Dam
Non-jurisdictional dams

A dam is non-jurisdictional when its size (usually "small") excludes it from being subject to certain legal regulations. The technical criteria for categorising a dam as "jurisdictional" or "non-jurisdictional" varies by location. In the United States, each state defines what constitutes a non-jurisdictional dam. In the state of Colorado a non-jurisdictional dam is defined as a dam creating a reservoir with a capacity of 100 acre-feet or less and a surface area of 20 acres or less and with a height measured as defined in Rules 4.2.5.1. and 4.2.19 of 10 feet or less. In contrast, the state of New Mexico defines a jurisdictional dam as 25 feet or greater in height and storing more than 15 acre-feet or a dam that stores 50 acre-feet or greater and is six feet or more in height (section 72-5-32 NMSA), suggesting that dams that do not meet these requirements are non-jurisdictional. Most US dams, 2.41 million of a total of 2.5 million dams, are not under the jurisdiction of any public agency (i.e., they are non-jurisdictional), nor are they listed on the National Inventory of Dams (NID).

Small dams incur risks similar to large dams. However, the absence of regulation (unlike more regulated large dams) and of an inventory of small dams (i.e., those that are non-jurisdictional) can lead to significant risks for both humans and ecosystems. For example, according to the US National Park Service (NPS), "Non-jurisdictional—means a structure which does not meet the minimum criteria, as listed in the Federal Guidelines for Dam Safety, to be included in dam safety programs. The non-jurisdictional structure does not receive a hazard classification and is not considered for any further requirements or activities under the NPS dam safety program." Small dams can be dangerous individually (i.e., they can fail), but also collectively, as an aggregation of small dams along a river or within a geographic area can multiply risks. Graham's 1999 study of US dam failures resulting in fatalities from 1960 to 1998 concluded that the failure of dams between 6.1 and 15 m high (typical height range of smaller dams) caused 86% of the deaths, and the failure of dams less than 6.1 m high caused 2% of the deaths. Non-jurisdictional dams may pose hazards because their design, construction, maintenance, and surveillance is unregulated. Scholars have noted that more research is needed to better understand the environmental impact of small dams (e.g., their potential to alter the flow, temperature, sediment and plant and animal diversity of a river).
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