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1 1 I 0 Introduction Habitat disturbance can occur when heavy equipment is used to move streambed and riverbank materials into temporary berms or dams and when such materials are disposed of on riparian areas. The movement of streambed and bank materials can suspend fine materials, creating sedimentation problems downstream, and can destroy benthic communities that fish depend on for food. Disposal of streambed materials on riparian lands can adversely impact wetlands and related stream"side habitats. Riparian habitats can also be disturbed by construction equipment access to the diversion structures. It has been estimated that typical diversion structure maintenance activities cost irrigators between $500 and $1,500 annually. However, because there has been little or no monitoring or regulation of these activities, there is little known about their annual extent or cumulative effects. Water rights administration of diversions has generally not been concerned with these matters of structural detail or environmental impact. Accordingly, the extent to which these maintenance activities may have adverse impacts on endangered fishes has not been well understood. In recent years the Yampa River mainstem has become an increasingly popular fishing and boating destination. Popular non-native sport fish in the river include trout, northern pike, and bass; habitat disturbance and flow modification associated with diversion structure maintenance have potential impacts on these fish populations. River rafting and kayaking are especially popular in the Steamboat Springs area and from Craig downstream through Dinosaur National Monument; some of the larger irrigation diversions (e.g. the Maybell Canal) are not passable to boaters except during peak runoff. The leakage and overflow present at many temporary diversion structures is also problematical from a water rights administration perspective. This stems from the fact that the Diversion Engineer cannot administer a water right "call" on behalf of a water user unless that user's diversion structure is physically capable of taking all the flow in the river. If diversion structures are modified to provide bypass flows passable to fish, they may also need to be fitted with measuring devices to facilitate administration of bypasses. OBJECTIVES OF STUDY The objectives of this study are to develop an inventory of problematic mainstem Yampa River diversion structures in river reaches that are believed to contain endangered fish species; to make a reconnaissance-level evaluation of the potential effects such structures might have on flow conditions, fish passage, and aquatic and riparian habitats; to make preliminary recommendations concerning the structures thought to be most problematical; and to describe, in a preliminary way, the potential alternatives that exist to reduce any adverse effects these structures or the maintenance thereof might have. The river reaches included in the study are the mainstem of the Yampa River from its confluence with the Little Snake River upstream to the vicinity of the town of Mt. Harris. The lower terminus of this reach corresponds roughly with the eastern boundary of Dinosaur National Monument; there are no significant diversions that take place below this point. The upstream terminus of this reach is defined by the location of several large irrigation and industrial diversions; above these diversions the river is relatively clear and cold and is not believed to support populations of the endangered species. 1-5