There are mayors in some towns whose fondest municipal wish might be a fat federal grant to build a library or buy a new fleet of fire trucks. Dorothy Johnson, the mayor of Appleton, has something else on her mind.

"What this city needs right now," her honor said recently, "is to find some nice snail darters in Mud Creek."

It is not any particular affection for that tiny aquatic species that prompts this wish. What the mayor is really fishing for is the state and federal regulation that would follow if the snail darter, or any other legally protected flora or fauna, were found in Mud Creek, an appropriately named stream (average depth, six inches) that trickles sluggishly through a cornfield just outside Appleton's city limits.

Mud Creek is Appleton's last line of defense in a furious battle that may shape the city's future for decades. The city's weapons are environmental protection laws -- but the battle of Mud Creek is not an "environmental" matter at all.

Indeed, Appleton's Mud Creek offers a classic example of how federal regulation spreads power, throughout the society by giving citizens new forms of legal leverage over events. Organized interest groups, whether they are environmentalistgs or corporate trade associations, have been using this leverage for years; now others are discovering it, too.

"It became pretty clear in my mind what was really going on here," says one of the federal inspectors who have been drawn into the battle, "when I realized that the outfit that was raising the most hullaballoo about this creek was called the Save-the-Downtown Committee."

What is really going on here is an all-out effort by Appleton's downtown merchants to stop construction of a giant shopping mall planned for the cornfield through which Mud Creek flows.

About a year ago, when it seemed that all other efforts to block the project might fail, the merchants and the municipal government hit on Mud Creek. They have been battling ever since to save the stream and stop the shopping center.

To the company that wants to build the $40 million mall -- a Des Moines, Iowa, shopping center concern called General Growth Development -- Mayor Johnson's quip about the snail darter seems perfectly appropriate. General Growth sees the Mud Creek affair as a classic case of something developers call the "snail darter syndrome."

"This whole business has snail darter stamped all over it," growls James Long, who was Ronald Reagan's campaign chairman here last year and has been retained as General Growth's local lawyer. "What they did to us is, they just went shopping for anything they could find to harass us -- and they found this creek."

General Growth's complaint is echoed by business concerns and government agencies from coast to coast that have found themselves ensnarled in "environmental" litigation that actually seems to have a social or economic basis. The celebrated snail darter of Tennessee was identified as an endangered species but the real objective was to halt construction of TVA's Tellico dam. At the same time, Appleton's explanation for its tactics echoes the belief of many "public interest" activists that environmental laws often provide the only means for the public to take part in major decisions.

"This mall is going to be the most important land-use decision in the region for the next 20 years," says Peter Peshed, a lawyer from the Wisconsin "Public Intervenor" office who is helping the anti-mall forces. "And you had all the government agencies just avoiding the issues. We pulled in Mud Creek and forced them to pay attention."

General Growth is no stranger to creative uses of federal regulations. The firm, which operates profitable shopping centers in 22 states, has organized itself in such a way that its development arm is exempt from federal income taxes. And it has successfully overcome local objectors in building some of its other malls.

But Appleton has proved to be a particularly tough foe.

This is fitting, because Appleton, at least on paper, seems a particularly sweet spot for the 952,000-square-foot emporium General Growth wants to build. This prosperous city, retail hub for a half-dozen towns with a total population of 150,000, has somehow entered the 1980s without any of the sprawling suburban malls that loom at the fringes of most American cities.

Appleton's chief retail district is still right in the heart of town. The 10 central blocks of College Avenue, the city's main street, are dotted with locally owned department stores, restaurants, book stores and pharmacies -- creating the kind of neat, wholesome and tax-generating central business district that many other citie Appleton's size lost years ago.

"We have a nice, busy downtown," Johnson says. "No peep shows, nothing boarded up. But you see, if they put that mall out there, they're moving my tax base and my work force into a cornfield outside the [city] limits. This is why we have to fight."

The downtown merchants and the municipal government were off and fighting from the very day in September 1979 when General Growth announced its lavish plans for the new center on a 100-acre plot in the town of Grand Chute, less than a mile outside Appleton's border.

But it was soon evident that the mall's opponents had minimal legal ammunition. Grand Chute's municipal government, and the relevant county and state agencies, made it clear that the necessary permits and variances would be granted -- regardless of objections from downtown Appleton.

"It has been exasperating," says Mayor Johnson. "It is our downtown that is going to be most seriously affected, but my means of objecting were extremely limited. I had no way to force people to step back and take a careful look at this."

In desperation, the anti-mall forces resorted to various forms of guerrilla warfare. Two members of the Save-the-Downtown Committee bought stock in Sears, Roebuck & Co. and launched a futile proxy fight to convince the chain to stay out of the mall. A group of downtown backers hired a competing developer, National Rfedevelopment, to plan a competing mall in downtown Appleton, but this fell through when the Appleton Post-Crescent reported the National Redevelopment's vice president had a criminal record.

And then somebody on the city's side of the battle noticed something on maps of the mall site: a tiny stream passing right through the place where General Growth plans a 5,000-car parking lot. The maps call this waterway "Mud Creek."

It isn't much of a waterway. Not only does its depth average half a foot, but at most places along its 6.6-mile course, it is narrow enough for school-boys to jump across without a running start.

But the lawyers knew that it didn't take much of a stream to trigger the various laws protecting waterways and their inhabitants. One day last spring -- when Mud Creek was swollen with winter run-off -- anti-mall activists launched two canoes and paddled a few hundred years along a wide spot in the stream. This brought Mud Creek legal recognition as a "navigable stream" -- and brought General Growth a whole range of potential legal headaches. The law does not permit such a waterway to be paved over for parking.

The law does not permit such a waterway to be paved over for parking.

Next, the anti-mall forces asked the Army Corps of Engineers to assert jurisdiction over Mud Creek through federal laws protecting "wetlands." The corps declined -- its inspector concluded that the battle over the stream involved "l,ocal political issues primarily or totally non-aquatic-environment-oriented." But Peshek, the anti-mall lawyer, hints that a lawsuit may follow if the Army doesn't change its mind.

"Even after all the local and country [permits] are taken care of, "Peshek says, "Mus Creek provides a jurisdictional basis for us to keep going."

Despite the heat on both sides, the combatants in the battle of Mud Creek sometimes take a minute to contemplate the larger questions their dispute raises. Is it right that national environmental laws should become the chief weapons in a commercial struggle over a piece of suburban real estate? On the other hand, why is it that a city has no other weapons with which to force public consideration of an action that could affect it, esthetically and economically, for decades to come?

But these diversions into the cosmic aspects of the case do not come often. People here are too busy fighting the concrete question, shopping center or no shopping center?

Long, the developer's local lawyer, finds that kind of talk maddening. "This is just total waste of taxpayer's money, total obstructionism," he says. "Fortunately, General Growth has some resources and we aren't going to back down . . . There is just no way this can go on much longer."

Down at City Hall, Johnson disagrees. "We will keep fighting. We have no choice," she says. "If we could just find a snail darter! Where are you, snail darter? Where are you?"