As long as Congress has not exercised its power over commerce in a certain area, a state may regulate that area as long as such regulations do not conflict with the Dormant Commerce Clause of the U.S. Constitution.
Willson v. Black-Bird Creek Marsh Co., 27 U.S. (2 Pet.) 245 (1829), was a significant United States Supreme Court case regarding the definition of the Commerce Clause in Article 1 sec. 8, cl. 3 of the U.S. Constitution. Willson, the owner of a sloop who was licensed under federal navigation laws, broke through a dam that blocked his passage which was built by the Black-Bird Creek Marsh Co. and had been authorized to do so by Delaware law. The company brought a case against Willson, claiming Delaware authorized the building of the dam through a law which was passed under the police power of the state in order to clean up a health hazard and there was no legislation by Congress dealing with the same subject matter. Willson claimed that the law authorizing the building of the dam was a violation of the commerce clause. Chief Justice Marshall affirmed the lower court's decision, that because no federal law dealt specifically with the situation, and the state law did not violate Congress' Dormant Commerce Clause power, the state law was valid.