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Across North America, flow alterations and diversions have led to the depletion of stream flow-reliant ecosystems and ecosystem services. In western states and provinces, the law historically considered water left in the stream to be wasted. Western state laws encouraged full appropriation of rivers and streams, primarily to satisfy the need to divert water to arid areas for economic and domestic purposes. In Eastern states and provinces, stream flows have been altered and depleted through channelization, dams, levees and other structural changes. By the 1970s, "salmon populations were crashing, riparian habitat was being lost, and . . . legendary rivers like the Rio Grande had become little more than concrete-lined conduits." In the mid-twentieth century, citizens began to demand protection for the rivers they valued for fishing, swimming, boating, inspiration and aesthetic pleasure. Legislatures responded with statutory provisions for wild and scenic rivers, water quality requirements and constraints on the exercise of water rights. Oregon is credited with adopting the first protective instream flow legislation in the United States in 1955. Montana and Colorado followed suit in 1969 and 1973, respectively By the 1990s, instream flow laws had been adopted in many jurisdictions.
Statutory parameters and on-the-ground implementation vary widely between jurisdictions. In western North America, key differences in instream flow laws include: restrictions on the allowable sources of water that may be used for instream appropriations, and limitations on who may obtain instream flow rights and the purposes for which instream rights may be appropriated.
Relatively few river miles have been protected by state water law. For example, since the passage of its instream flow legislation in 1984, only two percent (247 miles (397 km)) of Nebraska's streams have received protection through instream flow appropriations, 239 miles (384 km) of which are on the Platte River. Other Rocky Mountain states have similar track records; only one percent of Idaho's 93,000 stream miles is protected. Instream flow legislation in the states and provinces of Eastern North America tends to be less clearly delineated. Eastern jurisdictions rely heavily on the common law riparian concept of reasonable use, which may implicitly protect instream values. The protection of instream flows in the east is also driven by statutes that define reasonable use, some of which explicitly protect fisheries, water quality and other instream values.
In the United States, federal legislation also plays an important role in protecting instream values-primarily through hydropower licensing requirements, water quality provisions and endangered species protection. Rivers designated as wild, scenic or recreational under federal or state law receive special attention. In many cases, federal funding for water transfers, fishways and restoration initiatives has also been key to restoring and maintaining instream flows.