At Intake, Traverella relative abundance declined from 60 percent in 

 August to less than 1 percent in November, while HydvopBuchc increased from 

 14 percent of the organisms to 54 percent during the same period (Report No. 5). 

 With the loss of i'ravcrrlLa, llydi'o['Bychc becomes the dominant food item in 

 the riffle habitat. Hydropsychids were the major food item of sturgeon 

 captured from the Yellowstone in May and June. Hoopes (1960) and Modde (1973) 

 also point out the importance of Hydropsychids in the sturgeon diet. 

 Hydropsychc, being sensitive to changes in velocity (Edington 1965 and 1968), 

 may be affected more adversely than most other riffle species. A reduction 

 in hydropsychids could, by altering food habits, be detrimental to fish that 

 feed in the riffles, especially the shovelnose sturgeon, which is highly 

 adapted for riffle feeding. 



In addition to reducing numbers of food organisms, a drop in discharge 

 concentrates fish into a smaller habitat. Forage fish and the remaining 

 benthic organisms become more vulnerable to predation from the high-density 

 fish community. Decreased food and space can limit the fish population growth 

 to the point that predator biomass may decline in amounts equal to the loss of 

 riffle insect biomass. 



SLOW-WATER HABITAT 



The specific habitat requirements of forage and game fish species of the 

 Yellowstone are not known, but large numbers of forage fish, burbot, sauger, 

 and catfish were collected in the backwaters and slow littoral waters. To 

 maintain the present populations of carnivorous species, it is necessary to 

 preserve the backwater habitats. Backwaters are conmon to the braided 

 section of the river. A dominant discharge (that high flow recurring every 

 1*2 to 2 years which, through a combination of magnitude and frequency, 

 accomplishes the most geomorphic work in a channel over the long term) is 

 necessary to retain the characteristics of the main and side channel 

 morphology. Periodic flows in the side channels prevent the invasion of 

 riparian vegetation. Without side channels, backwaters would fill with silt, 

 causing a reduction in wetted perimeter and depth, eventually destroying the 

 backwater habitat. Martin (see Report No. 2 in this series) reports a 43 

 percent reduction in the number of islands on the Bighorn River as a result 

 of reduced flows due to water impoundments. Islands became part of the 

 mainland, eliminating the side channels and backwaters. Similar results 

 could be expected on the lower Yellowstone following reduction of spring 

 flows, changing the river from a braided to a meandering stream. 



Even if sufficient flows were preserved to maintain the existence of 

 side channel flow during the spring runoff, it would still be necessary to 

 ensure that the needed volume of water is available to keep the backwaters 

 full during the remainder of the year to maintain slow-water habitat. 



The growth rate of most aquatic organisms is greatest during summer months 

 when water temperatures are at a peak (Hynes 1970). During this period of 

 receding flows and heavy water diversions for irrigation, food and habitable 

 area are of critical importance to the growth of insects and fishes. The 

 most severe impacts on aquatic conmunities are likely to occur if excessive 

 water withdrawals are made during August and September. 



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