Conventional hydropower accounted for more than 6% (258,749 gigawatt hours) of total net U.S. electricity generation in 2014. The United States has considerable hydropower potential beyond what is already developed. Although there is some interest in increasing the current level of hydropower investment, such investment remains limited, in part because of federal and nonfederal financial constraints, high uncertainty in electricity generation policies and markets (which may dissuade capital investment), and the environmental operating requirements required for hydropower projects. Some hydropower proponents are pursuing policies to reduce what they view as impediments to hydropower development. Other stakeholders prefer that hydropower investments only proceed when they are protective of other interests, including other water users and the aquatic environment and its species. At issue for the 114 th Congress is whether, and if so how, to change federal support for hydropower and what priority to give hydropower vis-à-vis other energy investments, and social and environmental concerns. These issues are ongoing and are shaped by the introduction and consideration of hydropower-related bills from past Congresses. For example, more than 25 bills dealing with various aspects of hydropower were introduced in the 112 th Congress, a quarter of which were state- or site-specific legislation. And more than 30 hydropower-related bills were introduced in the 113 th Congress. For example, Congress passed P.L. 113-20 (Bonneville Unit Clean Hydropower Facilitation Act), P.L. 113-23 (Hydropower Regulatory Efficiency Act of 2013) P.L. 113-24 (Bureau of Reclamation Small Conduit Hydropower Development and Rural Jobs Act), and P.L. 113-121 (Water Resources Reform and Development Act of 2014). So far, four hydropower-related bills have been introduced in the 114 th Congress: S. 1236 (Hydropower Improvement Act of 2015), S. 1264 (Renewable Electricity Standard Act), S. 1338 (Small Hydropower Dependable Regulatory Order Act of 2015), and S. 1270 (Reliable Investment in Vital Energy Resources [RIVER] Act). Over time, there has been a variety of hydropower legislative proposals and stakeholder opinions, which stem partly from differing views of the benefits and costs of hydropower development. Many have pointed out advantages and drawbacks of conventional hydropower. Cited advantages include its possible renewable energy status, zero to minimal greenhouse gas emissions during operation, and high operational efficiency. Supporters also note its ability to generally serve as a reliable and flexible domestic energy source. Generally speaking, hydropower generation can be dispatched on relatively short notice, and can supplement shortfalls in generation. However, others note that conventional large hydropower has relatively high initial capital costs, can be detrimental to surrounding ecosystems (e.g., fish and wildlife), may not be reliable during low water years and seasons, and may disrupt recreational or scenic values. The legislative environment for hydropower can be complicated. Whether the hydropower project investor is a federal or nonfederal entity dictates which laws apply and thus may affect which committees are involved in oversight or changes to such laws. Several federal government agencies own and operate large hydropower projects, while other agencies administer the process by which nonfederal hydropower projects are built, maintained, and operated. Large federal hydropower projects are managed primarily by the U.S. Department of the Interior's Bureau of Reclamation (Reclamation) and the U.S. Army Corps of Engineers (Corps). The power from these projects is generally marketed by the Department of Energy's Power Marketing Administrations (PMAs). FERC regulates investigation, construction, and operations of nonfederal hydropower projects as well as overseeing dam safety for nonfederal projects. This report explains how the federal government is involved directly in hydropower generation at federal facilities and in the regulation of nonfederal hydropower generation; the focus is on current roles and processes and common concerns and questions about changing those roles. Most of U.S. hydropower capacity is from conventional hydropower. Conventional hydropower plants take three general forms: storage (or impoundment), run-of-river (or diversion), and pumped storage. A storage plant uses a dam to store enough water in a reservoir so that, when released, it flows through a penstock to a turbine, spinning it, which in turn activates a generator to produce electricity. A run-of-river plant directs a portion of a river through a canal or penstock to generate electricity without the need for a reservoir. A pumped storage facility stores energy and generates electricity by pumping water from a lower reservoir to an upper reservoir during off-peak hours, releasing the stored water from the upper reservoir to the lower reservoir during periods of higher electricity demand. Conventional hydropower is a significant contributor to the national electric power portfolio. Hydroelectric facilities produced 6%-9% of U.S. electric generation between 1998 and 2014. When considered a renewable power source, hydropower was the largest contributor to renewable electric power generation in 2014, followed by wind and by wood and wood-derived fuels. The western states produced the most hydropower in 2014, but the top individual hydropower-producing states were Washington, Oregon, and New York. As the West continues to experience drought, hydropower generation may decline. For example, in 2014, hydropower in California (the fourth-largest producer of hydropower among the states) contributed half (10%) of its average generation (20%) to California's energy portfolio. Generally, reductions in hydroelectric generation are addressed by increasing the use of natural gas, which some argue is more expensive and produces more greenhouse gas emissions than hydropower. Facilities in western states, such as the Hoover Dam, are producing significantly less power than average due to drought conditions. There is an ongoing debate about whether hydropower should be characterized as "renewable." It has historically been characterized as renewable because it is a replenishable resource. More recently, others have asserted that it is not renewable because of its size (e.g., in the case of large-scale projects) and environmental impact, particularly on ecosystems, and in some cases induced evaporation (particularly at reservoirs at low elevations in dry climates) and greenhouse gas emissions during construction. However, for purposes of this report, hydropower is discussed as renewable because it does not originate from a fossil fuel (i.e., its source is not finite). The precise number of hydropower projects in the United States is unknown. Different databases yield different results based on selected criteria. As of 2015, 90 nonfederal hydropower projects are licensed to operate at Corps dams and 28 are licensed to operate at Bureau of Reclamation sites. The public sector (federal, states, cooperatives, and municipalities) owns the majority of hydropower capacity because it owns mostly large (>30 megawatt [MW]) hydropower plants (see Figure 1 ). The private sector (e.g., private utility, private nonutility, and industrial entities) owns the majority of hydropower projects, and many are small hydro (1 MW-30 MW) and low-power (< 1 MW) plants (see Figure 2 ). Small hydro and low-power plants make up of 83% of total plants but constitute approximately 10% of total hydropower production capacity in the United States. In contrast, large hydropower plants are responsible for approximately 90% of total hydropower capacity though they make up 16% of total plants. Without considering economics, current hydropower capacity represents a fraction of U.S. hydropower resources that technically could be developed at non-powered dams in addition to accessing new stream-reaches. The National Inventory of Dams identifies that of the more than 80,000 dams in the United States, 2.5% are used for hydropower. According to a 2012 Department of Energy (DOE) study, there are more than 54,000 non-powered dams with a total potential capacity of 12 GW, which would increase hydropower capacity by 15% (See Figure 3 ). The majority of this capacity (8 GW) is concentrated at 100 non-powered dams, 81 of which are operated by the Corps. Some have argued that many of the monetary costs and environmental impacts of construction already have occurred at non-powered dams, so using non-powered dams for hydropower is an option for increasing the U.S. renewable energy portfolio. However, the actual cost and economic feasibility of powering these dams is unknown, and additional costs for site-specific studies may be required before hydropower capacity could be added. The federal government owns and operates approximately half of all U.S. hydroelectric generating capacity. This capacity is principally at large multi-purpose dams owned and operated by the Corps or Reclamation. Combined, the Corps and Reclamation operate almost all federal hydroelectric dam capacity (90% of federal capacity). Other federal entities operating hydroelectric generation facilities include the Tennessee Valley Authority, the Bureau of Indian Affairs, and the International Boundary and Water Commission. Other entities also own facilities that generate hydroelectric power, but they are relatively small. While the electricity generated by these facilities is owned by these federal agencies, the Power Marketing Administrations (PMAs), which are part of the U.S. Department of Energy, are generally responsible for selling and distributing this power. Federal hydropower capacity varies substantially by state and region. For instance, 90% of all federal capacity is found in 13 states, and the majority of this capacity is in the West. Washington and California contain the greatest federal hydroelectric capacity, while Oregon, Arizona, Montana, and Idaho also have significant federal hydropower capacity.