The Dakota Mainstem Water Supply Project Feasibility Study Act aims to assess the possibility of building a water supply project that would provide municipal, rural, and industrial water to specific areas in South Dakota, Iowa, Nebraska, and Minnesota. This study is intended to ensure that these regions have adequate water resources to meet their needs. Key provisions of the act include the requirement for the Secretary of the Interior to conduct a feasibility study in collaboration with the Dakota Mainstem Regional Water System.