To Gilbert Gude, former congressman from Maryland, the Potomac River's origin is not just a stumper a la Trivial Pursuit. (For the record, the source is a West Virginia spring near a ridge in Great Backbone Mountain, just the other side of Maryland's southwestern boundary.) In 1975, as part of an unsuccessful effort to have the Potomac designated a national river, he devoted a congressional recess to hiking its 400-mile length. What he found at the river's headwaters was a world shaped by the economics of coal and the vagaries of mining disasters, "a piece of Appalachia . . . markedly different from the foothills and coastal plains of the east."

Now director of the Congressional Research Service in the Library of Congress, Gude has returned to the North Branch Valley several times, interviewed the residents and sifted through a daunting array of reference material. (His bibliography runs to 12 pages of small print.)

Amid all these data Gude keeps a nice eye out for what writer Evan Connell calls "the luminous detail" -- that extra ray of small-scale fascination that gives an anecdote staying power. While sketching the career of E.E. Sollars, company doctor for Kempton, Md., Gude breaks off from a report of Sollars' obstetric fees ($10 for delivering a baby, $15 if nursing care was needed) to give us this bright aside: "One couple who saved up the money kept it under the living room rug until they worried that someone would notice the bulge, whereupon they changed the ones to a single ten."

In one of his best passages, Gude assembles a welter of facts into the town's demographic portrait. In Kempton there was Main Street -- and everywhere else. "Second, Third, Fourth, Fifth, and Sixth streets ran up the hillside. The streets were unpaved and difficult to travel in wet or icy weather. At any given time several factors in a culture will determine which building material is most desirable, but wood seems to have always been universally considered a second-class material for sidewalks. As wooden sidewalks ran beside the streets, the quick echo of his shoe striking the board would remind a citizen that he was not in the best part of town. From time to time boards would disappear from the walks; a person walking at night soon learned where the boards were missing. The company's all-round man responsible for repairs could never find the missing boards, and so one thing seemed certain: if a miner removed a board for one purpose or another -- to build a shelf or start a fire -- he would never take it from the front of his own house."

At the book's center the publishers have inserted a sheaf of photographs by John Vachon, who came to Kempton on archival assignments from the Farm Security Administration in 1939.

The local miners were on strike when Vachon arrived. But this was an annual custom of theirs, and they look plumper and cheerier than the rural ascetics who populate Walker Evans' and Dorothea Lange's contemporaneous work. The most memorable photo shows a towheaded boy holding a chunk of coal in both hands. His expression straddles the line between laughter and tears, as if he knows that coal can both make and break him.

The residents' vibrant photographic presences underscore the one striking absence from Gude's text: their words. With rare exceptions Gude has chosen not to let the people of Kempton and nearby Elk Garden, W. Va., speak for themselves. In his preface he characterizes their way of talking -- "plainly, most often matter-of-factly, and in an idiom shy of metaphor" -- but the book lacks a directness that samples of this laconic style could provide.

Today Kempton seems to be holding its own, and Elk Gardeners are hoping that nearby Bloomington Dam and Lake will attract tourist dollars. There is also a regional feeling that coal is on the way back, and North Branch Valley's got it. In any case, "Where the Potomac Begins" is an interesting and moving book, one that succeeds in capturing the texture of a vanished way of life. Anyone who reads it will look at that sluggish tidal river that flows past Washington's monuments in an altogether different light.