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Messrs. Lochhead, Simpson, and Robotham <br />Comment: "What is the relationship between flow, ice scouring, acres of <br />channel cleared, and benefits to listed species ?" <br />Flow moves ice and sediment, and scouring is more effective at higher flow. <br />Johnson (1994) reported that scouring during ice breakup was an important <br />mortality factor of established cottonwood seedlings and that flows greater <br />than 3,000 cfs were most effective during this period. It is not known if he <br />estimated acres. The relationships between wide channels and species are <br />noted in the response to a previous question, above. <br />Page 7. 2.3. Late Spring Pulse Flows <br />Comment: "The Service also should explain why specific channel <br />maintenance flows are recommended when many knowledgeable observers agree <br />that channel width from Overton to Grand Island is generally in a state of <br />quasi - equilibrium ?" <br />Some observers have concluded that channel width is in equilibrium, while <br />others have concluded that channel width has not yet attained equilibrium in <br />some reaches. Even if all observers agreed that equilibrium has been <br />attained, it would be reasonable to recommend channel maintenance flows in <br />order to protect the existing channel characteristics (assuming they were <br />sufficient for endangered species habitat requirements). Conclusions differ <br />perhaps due to time period, location examined, and techniques used. <br />At best, the designated critical habitat reach may have achieved a state of <br />quasi - equilibrium, and no long -term reductions in width will occur. However, <br />the available information does not allow a definitive conclusion regarding <br />equilibrium, and additional reductions in width may still occur in the lower <br />portion of the Overton to Grand Island reach, even though bed material <br />transport is roughly in equilibrium. The reasons for this conclusion were <br />described in Butler (1994). For example, the conclusion by Lyons and Randle <br />(1988) concerning bed material transport equilibrium was attained in part by <br />using the same sediment rating curve at both ends of the reach in computing <br />their mass balance. In addition, the bed load transport computation alone is <br />not able to detect possible channel incision. The Service believes these <br />limitations preclude drawing the conclusion from this data that channel width <br />is in equilibrium. <br />DI <br />Dr. Carter Johnson (1994) concluded that the channel remained near <br />quasi - equilibrium during 1969 -1986. However, the frequency of high flow <br />episodes during that time period is atypical of the longer period of record <br />under the present water development levels. Dr. Johnson (pers. comm.) <br />indicated channel narrowing at the rate of about 1 percent per year over the <br />recent 4 -6 years. Field biologists concur that a noticeable invasion of <br />vegetation on river sandbars has occurred during recent years. Analysis of <br />past and future aerial videography conducted by the Service and of future data <br />collected by Dr. Johnson should provide rather conclusive evidence of whether <br />channel width has changed in recent years and will continue to change in the <br />future. <br />