For more than 77 years, the iconic Hoover Dam has been producing dependable and cheap electricity for millions of customers in the Southwest.

In today's climate, the fact that hydropower is both renewable and carbon-free makes the federal dam and others like it across the West that much more integral to any national policy to control CO2 emissions.

Hoover dam

But the water level of Lake Mead behind the dam that straddles Arizona and Nevada has been in a steady decline and is already affecting electricity production at the man-made wonder, which was both the largest concrete structure ever built and the largest hydro plant in the world when it was turned on in 1936.

As a result, each of the power plant's 17 generating units with a nameplate capacity of 2,074 megawatts was derated in June; the current capacity is 1,592 MW and is projected to decline later this year. So the dam mostly provides power only during periods of peak demand, according to Mark Cook, engineering group supervisor at the Hoover Dam. "We pretty much shut down at night."

"It's kind of the perfect storm, or the perfect lack of a storm," Cook said during a recent tour of the power plant and massive dam that holds back the receding waters of Lake Mead, the largest reservoir in the United States.

Water is king

The Hoover Dam was built primarily to corral water for the booming population of the Southwest -- especially in Los Angeles and Las Vegas -- and to irrigate millions of acres of farmland. The hydroelectric power is a byproduct, the revenues from which allow the dam to be self-sustaining.

Hoover dam interior

Like much of the West, the Colorado River Basin that feeds Lake Mead is in the 14th year of drought. A stark white "bathtub ring" of mineral deposits encircles Lake Mead, evidence of water levels that are dropping at a rate of 1½ to 2 feet per month, according to Cook.

The drought has affected other federal dams, such as the Glen Canyon Dam in northern Arizona that holds back Lake Powell. For the first time ever this year, the amount of water released into Lake Mead was reduced from 8.23 million acre-feet to 7.48 million acre-feet, further putting pressure on the Hoover Dam's ability to fulfill its water and power missions (Greenwire, Aug. 16, 2013).

As the Lake Mead water level falls -- it was 1,083 feet, or 40 percent of the reservoir's capacity, on Friday -- there is less pressure as the water enters the intake towers and falls toward the turbines that run the electricity generators, Cook said. That drives the derating of the generators.

A Bureau of Reclamation projection published this month doesn't provide much comfort. It sees Lake Mead's water level falling to 1,064 feet by May 2016 and electricity production of 1,120 MW that month.

Asked about how low the water can go before producing power is no longer possible, Cook said, "We don't have a good answer for that," because it's never been experienced. But the official published number for the power plant's minimum generation pool is 1,050 feet, he said.

That number is about to be revised to 950 feet, he added.

Better technology improving power output

The engineers at Hoover are changing out the turbines and are in the process of a "major replacement" project, Cook said. It involves installing so-called wide-head turbines that operate more efficiently when water levels are low or high "so we're able to capture more of that energy than we ever have before."

"We also redid the control systems for the generators to increase their capabilities so we can bring them online faster, we can ramp them up and down faster," Cook said.

Hoover dam graph

The power plant is "more efficient than any time in its history. We're producing more energy than we otherwise would have with the amount of water" in Lake Mead, he said.

In theory, the Hoover Dam power plant can operate in perpetuity as long as there is water to fuel the turbines. "That's what the plan is," Cook said. "There's no plan to decommission this. We take a lot of care to change up bad pieces or maintain the pieces that we don't ever want to wear out."

Electricity is produced at the same time as the dam fills orders for downstream customers. Ranked by amount, that power is sold at a wholesale rate of 2.018 cents per kilowatt-hour to the Metropolitan Water District of Southern California, the state of Nevada, the state of Arizona, the city of Los Angeles, Southern California Edison Co. and 10 cities downstream.

In a pinch, Cook and his team "can push the full amount [of water] through at any given point in time" in an emergency -- such as if the 3,875 MW Palo Verde nuclear plant in Arizona went offline. "But we don't have the water to do it on a long-term basis."

Where the power plant once produced more than 4 billion kWh a year, that output was 3.7 billion kWh in 2009 and has been slipping since.

Randy Wilkerson is the spokesman for the Western Area Power Administration, which markets the low-cost power from Hoover and federal dams in 11 states. The cost "is why people love hydropower," Wilkerson said.

"The long-term concern though is: What is the effect of climate change on all this? Is that playing into this, or is this just part of a natural cycle?"