Folks from the Pacific Northwest and back East have been known to scoff at what passes for a "river" in the Southwest. The Los Angeles River is better suited to high-speed Hollywood car chases (Italian Job, Terminator 2, To Live and Die in L.A.) than traditional river activities. New Mexico's Santa Fe River is often a stagnant trickle, and the Santa Cruz River in southern Arizona is just a parched and sandy, concrete-confined wash through Tucson during most of the year. These streams are in a tattered state because humans have locked them into flood-control structures, sucked them dry with groundwater pumping, and wrecked their banks with overgrazing and development. But even in pre-industrial times, they would have been considered only minor streams, not rivers, in most of the country.

After a summer thunderstorm, however, the Santa Cruz roars through Tucson in a torrent, putting other rivers to shame. It's been known to swell to the size of the Colorado River through the Grand Canyon, even reaching 50,000 cubic feet of water per second on one occasion, which rivals the Missouri River as it meets up with the Mississippi.

About 95 percent of Arizona's rivers and streams -- or virtually everything in the state except the Colorado, Gila and Salt Rivers -- are ephemeral or intermittent, running only part of the year. The percentage is nearly that high in Nevada, New Mexico and Southern California, compared to 59 percent of streams in the nation as a whole.

But these non-perennial streams offer the same benefits to land, wildlife and plant life as year-round rivers, according to an Environmental Protection Agency study published last fall.

When functioning properly, even sandy arroyos can slow high-water flows enough to keep streambanks from washing out. Ephemeral and intermittent streams in the desert can replenish the aquifers that serve people. And they generate the desert's richest collections of trees and shrubs, offering food, cover, nesting and movement corridors for wildlife.

Though these streams are often bone-dry, they have long been considered "waters of the United States," protected by the federal government.  If someone wanted to fill in an arroyo, the Corps would visit the site and determine its ordinary high-water mark. Land within that area was then deemed "jurisdictional," meaning that anyone wanting to build there needed a permit under Section 404 of the Clean Water Act. Anyone wishing to discharge pollutants needed a permit from the Environmental Protection Agency, under Section 402 of the act.

Today, the Army Corps issues about 100,000 404 permits annually, giving each of the agency's staff of about 1,200 regulators dozens of cases each year.

Permits are seldom denied, but most are issued with conditions attached. If a developer or road builder clears trees along a wash, for example, he might have to plant three trees for every one destroyed, or pay a conservation group to restore other areas, or develop in such a way as to protect the most significant portion of a wash.

Getting the permits can be exhausting and expensive. A 2002 peer-reviewed study found that the typical 404 permit costs $271,596 and takes 788 days to obtain. (The Corps itself says that a typical permit costs about $24,000 and takes 187 days.)

In any event, says Jonathan Adler, a law professor at Case Western Reserve University who is generally sympathetic to property-rights issues, the hassles associated with Section 404 "did more than any other single federal program to stoke the fires of property-rights activism."