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Water law is the field of law dealing with the ownership, control, and use of water as a resource. It is most closely related to property law, but has also become influenced by environmental law. Because water is vital to living things and to a variety of economic activities, laws attempting to govern it have far-reaching effects.
The history of people's relation to water illustrates varied approaches to the management of water resources. "Lipit Ishtar and Ur Nammu both contain water provisions, pre-date Hammurabi by at least 250 years, and clearly provide the normative underpinnings on which the Hammurabi Code was constructed" . The Etruscans had a deep knowledge of hydrology and hydraulics, a knowledge which they put to good use in their many land drainage schemes. The lower lying portions of Rome such as the area between the Capitol and Velia was formerly marshland. Settlement of the low-lying ground would never have been a possibility without the hydraulic engineering skills of the Etruscans. This took place around 625 BCE when, according to archaeological evidence a network of drainage channels was dug through the marshy ground, and at the same time, the stream that separated the two hills of the Capitoline and Palatine was regulated, its embankments were strengthened, and it was finally covered over.
Without enclosure and drainage, more than half of the area of the present-day Netherlands would be flooded with every high tide, every wet season, or permanently. The struggle for the country's survival has largely determined its appearance and left numerous marks on it. In the triangle between the cities of Amsterdam, Haarlem and Leiden the 45,000-acre (180 km2) Haarlemmermeer polder demonstrates an extraordinary step in the scale of land drainage. After centuries of preliminaries, a lake of unprecedented size was drained in three years (1849–1852) and transformed into valuable agricultural land.
In 1899, construction of the first Aswan Dam was begun to address agricultural and energy shortages exacerbated by population growth in Egypt and the Sudan. Completed in 1902, its height was raised in subsequent building campaigns of 1907-12 and 1929-34. With the signing of the Nile Water Agreement by Egypt and the Sudan in November 1959, work began on the second Aswan dam. The second dam submerged much of Lower Nubia displacing 90,000 Egyptian peasants and destroying monuments and archaeological sites from the First to the Third Cataracts of the Nile River. The Aswan High Dam captures floodwater during rainy seasons and releases the water during times of drought. The dam also generates more than 10 billion kilowatt-hours every year. The project prevents the natural silting process which enriched Egyptian agriculture and farmers must now use about one million tons of artificial fertilizer as a substitute for natural nutrients that once fertilized the now arid floodplain.
Difficulties of water rights
Water has unique features that make it difficult to regulate using laws designed mainly for land. Water is mobile, its supply varies by year and season as well as location, and it can be used simultaneously by many users. As with property (land) law, water rights can be described as a "bundle of sticks" containing multiple, separable activities that can have varying levels of regulation. For instance, some uses of water divert it from its natural course but return most or all of it (e.g. hydroelectric plants), while others consume much of what they take (ice, agriculture), and still others use water without diverting it at all (e.g. boating). Each type of activity has its own needs and can in theory be regulated separately. There are several types of conflict likely to arise: absolute shortages; shortages in a particular time or place; diversions of water that reduce the flow available to others; pollutants or other changes (such as temperature or turbidity) that render water unfit for others' use; and the need to maintain "in-stream flows" of water to protect the natural ecosystem.
One theory of history, put forward in the influential book Oriental Despotism, holds that many empires were organized around a central authority that controlled a population through monopolizing the water supply. Such a hydraulic empire creates the potential for despotism, and serves as a cautionary tale for designing water regulations.
Water law involves controversy in some parts of the world where a growing population faces increasing competition over a limited natural supply. Disputes over rivers, lakes and underground aquifers cross national borders. Although water law is still regulated mainly by individual countries, there are international sets of proposed rules such as the Helsinki Rules on the Uses of the Waters of International Rivers and the Hague Declaration on Water Security in the 21st Century.
Long-term issues in water law include the possible effects of global warming on rainfall patterns and evaporation; the availability and cost of desalination technology; the control of pollution, and the growth of aquaculture.
Water law by country
Under the Constitution Act, 1867, jurisdiction over waterways is divided between the federal and provincial governments. Federal jurisdiction is derived from the powers to regulate navigation and shipping, fisheries, and the governing of the northern territories, which has resulted in the passage of:
Provincial jurisdiction is derived from the powers over property and civil rights, matters of a local and private nature, and management of Crown lands. In Ontario, Quebec and other provinces, the beds of all navigable waters are vested in the Crown, in contrast to English law. All provincial governments also govern water quality through laws on environmental protection and drinking water, such as the Clean Water Act in Ontario.
Water law in Australia varies with each state.
A newly formed Tasmanian Water Corporation has compulsorily acquired all drinking water supply infrastructure without payment and does not have direct accountability 
Water law in the United States
In the United States there are complex legal systems for allocating water rights that vary by region. These varying systems exist for both historical and geographic reasons. Water law encompasses a broad array of subjects or categories designed to provide a framework to resolve disputes and policy issues relating to water:
- Public waters, including tidal waters and navigable waterways.
- Other surface waters—generally water that flows across non-public land from rain, floodwaters, and snowmelt before those waters reach public watercourses.
- Groundwater, sometimes called subterranean, percolating, or underground water
- Public regulation of waters, including flood control, environmental regulation—state and federal, public health regulation and regulation of fisheries
- Related to all of the above is interplay of public and private rights in water, which draws on aspects of eminent domain law and the federal commerce clause powers
- Water project law: the highly developed law regarding the formation, operation, and finance of public and quasi-public entities which operate local public works of flood control, navigation control, irrigation, and avoidance of environmental degradation
- Treaty Rights of Native Americans
The law governing these topics comes from all layers of law. Some derives from common law principles which have developed over centuries, and which evolve as the nature of disputes presented to courts change. For example, the judicial approach to landowner rights to divert surface waters has changed significantly in the last century as public attitudes about land and water have evolved. Some derives from state statutory law. Some derives from the original public grants of land to the States and from the documents of their origination. Some derives from state, federal and local regulation of waters through zoning, public health and other regulation. Non-federally recognized Indian tribes do not have water rights.
Water law in the European Union
For countries within the European Union, water-related directives are important for water resource management and environmental and water quality standards. Key directives include the Urban Waste Water Directive 1992  (requiring most towns and cities to treat their wastewater to specified standards), and the Water Framework Directive 2000/60/EC, which requires water resource plans based on river basins, including public participation based on Aarhus Convention principles. See Watertime — the international context, Section 2.
- Clean Water Act
- Clean Water Act (Ontario)
- Clean Water Protection Act
- Colorado River Water Conservation District v. United States
- Drinking water
- Drainage law
- Environmental Law
- Food safety
- Navigable servitude
- Right to water
- Water right
- Thompson, Olivia N., "Binational Water Management: Perspectives of Local Texas Officials in the U.S.-Mexico Border Region" (2009). Applied Research Projects. Paper 313. http://ecommons.txstate.edu/arp/313
- See The Beds of Navigable Waters Act in Ontario and Civil Code of Quebec, s. 919, in Quebec
- Flora Fox Regional News, Kingborough, Hobart Southern Tasmania http://florafox.tumblr.com/post/3418412236/water-sewer-governance-tasmania
- Hildering, A. (2004), International Law, Sustainable Development and Water Management, Eburon Academic Publishers, Delft, The Netherlands, 2004 
- International Law Association Water Resources Committee (2004), Final Report presented at the Association's 2004 Conference in Berlin 
- UNEP (2002), Vital Water Graphics — An Overview of the State of the World's Fresh and Marine Waters. UNEP, Nairobi, Kenya. 
- Sax, J. L., et al.. Legal Control of Water Resources: Cases and Materials (4th edition). Thomson/West (2006), ISBN 978-0-314-16314-1.
- George Vranesh, Colorado Water Law. Revised Edition, University Press of Colorado (2000), ISBN 978-0-87081-543-0, 2003 supplement (March, 2004), ISBN 978-0-87081-755-7
- Washington Office of Attorney General. "An Introduction to Washington Water Law."
- Paul Stanton Kibel, Grasp on Water: a Natural Resource that Eludes NAFTA's Notion of Investment, Ecology Law Quarterly (2007)
- Water Law
- UNEP Environmental Law Programme
- International Water Law Project
- UNESCO Centre for Water Law, Policy and Science, Dundee University, UK
- Centre for Comparative Water Policies and Laws, University of South Australia
- Bibliography on Water Resources and International Law. Peace Palace Library
- New Mexico Office of the State Engineer, Litigation and Adjudication Program
- In Historic Vote, UN Declares Water a Fundamental Human Right - video report by Democracy Now!
- The Water Report