Many of the problems with modern hydropower projects stem from their massive size. Focusing on smaller-scale hydroelectric development and building lower dams can mitigate most of these impacts. Lower dams reduce the amount of water stored and the area flooded. Because small dams store little water, they create little change in river flow and have minor effects on the temperature or chemistry of a river. They also displace few or no people.

One common alternative to erecting large dams on the main channel of a river is to erect smaller power plants. These run-of-the-river power plants, which provide little or no water storage in a reservoir, divert a portion of river flow through pipes that pass directly through a turbine (Figure 10.45). These systems have their own impacts, but there are ways to mitigate them. First, water must flow continuously through the main river channel at reasonable water levels to maintain ecosystem health and biodiversity. In addition, fish still need a bypass system to permit them to swim beyond the diversion dam. However, with a low dam, it’s much easier to build such a bypass system with relatively natural structures and flows (Figure 10.46).

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Many dams were built in the early 20th century for water storage and flood protection—not hydropower production. In such situations, we can add hydropower turbines without building new dams. Researchers at the Oak Ridge National Laboratory point out that there are 54,000 unpowered dams in the United States alone (Figure 10.47), and they estimate that the top 100 could add 8 gigawatts of installed hydroelectric capacity to the U.S. electrical grid. That’s equivalent to four Hoover Dams (see Figure 10.12, page 304). The greatest potential for such developments is found in the nation’s largest rivers, including the Mississippi, Ohio, and Arkansas Rivers—notably, rivers mainly found in regions of the country not particularly rich in wind or solar energy.

It’s not always necessary to build a dam to harness the kinetic energy of a river (Figure 10.48). In 2009 one such electric turbine was placed directly in the Mississippi River at Hastings, Minnesota. The turbine actually sits beneath the outflow of an existing, conventional hydroelectric power plant. Tests of the turbine installed at Hastings, which spins at 21 revolutions per minute, showed that over 97% of fish were able to pass through it without injury. Similar projects are planned for many of the larger rivers of the eastern half of the United States, with a total potential generating capacity of 500 megawatts.