HighlightsPrivate and public irrigation development was important for expanding agricultural production in the western U.S.The Twin Falls Canal Company is an excellent example of a successful Carey Act project.Cooperative efforts during the last 30 years have dramatically improved the water quality of irrigation return flow.Electricity generated by six hydroelectric facilities improves the sustainability of the irrigation project.Abstract. Private and public irrigation development projects were fundamental to bringing irrigation to arid regions of the western U.S. The Twin Falls Canal Company in southern Idaho provides a case study of private and public irrigation development because the project was developed by private investors under the Carey Act and receives a portion of its irrigation water supply from U.S. Bureau of Reclamation reservoirs. The project survived initial financial struggles and waterlogged soil to focus on sustaining crop production by reducing chronic furrow irrigation erosion and nutrient losses in irrigation return flow. Average sediment loss from the project was 460 kg ha-1 in 1970. A cooperative effort by the canal company, state and federal agencies, and farmers improved water quality by installing sediment ponds on fields, applying polyacrylamide with furrow irrigation, converting from furrow to sprinkler irrigation, and constructing water quality ponds on irrigation return flow streams. From 2006 to 2018, more sediment and total phosphorus flowed into the watershed than returned to the Snake River, and the project removed 13,000 Mg of sediment and 30 Mg of total phosphorus from the Snake River each year. However, nitrate-N from subsurface drainage was lost at 10 kg ha-1 each year, or 800 Mg year-1, for the entire watershed. While sediment and phosphorus concentrations in irrigation return flow have decreased, these concentrations were still greater than the irrigation water, indicating that more can be done to reduce the project’s influence on water quality in the Snake River. Keywords: Nitrogen, Phosphorus, Sediment, Soluble salts, Water quality.