If you asked me what America's best idea was, I'd say it’s the Bill of Rights. But according to Lord James Bryce, eminent British historian and ambassador to the United States in 1912, our national parks are “the best idea America ever had.” Wallace Stegner echoed that sentiment in 1982.

And now we have noted American documentary filmmaker Ken Burns (The Civil War, Baseball and Jazz) with The National Parks: America's Best Idea. The 12-hour series is scheduled for six nights on PBS starting Sept. 27

Stegner generally knew a good idea when he saw one. But even the best ideas need hard work to become – and remain – a reality. Because he thought the parks were suffering from neglect, one of Stegner’s contemporaries, the great Western writer Bernard DeVoto, proposed in 1953 that they should be closed and sealed off by the U.S. Army.

DeVoto argued that the National Park Service – suffering from a “fiscal anemia” that caused low staffing levels -- simply couldn't maintain the parks, let alone accommodate the surge in tourism that came after World War II. The Park Service had the same-sized staff in 1932 as in 1952, although visitor counts soared from 5 million to 35 million in those 20 years.

DeVoto's essay in Harper's Magazine caused a public outcry that pushed Congress to fund the Park Service more adequately. Even now, however, 56 years later, we still read of maintenance backlogs, recently estimated at $8 billion to revamp everything from crumbling roads to antiquated sewage-treatment plants.

And it's not as though national parks are getting more popular these days; Park Service figures show that the number of recreation-days peaked at 287.2 million in 1987 and dropped to 274.9 million last year. Even as America's population increased by 24 percent, park visitation declined by 3.6 percent. Overnight stays dropped 20 percent from 1995 to 2005, while camping declined even more, by almost 24 percent.

There are several theories about this decline. Some say that modern kids are so wired into texting and video games that they don't want to venture into the Great Outdoors at a national park. On the other hand, retiring Baby Boomers, a big population segment, have reached an age where tents and sleeping bags aren't nearly as appealing as motel rooms. In a nation with a growing minority population, national parks still appeal mostly to white folks, despite the Park Service's efforts to broaden its staff and appeal.

Doubtless there is some truth in all those factors, as along with rising gasoline prices and associated environmental and national-security concerns. Who's the better citizen? The one who saves gasoline and minimizes his carbon footprint and reduces oil imports from nations that hate us? Or the one who accumulates national park stickers on the back of a 24-foot travel trailer towed by a 9-mpg pickup?

The chambers of commerce of the West's gateway tourist towns are doubtless rooting for the latter. That's an angle that Burns will likely explore, because commercial interests have always been intertwined with our national parks, especially in the West. A century and more ago, Western railroads wanted to increase their passenger business, and there's nothing like a national park to inspire travel. Glacier National Park is essentially a creation of the Great Northern Railroad; Grand Canyon owes much to the Santa Fe, as does Yellowstone to the Northern Pacific. The railroads lobbied for the creation of parks, they promoted travel to the parks, and the park concessions were often built and operated by railroad subsidiaries.

But national parks can serve other purposes besides commerce and recreation. In fact, there's a modern use for the parks that America's Best Idea is unlikely to discuss: They can be an excellent way to deter water development. Consider two examples in Colorado.

Back in 2004 in Colorado's San Luis Valley, Great Sand Dunes National Monument expanded into Great Sand Dunes National Park and Preserve. The main impetus for expansion came from local activists, who wanted the federal government to buy the 97,000-acre Baca Ranch to keep its owner from tapping the deep aquifer beneath the Closed Basin.

Like Great Sand Dunes, Black Canyon of the Gunnison was declared a national monument in 1933 by President Herbert Hoover in his waning lame-duck days in office; it became a national park in 1999.

Its water story is more complex. At issue, though, is a simple question with a difficult answer: How much water does it take to maintain the park in something of a natural state?

According to naturalists, the 2,700-foot-deep canyon needs periodic flows of high water to sweep away the sand bars and related vegetation at the bottom, which wouldn't be there if the Gunnison River followed a natural flow regime.

It doesn't, because there are three large dams immediately upstream, as well as one on a major tributary, the Taylor River. Further, it didn't have a natural flow even back in 1933, on account of a 1909 water diversion, the Gunnison-Uncompahgre Tunnel, which takes water from the Gunnison River above the Black Canyon and conveys it 5.8 miles to irrigate 76,000 acres in the Uncompahgre valley above Montrose.

The federal government finally got around to “quantifying its reserved water right,” and in 2003, the feds and the state of Colorado came to an agreement on flow through the Black Canyon. It was challenged in federal court as insufficient, and earlier this year, a new and higher flow regime was agreed to.

This will help protect the Gunnison River from potential diversions to Colorado's ever-thirsty Eastern Slope -- but how much difference will it make to the park? From the rim of the Black Canyon, you can barely tell that there's a river down there, let alone determine how natural its channel might be. And of the 150,185 park visitors in 2008, only 1,256 (less than one percent) got backcountry permits to descend into the canyon where they might see the sandbars.

In other words, hardly anyone will notice. But just as the case with Great Sand Dunes, a national park comes in handy when you're trying to hold on to your water. It’s one of those good ideas that Ken Burns probably won't be talking about.