Seventy-five years ago, President Franklin Roosevelt declared Hoover Dam -- then called Boulder Dam -- "a marvel of the 20th century." But I predict that when the dam turns 100 in 2035, no one will be celebrating what now appears to be a 20th century folly.

The third decade of the 20th century and the first decade of the 21st have two things in common: a drought of water and a dearth of economic prosperity. Hoover Dam was supposed to prevent both from ever happening again, yet here we are, faced with a shrinking reservoir even as nearby Las Vegas faces diminishing prosperity.

If the water level of the reservoir continues to fall toward empty -- as thoughtful scientists from the Scripps Institute have predicted -- the Colorado River downstream will behave much as it did before Hoover Dam was built. It will once again become a fluctuating river that is incapable of supporting a robust agricultural empire or the metropolitan sprawls of Las Vegas, Los Angeles and Phoenix.

It was a heady dream, the idea of forcing a river like the Colorado to do our bidding. Now, however, the reservoir is down 133 feet to its lowest level ever. That’s disheartening enough, but imagine what it will be like if the reservoir succumbs to any of the other challenges it faces such as structural fatigue or sediment fill.

Lake Mead is the largest manmade reservoir in the United States, and its upstream neighbor, Lake Powell, is the second largest. But among the rivers in the 50 states, the Colorado River only ranks 25th by volume. It took years for the two reservoirs to finally top off in tandem. That finally happened in the early 1980s, only because there was enough surplus water available for long enough to make the dream come true.

That is clearly not the case anymore. Consumption of water from both dams now exceeds supply, according to the Bureau of Reclamation, which is conducting a two-year study to assess supply and demand. When you think about it, it seems ludicrous to believe that the 25th largest river could ever keep filling up the country’s two largest reservoirs, simply to satisfy a segment of the population that is hell-bent on developing the desert Southwest’s remaining empty places. I trust we have all come to realize that any water that is saved through conservation programs is quickly diverted into yet more development projects.

The Colorado River has a yoyo personality, and some temporary reservoir recovery may eventually occur. But does anyone really doubt that the day of reckoning is coming? Usually, the region’s nasty droughts only last four or five years. That certainly was the hope in water-year 2005, when the basin jumped to an annual yield of 105 percent. Unfortunately, the anticipated recovery fell flat over the following four water years, with the 10-year annual yield averaging only a dismal 69 percent. The Colorado River would have to increase its annual yield to 131 percent to refill both reservoirs in the same amount of time.

The last decade has been the most severe 10-year drought the Colorado River has experienced since records began in the 1880s; even the 1930s Dust Bowl decade had a better hydrologic conclusion. And now we are on the cusp of an early and strengthening La Niña, which bodes ill for the health of the Colorado River’s reservoirs.

Congress was not ignorant of the Southwest’s chronic water scarcity. As early as 1953, Northcutt Ely, son of the city manager of Boulder City, told Congress that although creating Lake Powell would double the storage capacity of the Colorado River, it would only be a good idea for the first 50 years. After that, he said, it would be impossible to maintain both reservoirs at full capacity.

Congress decided to go ahead and build the new dam called Glen Canyon anyway, putting off the issue of water scarcity for as long as possible. In 1968, Congress passed legislation encouraging the importation of water from other river basins to supplement the needs of the seven Colorado River states and Mexico. But no schemes for importing water have arrived ahead of schedule and for a fair construction price, the way Hoover Dam did. River communities outside the area are not that eager to share their water with the Southwest’s unquenchable desert dwellers.

Hoover Dam may have been an engineering marvel, but it’s time to make some serious adjustments and adapt ourselves to the limits of a drying desert.

John Weisheit is a contributor to Writers on the Range, a service of High Country News (hcn.org). He is the conservation director of Living Rivers, based in Moab, Utah.