9489 - Laserfiche WebLink

Home Browse Search

I CI I I [I II [I [I 11 ,I (I L 'I t ,~ 'I \1 ,I .1 ,I I I Distribution and Relative Abundance Species composition and relative abundance varied significantly in relation to location and habitat availability. Habitat characteristics changed dramatically as one moved downstream and typically fell into three primary categories: (1) the tailwater downstream of Davis Dam, (2) the dredged river channel, and (3) the backwater complexes located near Lake Havasu. The study area was divided into two 40-km reaches for purposes of comparing potential differences in the up- and downstream fish communities. The upper reach was dominated by channel and tail water habitat while the fish community in the lower river reach was influenced by the reservoir and large backwaters. Physical Description of the Upper and Lower 40-km River Reaches The first characteristic of the upper reach was Davis Dam's tailwater, which included a 20-km reach of physically degraded channel downstream of Davis Dam. This reach was represented by a relatively broad, and in some areas, braided river channel composed primarily oflarge gravels and rock. Channel depth varied from swift runs to scoured pools, some exceeding 20 m in depth. There were two relatively large (>10 ha) backwaters, one at Laughlin Lagoon and the other at Boy Scout Camp. Both were relatively shallow (<1m) and experienced daily river stage fluctuations (1.5 m), which exposed large expanses of mud flats. Fluctuations caused by hydropower were less evident in downstream areas. The 40-km reach between the old Fort Mohave ruins to Topock Gorge was dredged in the 1950's and 1960' s. Here the river was bordered by levees and the channel was seldom wider than 150 m. The riverbed seldom exceeded depths greater than 3 m and was composed primarily of drifting sand. This reach extended through the broad floodplain where the river channel historically extended nearly 3 km wide during spring runoff in the Topock Marsh area. Dredging and levee construction had either drained or isolated these wetlands. Backwaters or floodplain wetlands were absent from the river until one reached Park Moabi, located some 15 km downstream of Needles, California. Large shallow backwaters that were typically bordered by large expanses of cattail dominated the third and last zone. The first backwater or lagoon was found at Park Moabi and represents the old Colorado River channel prior to dredging. The second occurred at Golden Shore, which again was a remnant of the historic channel. The river then entered the confines of To pock Gorge, which eventually opened up to large numbers of backwaters and a vast marshy delta before the river entered Lake Havasu. Differences in Fish Distribution and Relative Abundance Habitat availability influences fish composition and relative abundance. This was evident not only for recreational species but also in the distribution and relative abundance of native suckers. Channel habitat dominates the upper 40-km reach where flannelmouth suckers are concentrated near Davis Dam. The flannelmouth sucker was the predominant species captured in that reach (3.5 fish/I 00 m2) and exceeded all other species (Figure 1.3). Nearly all (93%) flannelmouth suckers were captured in the upper reach. By contrast, nearly all razorback suckers were captured in the lower 40-km reach. Here, backwaters were prevalent and predators (i.e., bullhead, sunfish, black bass, and striped bass) were typically more than twice as abundant compared to the upstream reach (Figure 1.3). This suggests each sucker species faced a different predator community in terms of composition and abundance. Predators were twice as abundant in the lower 40-km reach and were dominated by redear sunfish, largemouth bass, bluegill, striped bass, smallmouth bass, and bullhead spp. Predators were far less abundant in the upper reach that was inhabited by flannelmouth sucker and only channel catfish were slightly more abundant (Figure 1.3). 9