Current environmental mitigation practices at nonfederal hydropower projects were analyzed. Information about instream flows, dissolved oxygen (DO) mitigation, and upstream and downstream fish passage facilities was obtained from project operators, regulatory and resource agencies, and literature reviews. Information provided by the operators includes the specific mitigation requirements imposed on each project, specific objectives or purposes of mitigation, mitigation measures chosen to meet the requirement, the kinds of post-project monitoring conducted, and the costs of mitigation. Costs are examined for each of the four mitigation methods, segmented by capital, study, operations and maintenance, and annual reporting costs. Major findings of the study include: the dominant role of the Instream Flow Incremental Methodology, in conjunction with professional judgment by agency biologists, to set instream flow requirements; reliance on spill flows for DO enhancement; and the widespread use of angled bar racks for downstream fish protection. All of these measures can have high costs and, with few exceptions, there are few data available from nonfederal hydropower projects with which to judge their effectiveness.