The Poteau River in southeastern Oklahoma contains a diverse mussel community, including 11 identified as species of greatest conservation need. The river is impounded by Wister Reservoir, a 2970-ha impoundment. In an attempt to improve water quality in the reservoir, a proposal was submitted to siphon eutrophic water from the bottom over the dam into the Poteau River. The movement of suspended sediments is expected to pose a significant threat to mussel assemblages below Wister Reservoir. To assess future project impacts on mussel assemblages, water quality and sediment analysis was conducted prior to the enactment of the proposal of the reservoir and in the river below the dam. Furthermore, a survey was conducted on mussels and potential host fishes in the first 27 km downstream of Wister Lake. Analysis of lake sediment samples showed elevated levels of phosphorus (>300 mg kg_1), nitrogen (>900 mg kg_1), and arsenic (>10 mg kg_1). Additionally, water discharged below the dam had dissolved oxygen levels below 4 mg L_1 for extended periods. A decline from earlier studies in both mussel abundance and diversity were documented. Of the 35 mussel species previously identified from the Poteau River, only 16 were collected during this study. None of the seven mussel species previously identified as greatest conservation need were collected. The proposed project poses a risk to mussel assemblages through a continued degradation of water quality.