Final answer:
When a spill that can affect navigable waterways occurs, the National Response Center (NRC) must be notified as they coordinate with relevant federal agencies.
Step-by-step explanation:
If any spilled material could enter a navigable waterway, the proper authority to notify is the National Response Center (NRC). The NRC serves as the primary federal point of contact for reporting oil spills and chemical releases. Since they coordinate with various agencies such as the Environmental Protection Agency (EPA) and the Department of Transportation (DOT), notifying the NRC ensures that the proper procedures are followed to minimize environmental impacts and comply with federal regulations.