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Lester Allan (or Allen) Pelton (September 5, 1829 – March 14, 1908) was an American inventor who contributed significantly to the development of hydro-power and hydroelectric power in the old West and world-wide. In the late 1870s, he invented the Pelton water wheel, at that time the most efficient design of the impulse water turbine. Recognized as one of the fathers of hydroelectric power, he was awarded the Elliott Cresson Medal during his lifetime and recently was inducted into the National Inventors Hall of Fame. 

Pelton's ideas for improving the turbine water wheel came from his studies of mining equipment and operations in California's gold rush country. Summary descriptions of the local technology observed by Pelton, and of the science by which his turbine water wheel extracts kinetic energy from a coursing mountain stream follow.

Steam-heat powered much of local mining activities but required a lot of wood for fuel; nearby forests were routinely decimated. Turbine water wheels also were used to supply power, but these were inefficient in converting the kinetic energy of mountain streams to horsepower. D.P. Stern reports: "According to a 1939 article by W. F. Durand of Stanford University in Mechanical Engineering, Pelton's invention started from an accidental observation some time in the 1870s. Pelton was watching a spinning water turbine when the key holding its wheel onto its shaft slipped, causing it to become misaligned. Instead of the jet hitting the cups in their middle, the slippage made it hit near the edge; rather than the water flow being stopped, it was now deflected into a half-circle, coming out again with reversed direction. Surprisingly, the turbine now moved faster... ..That was Pelton's great discovery. In other turbines the jet hit the middle of the cup and the splash of the impacting water wasted energy." 

Experimenting and modelling, Pelton improved upon the efficiency of the Knight wheel (developed earlier by the Knight Foundry at nearby Sutter Creek). The Knight wheel received the streamflow jet slightly off-center and at an angle into a single turbine cup. Alternatively, the Pelton wheel—by deploying a split double cup (in effect two cups side-by-side), then splitting the impinging water-jet directly onto the common vane of the double cup—captured a stream's kinetic energy more efficiently. There were two prime results of Pelton's design: it consolidated the introduction of a new physical science into the ancient human quest to develop hydropower, i.e., the science of the impulse turbine as opposed to the reaction turbine; and it revolutionized the use of turbines adapted for high head (i.e., elevation energy) sites. Before Pelton, almost all water turbines were reaction machines powered by water pressure, or head, while Pelton's wheel was powered by the kinetic energy of a high velocity water-jet  which could be conveniently developed from a small mountain stream.

(The Pelton design can be approximated by cupping both hands vertically then bringing them together, aligning the fingernails 'back-to-back' to represent the splitter vane between the cupped hands.  The split double cup, also called the split bucket, may be seen here; or here—showing the original Pelton model at the original site of manufacture, Miners Foundry - Allans (sic) Machine Shop, Nevada City, California.) 

Pelton died in California on March 14, 1908, at age 78; he was buried at his family site in Vermilion, Ohio. His Pelton Runner design is still used today to produce hydroelectric power in the United States and around the world, as shown here. Later designs such as the Turgo turbine, first patented in 1919, and the Banki turbine were inspired by the Pelton wheel.

In 1895 The Franklin Institute, Philadelphia, US, awarded Lester Pelton the Elliott Cresson Medal—now renamed the Benjamin Franklin Medal—for his accomplishments of invention in technology. Pelton was inducted into the National Inventors Hall of Fame in 2006.  There are memorials and monuments celebrating Pelton and the Pelton Runner mounted in Camptonville, California, in the Miners Foundry in Nevada City, California, and at the Smithsonian Museum in Washington D.C., California Resort at Disney Land, among other sites.