Which act resulted in maintaining the integrity of U.S. waterways by regulating discharges?

The Federal Water Pollution Control Act is essential in the realm of environmental regulation as it lays the groundwork for a series of water quality standards and regulations aimed specifically at protecting the integrity of United States waterways. This act establishes the framework for regulating the discharge of pollutants into navigable waters and sets the basis for the National Pollutant Discharge Elimination System (NPDES).

Through the NPDES, the act empowers the Environmental Protection Agency (EPA) to issue permits that control the quantity and quality of discharges, thereby ensuring that waterways remain clean and safe for aquatic life and human use. This regulation is critical in preventing the contamination of surface waters and maintaining overall water quality across the nation.

The other options, while significant in their respective areas of environmental protection, do not specifically pertain to regulating discharges into U.S. waterways. The Comprehensive Environmental Response, Compensation, and Liability Act focuses on cleaning up hazardous waste sites, the Oil Pollution Act of 1990 primarily addresses oil spills and their impacts, and the Refuse Act relates to the control of refuse and waste in navigable waters, but the Federal Water Pollution Control Act is the cornerstone legislation for managing water pollution comprehensively.