logs). Duties arising from the model include the following: Pass on the flow of water without obstruction, pollution or diversion affecting the rights of others. Maintain the bed and banks of the watercourse and to clear any debris, whether natural or artificial, to keep any culverts, rubbish screens, weirs and mill gates clear of debris. Be responsible for protection of land from flooding and cause no obstructions, temporary or permanent, preventing the free passage of fish. Accept flood flows even if caused by inadequate capacity downstream, but there is no duty to improve the drainage capacity of a watercourse. United States The United States recognizes two types of water rights. Although use and overlap varies over time and by state, the western arid states that were once under Mexico and Spain generally follow the doctrine of prior appropriation, also known as "first-come, first-served", but water rights for the eastern states follow riparian law. Riparian rights Under riparian law, water is a public good like the air, sunlight, or wildlife. It is not "owned" by the government, state or private individual but is rather included as part of the land over which it falls from the sky or then travels along the surface. In determining the contours of riparian rights, there is a clear distinction between navigable (public) waters and non-navigable waters. The land below navigable waters is the property of state, and subject to all the public land laws and in most states public trust rights. Navigable waters are treated as public highways with any exclusive riparian right ending at the ordinary high water mark. Like a road, any riparian right is subordinate to the public's right to travel on the river, but any public right is subject to nuisance laws and the police power of the state. It is not an individual right or liberty interest. Because a finding of navigability establishes state versus federal property, navigability for purposes of riverbed title is a federal question determined under federal law. The states retain the power of defining the scope of the public trust over navigable waters. A non-navigable stream is synonymous with private property, or jointly-owned property if it serves as a boundary. The state could choose to divest itself of title to the streambed, but the waters and use of the waters remains subject to the Commerce Clause of the United States Constitution which holds an easement or