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30 ROAD GRAVEL PIT d~ Rive o~°~a Former Island y Boundary "~ ~ __ _ Gravel ,~, ~ ~,- -- ----- ---~ Pit ~, -' Emergent --------'0~ Vegetation ~~ Gravel Berm 0 250 Meters * N Colorado black bullhead (Ictalurus melas) (Table VII). The native fishes were 26.7 of the fish composition in this artificial feature; roundtail chub, flannelmouth sucker and bluehead' sucker were found primarily at the mouth of the backwater near the swift main channel current. One adult Colorado squawfish and one adult razorback sucker were caught in a trammel net in about 2 m of water within the pit in June 1981. A second squawfish was taken in a fyke net set over- night to catch fish moving into the pit. Fish composition when the pit was isolated in summer 1981 was dominated by green sunfish, fathead minnow, red shiner, and topminnows (Gambusia affinis). The second type of gravel pit is open to the river only during runoff, and is represented by several pits (Table VI), including Walter Walker Wildlife Area (WWWA). A single out- let, a 5-m cut in a gravel berm, separates this 18.3-ha pit from the river (Figure 6). The river flows through this single opening only when flow exceeds about 7,500 cfs. The river normally flows into the pit from May to mid-July except for low water years such as 1981, when flow into the pit occurred for only 2-3 weeks. Much silt is deposited in this gravel pit during this overflow period, as the energy of the Figure 5. A gravel pit excavated from a midchannel island on the Upper Colorado River, river mile 175.5. The 1978 pit is nearly filled with silt and sediment. Table VI. Area, depth and substrate of four gravel pits open to the Upper Colorado River near Grand Junction, Colorado. River Name Area Depth (m) Substrate Mile (ha) Max. Ave. 163.6 Walter Walker 18.30 1.9 1.1 silt/gravel 168.0 Connected Lakes 4.00 2.2 1.0 silt/gravel 175.5 30 Road Pit 1.00 5.5 0.6 silt/gravel 177.8 Clifton Ponds 1.10 3.1 0.5 silt/gravel Means: 6.10 3.2 0.8 silt/gravel 530 k I Table VII. Percentage composition of native and non-native fishes in the 30 Road Gravel Pit of Colorado River (n=569). the Upper Native Species Percentage Non-Native Percentage Composition Species Composition roundtail chub 15.2 common carp 23.7 flannelmouth sucker 5.4 red shiner 15.6 bluehead sucker 5.0 fathead minnow 11.7 speckled dace 0.7 sand shiner 7.5 Colorado squawfish 0.3 black bullhead 7.2 razorback sucker 0.1 green sunfish 4.5 Total: 26.7 largemouth bass 2.2 white sucker 0.5 channel catfish 0.3 black crappie 0.1 Total: 73.3 S '2l