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1 ' Chapter 4 Mitigation and Modification Alternatives CLASSIFICATION OF DIVERSION STRUCTURES Each of the diversion structures examined first-hand can be classified as being one of ' three types, based upon its physical configuration. The diversion structures not visited or reviewed in detail would likely fall into one these same categories. The names of one or more structures which are characteristic of each type are noted in the descriptions of each general ' type below. More detailed descriptions of each of these named examples are provided in Appendix F. Streambank Structures The simplest of the diversion structures typically consist of: 1) a movable pump on the streambank with a suction line dropped into the river (e.g. the Sunbeam Ditch and Five Fifty- Five Ditch); 2) a constructed side channel to a pumping station (e.g. the Hayden Station or Craig Station diversion pipelines); 3) a pumping station on the streambank (e.g. the Maybell ' Mill Ditch); or 4) a direct gravity diversion at the streambank (e.g. the Gibraltar Ditch). A variation on these simple diversion types is one at which periodic instream excavation ' or berming takes place immediately adjacent to the diversion point. These structures are temporary, requiring frequent reconstruction and annual maintenance. This structure usually takes the form of movable pump situated on the bank adjacent to a short instream berm, often constructed by equipment positioned on the top of the bank. The function of the berm varies by location and can serve different purposes such as diversion of flow to suction area, bank erosion protection, disposal of river alluvium excavated from a suction area, concentrating flow, controlling the location of sediment deposition, etc. The Old Sweeney Ditch and the ' Henry Sweeney Ditch are examples of these types of structures. ' Since these diversions involve only minor instream structural components they are not very susceptible to engineering modification to improve passage or navigability or to reduce habitat disturbance. At most, only very minor physical modification needs are identifiable. As ' long as the area of habitat disturbance remains restricted, the mitigation considerations are minimal for these structures. Temporary Instream Structures This diversion type consists of gravel alluvium pushed into place from the streambed to either completely or partially block the channel and direct lowflow to the headgate or pump. Variations on it include the use of rock lowflow section (e.g. the Shelton Ditch) and/or the use of a semi-permanent rock core beneath the gravel dam. Diversion structures for the Williams ' and Behrman ditches are typical of this type of diversion used for gravity and pump applications, respectively. Photographs of the Shelton and Williams ditches and an unnamed diversion downstream of the Cross Mountain Pump-Ground are provided in Appendix E. 4-1