gravel pits can be hydraulically similar to some natural backwaters. As gravel pits are common in the Colorado River floodplain of the Grand Valley and since such pits will continue to be developed in the future, it seems prudent to investigate the usefulness of these gravel pits as habitat for the spawning and rearing of endangered fishes. During 1982 routine physicochemical and biological surveys were completed on several river backwaters and gravel pits along the Colorado River in the Grand Valley (Figure 2). These investigations were performed to determine the characteristics of habitats used by endangered fishes. Four river backwaters and seven gravel pits were examined on an approximate weekly schedule. Physicochemical analyses included dissolved oxygen (DO), salinity, conductivity, turbidity, and observations on water elevation (stage) and communication between the study area and the Colorado River. Biological sampling of gravel pits was performed using trammel nets and seines during daylight (3x5 mm-mesh), electrofishing during darkness, and qualitatively sampling for larval fishes with a fine-mesh hand net. The biological sampling program for river backwaters differed somewhat from that for gravel pits. Because river backwaters that contain young Colorado squawfish normally contain too few squawfish to allow extensive study of the interactions of fishes and their environment, the natural squawfish populations in study backwaters were supplemented using Colorado squawfish raised at Dexter (New Mexico) National Fish Hatchery. These hatchery fish were about 3 months old and 35-85 mm long when stocked into the experimental backwaters on 6 October 1982. Each fish was marked with a coded-wire microtag before stocking. The microtags consisted of a small ( 1x0.5 mm) piece of magnetized wire that was mechanically inserted in the cartilage of the fish's head. An electronic device detected the presence of a tag when the fish was passed through the detector. The fish need not be killed in order to detect the presence of a tag. Some of these tagged hatchery Colorado squawfish were also planted in gravel pits. -5-