Less than a month ago Paul Dresler, a government marine biologist, reported finding for the first time a concentration of potentially troublesome Asian clams in the Potomac River near the Wilson Bridge, Hains Point and the 14th Street Bridge.

The clam, called Corbicula manilensis, was found at concentrations of up to 62 per square foot and could clog water intakes and contaminate sand and gravel dredged for concrete, although no problems have been reported so far.

A few days after Dresler's finding was published, Allen Sinnott, a government hydrologist, reported discovery of a vast underground reservoir -- containing an estimated 140 trillion to 350 trillion gallons of good quality water -- covering a 108,000 square mile region of Virginia, Maryland, the District of Columbia, West Birginia, New Jersey, Delaware, New York, Pennsylvania, Vermont and Massachusetts.

About half of the underground reservoir could serve as a reserve water supply during such emergency periods as regional droughts, Sinnott said.

The two findings are unconnected except for one link: Both are products of the U.S. Geological Survey, an organization that describes itself as "the federal government's largest earth science research agency."

To put it simply, the Geological Survey is in the business of finding out everything there is to know about the earth -- and more recently some of the planets. Its projects range from major research efforts with millions of dollars riding on the results to arcane scientific studies with little apparent practical significance.

Established on March 3, 1879, during the administration of President Rutherford B. Hayes, the U.S. Geological Survey will celebrate its 100th anniversary Saturday with a 10 a.m. ceremony and open house at its headquarters in Reston.

A Navy musician has written a special march for the occasion to be played in public for the first time by the U.S. Navy Band. President Carter and the Secretary of the Interior have been invited to attend, and a special postal cancellation will be used beginning today in honor of the Geological Survey's Centennial.

"To make decisions for rational use of lands, to explore for and develop our resources wisely, to mitigate the effects of geological hazards -- these and other related problems are formidable and sobering challenges to all Americans," said the Survey's director, H. William Menard, discussing his agency's centennial.

In March 1879, when Congress forst appropriated funds for the Survey, the nation was emerging from the post Civil War era and the West was opening for settlement. Everyone needed maps, geological data, information on the availability of water and on the suitability of a particular region for cattle ranching, homesteading, sheep farming or mining.

Until then, such assignments had been handled -- if at all -- by state agencies, the U.S. Army and a few ad hoc federal agencies. The Survey was the first federal agency to be charged with a comprehensive "classification of the public lands, and examination of the geological structure, mineral resources and products of the national domain" in that vast, and for the most part, uncharted region.

In the century since then, it has evolved into a major government agency with a budget of $640 million and more than 13,000 scientists, engineers and technicians and administrators. The Survey has its national center in Reston, regional centers in Denver and Nenlo Park, Calif., and hundreds of subcenters throughout the United States and overseas.

Based on its data, decisions were and continue to be made on where to locate dams, mines, housing developments, highways and airports and where to drill for oil.

"There is hardly anything that has to do with the environment that we don't collect some basic data on," said Frank Forrester, the Survey's publicity director. "It's up to us to come up with the very basic and fundamental information, but we try to stay out of the fights between the environmentalists and the developers."

From an agency that in its beginning concentrated on mapmaking, the Gelogical Survey has broadened its scope to include such activities as searching for strategic minerals during the World Wars, pioneering in the use of aerial photography for mapmaking for the Tennessee Valley Authority in the 1930s, measuring the rate of flow in the nation's rivers and managing petroleum exploration in Alaska's National Petroleum Reserve.

In recent years, the Survey has published books reports and studies on such topics as:

The type of building stone used in historic monuments and government buildings in Washington;

The annual flow of water in the Potomac River,

How much water an inch of snow covering one acre of ground would produce;

The environmental impact of a proposed coal stripmine expansion in Montana;

A color map of Yemen made from satellite images taken from 570 miles above the earth;

The likelihood of future eruptions on Mount St. Helens and Mount Baker, two volcanoes in Washington State;

The predictability of California mudflows,

And a count of small aquatic insects in a Boone County, W. Va., stream that had to be rerouted because of highway construction.

For the year 1978, nfothe Survey found the average rate of flow in the Potomac was 9.5 billion gallons a day, or about 61 percent higher than the normal annual average of 5.9 billion gallons a day.

The highest daily flow occurred March 16, when 90.4 billion gallons were monitored, the lowest Nov. 9, when the count was 1.4 billion gallons, according to Myron Lys, a hydrologist at the Survey's Towson, Md., office. Water diverted from the river for municipal use in the Washington area averaged 309 million gallons a day, with a high of 397 million gallons on July 24.

In January, the Survey reported that an inch of snow falling evenly on one acre of ground is the equivalent of 2,715 gallons of water, based on a rule of thumb that 10 inches of snow equals one inch of water.

However, the Survey hydrologists added that the figure could vary substantially depending on whether the snow is dry and powdery or heavy and wet. An inch of wet snow could yield as much as 5,300 gallons of water, but an inch of light powdery snow might yield only about 1,300 gallons.

Besides the Centennial Day activities Saturday in Reston, other events include lectures and symposiums in the Washington area and elsewhere, exhibits and publication of a four-volume history of the Survey.

"While centennial activities will reflect Survey contributions to the nation's growth, they will also serve to focus on the challenges of the future to the earth science community," said Menard, the 10th director of the Survey.