Congress is known for having arcane battles, but the biggest fight these days in water law is over a single word — and Texas environmentalists and ranchers are anxiously awaiting the outcome.

At stake is the future of the Clean Water Act, a 1977 law designed to reduce pollution in America's waterways. The law, which was built on a 1972 law of a different name, refers to "the discharge of pollutants into the navigable waters" of the United States.

But what does "navigable" mean? The Supreme Court has weighed in on the question twice in the last 10 years, and while appearing to narrow the term's definition, it has also left everybody confused. Now Congress is considering whether to remove the word "navigable" altogether, leaving the phrase to read, simply, the "waters of the United States." The Senate passed a bill out of committee a year ago that would strike "navigable," and a similar bill was introduced last month in the House.

Landowners and farmers hate the idea; they describe it as government overreach, certain to lead to needless regulation and extra costs. Environmentalists say the opposite: that many of the small, non-"navigable" streams that currently escape regulation are an essential part of the nation's drinking supply and need oversight.

Texas has a particular stake in the outcome, says Luke Metzger, director of the advocacy group Environment Texas, because it has "some of the most polluted rivers in America." The Brazos, for example, is the 13th-most-polluted river in the country in terms of discharges of cancer-causing chemicals, according to a recent report by Metzger's group.

The Clean Water Act does cover big rivers like the Brazos, but many of the tiny streams that feed into it are left alone. This was not always the case: For the first few decades after Clean Water Act was enacted, many of those small streams were regulated. But two Supreme Court rulings — Solid Waste Agency of Northern Cook County v. United States Army Corps of Engineers, in 2001, and Rapanos v. United States, in 2006 — have muddied the waters, so to speak. The Clean Water Act itself defines the term "navigable" as meaning "waters of the United States, including the territorial seas." But the Supreme Court rulings have suggested that the word "navigable" must be applied more narrowly. Thus, environmentalists say, three crucial types of water bodies, as well as some wetlands, are vulnerable to pollution: intermittent streams (which dry up in the summertime), ephemeral streams (which run only run after a big storm) and headwater streams.

Due to its hot climate, Texas also has plenty of streams that dry up in the summer and are therefore not "navigable." But they do provide drinking water to large numbers of Texans. According to the Environmental Protection Agency, more than 11.5 million people in Texas get a portion of their water from intermittent, ephemeral or headwater streams. Brewster County, along the Mexican border, for example, gets nearly all of its drinking water from such waterways (see this county by county map of Texas). By removing the word "navigable" and replacing it with "waters of the United States," such streams would fall under the act's pollution protections, environmentalists say.

Landowners, farmers and their allies in Congress think this is an awful idea and do not mince words. U.S. Sen. Kay Bailey Hutchison has said the bill, if it passes, would constitute the "largest federal land grab in our nation's history." She was writing in The Dallas Morning News last summer, shortly after the Senate version of the bill, called the Clean Water Restoration Act, passed out of the Environment and Public Works committee. It still awaits action by the full Senate.

Ranchers fear that the drainage ditches and stock tanks will be subject to regulation if "navigable" is struck from the law. "Ranchers would have to go get a permit from federal government before they could build any sort of livestock watering tank," said Jason Skaggs, the executive director for government and public affairs at the Texas and Southwestern Cattle Raisers Association.

Kirby Brown, the vice president for public policy for the Texas Wildlife Association, which represents many private landowners, argues that striking the term navigable would "remove the basis of the law itself, which is the Interstate Commerce Clause." Brown emphasized that his group wants to see wetlands protected. But he thinks there are other ways to achieve this, such as positive incentives that "provide ways for landowners to participate and be paid to protect riparian areas." He cited efforts by the Natural Resources Conservation Service, a federal body, and the Department of Agriculture. The Clean Water act has done a fine job over the years, he said, and there is no need to change it.

Environmentalists say that the notion that the EPA will start regulating small puddles and stock tanks is off-base. "EPA has no interest, let alone the resources, to start doing that. So we think that's a really absurd assertion," Metztger said. The idea, environmentalists say, is to bring the Clean Water Act back to where it was before the Supreme Court started tinkering in 2001.

U.S. Rep. James Oberstar, D-Minnesota, who introduced the bill in the House last month, insists that there are sufficient protections for ranchers. The bill would preserve existing exemptions for "farm or stock ponds or irrigation ditches," as well as for "normal farming, silviculture and ranching activities," he said in a letter aimed at reassuring the nation's farmers.

Most farmers, including Skaggs, are not satisfied. Doing away with "navigable," Skaggs said, "brings every water body in the state of Texas under the jurisdiction of the federal government."

The House bill is currently lodged in the Transportation and Infrastructure Committee, which Oberstar chairs. U.S. Reps. Solomon Ortiz, D-Corpus Christi, Pete Olson, R-Sugar Land, and Eddie Bernice Johnson (D-Dallas) are the three Texans on what Piper Crowell, the clean water advocate for Environment America (the national arm of Metzger's organization), describes as a "tough" committee. It is unusually large, with plenty of rural representation as well as vulnerable Democrats.

Nonetheless, Crowell said, "we know clean water is a very good vote for people to take."