What's In A Name?
Hoover Dam is named for Mr. Herbert Hoover, the Nation's 31st president. When construction of the dam was initiated, on September 30, 1930, Secretary of the Interior Ray Lyman Wilbur ordered that the dam to be built in the Black Canyon of the Colorado as part of the Boulder Canyon Project Act be called "Hoover Dam". By a Congressional Act of February 14, 1931, this name was made official.
After Mr. Hoover left office, the names "Boulder Canyon Dam" and "Boulder Dam" were frequently used when referring to the dam, allegedly because the new Secretary of the Interior did not like Mr. Hoover. However, the name of the dam was never officially changed from "Hoover." In the 80th Congress (1947), a number of bills were introduced to "officially" restore the name of Hoover Dam. On March 4, 1947, House Resolution 140 was introduced for this purpose.
Source: http://www.usbr.gov/lc/hooverdam/History/articles/naming.html