What is a "small" dam?
There is no universal specification for a "small" dam. Some state agencies consider dams less than 15 feet high as small. The National Inventory of Dams, maintained by federal agencies, uses a combination of height and impoundment size as a cutoff for inclusion. The focus of TU's Small Dams Campaign is easier to describe by the types of dams on which it does not focus. This campaign does not focus on large-scale hydroelectric facilities, or on dams that provide flood control for large regions. Most of the dams that are possible candidates for selective removal are about 25 feet high or less.
TU advocates for the removal of old, unsafe, uneconomical dams that are doing more harm than good. Selective dam removal, thoughtfully carried out, is increasingly recognized as one of the most cost-effective fisheries and river restoration methods available.
Dams provided the energy that made possible the early development of the U.S. Many communities, especially along the northern tier of the country, grew up around a small dam that provided mechanical power for milling and, perhaps later, hydroelectric power. But, over the decades, society's needs changed, and tens of thousands of these small structures became obsolete, no longer serving the purpose for which they were designed.
Dams degrade under the pressures of time, gravity and flowing water. Dams typically have a life expectancy of around 50 years, but many of the smallest structures are over a century old. Lacking continual maintenance and repair, many of these old dams today pose public safety hazards.
Bringing old structures up to modern safety standards can be a financial burden to dam owners, especially if the owners are individuals or small communities. Repairing a small dam to meet safety standards commonly costs three to five times the cost of removing the structure. Typical costs for repairing an aging small dam can be $300,000 or more. Increasingly, dam owners are looking for alternatives to keeping old and expensive structures.
Obsolete dams also are one of the most visible of human impacts on rivers. Typically built on river stretches with a rocky bottom and fast-moving water - attributes shared by good fish spawning habitat - dams can harm water quality and fisheries. In recent years, scientists have developed a clearer understanding of the detrimental effects of dams on rivers, many of which are not immediately obvious to the casual observer. Small dams can:
More than 500 dams have been removed in the past century in the U.S. The majority of them have been less than 15 feet high. However, by some estimates, tens of thousands to possibly millions of small dams still remain in rivers and streams in the U.S. Most small dams do not protect communities from flooding and do not produce hydropower cost-effectively. But the cumulative impact of all these structures can be devastating to rivers and fish.
This is especially true for dams in headwater areas where rivers and streams originate. These small water bodies, often spring-fed coldwater streams (46 to 60 degrees F) that support coldwater species such as trout, are especially sensitive to temperature changes. Water is warmed as it sits impounded behind small dams, hindering the survival of coldwater fish above and below the dam. Selectively carried out, the removal of small dams can save money, permanently remove a public safety hazard, and provide additional financial benefits associated with rejuvenated fisheries, improved water quality, and an opportunity for community revitalization around a restored river. These and other converging societal, economic and environmental factors can truly make small dam removal a "win-win-win" proposition - for communities, dam owners and fish.
TU does not advocate for the removal of all dams. Many continue to provide important societal benefits. In cases of functioning, economically viable hydropower dams, for example, Trout Unlimited volunteers and staff work with dam owners and regulatory agencies to provide more "fish friendly" operations.