The short answer is that the washes were named after the hotel-casinos, rather than the hotel-casinos being named after the washes.
Indeed, the Strip cross thoroughfares -- Sahara, Flamingo, and Tropicana, among others -- were named after the hotel-casinos that they passed; the washes were named after the avenues, which they roughly follow.
Nothing in Las Vegas Valley had anything to do with flamingos, the large pink wading birds that live near shallow lakes, lagoons, mangrove swamps, and tidal flats, as well as on sandy islands above the low-tide mark, before the Flamingo Hotel-Casino opened in 1946.
Likewise, all Las Vegas knew about a "Tropicana" before the casino of the same name opened in 1958 was, as far as we can tell, that it was a nightclub in Havana’s Marianao neighborhood that opened in 1939 (and still exists) and one that opened in Manhattan as a Cuban music club in 1945.
The slightly longer answer is that Las Vegas is situated on a desert alluvial fan characterized by parallel stream networks and wide shallow channels (washes) that all empty into Las Vegas Wash, which drains the entire area into Lake Mead (and drained it for countless millennia into the Colorado River before Hoover Dam was built).
Las Vegas Wash is a 12-mile-long channel that’s fed by runoff from the Spring Mountains (after which the cross-street Spring Mountain Road is named) via Duck Creek, Las Vegas Creek, Flamingo Wash, Tropicana Wash, Tule Springs Wash, and a number of other natural washes and streams and man-made channels, tunnels, and detention basins to control floods in the metropolitan area.
Flash floods in Las Vegas can be very dangerous; in the past 80 years, a full dozen floods have devastated property and claimed dozens of lives. The largest, known as the Hundred Year Flood, occurred on July 8, 1999, when three inches of rain fell in 90 minutes and caused President Clinton to declare Clark County a federal disaster area.
Some of these washes are dammed. For example, Upper Flamingo Wash Dam, between Hacienda Ave. and Russell Road just east of Durango, was completed in 1992. It has an average surface area of 90 acres and drains an area of 19.1 square miles. The Tropicana Wash Detention Basin is between Decatur and Valley View boulevards just south of Russell Road and drains an area of 172 square miles.
For a look at the man-made part of Clark County’s flood-control efforts, read our book Beneath the Neon: Life and Death in the Tunnels of Las Vegas.
