The most contemplative hours of my life have been spent around streams and rivers -- in woods, wilds or meadowland. I have long delighted in the gentle, almost never fatal, pursuit of trout with a fly rod.

When I was young, dams -- small mill dams, usually no more than four feet high -- delighted me. They carved pools in which fish proliferated and waterfowl prospered, and when frozen, gave us grand skating. Huge dams, chronicled by newspapers and magazines, were majestic, towering achievements -- triumphs of man over nature.

I have come to detest dams, almost universally.

Some facts:

* In the U.S., 600,000 miles of once free flowing rivers are now stilled behind dams.

* There are more than 75,000 dams in the inventory kept by the Army Corps of Engineers -- which includes only dams over six feet high.

* Only 2.9 percent of those dams have hydroelectric generation as their main purpose (hydro power is a relatively minor 8.62 percent of the U.S. total, the Department of Energy reports).

* Only 13.7 percent are mainly for irrigation; 14.6 percent are intended for flood control (which often backfires); 10 percent impound drinking water. Recreation accounts for 31.3 percent. The others are simply there -- outdated or built for no enduringly sound reason.

Those figures and vastly more information are contained in Watershed: The Undamming of America, by Elizabeth Grossman (Counterpoint, 238 pages, $26). I cannot remember when I last read a more cogent, compelling and responsible book about the ecology of this nation.

Grossman is no dam-smashing Druid, no limnological Luddite. "The vast enterprise of dam building in the past hundred years," she writes, "paralleled the progress of the very American 20th century. Jobs were created, farms and ranches irrigated and factories built that helped to win world wars and furnish the American dream. Cities rose in the desert, floodplains were drained and rivers channeled to suit civic vision."

But man makes mistakes -- immense mistakes were made about how land, water, gravity and weather work. Enormously lucrative cultures of dam building grew, with gigantic government subsidies and massive payrolls.

The inventory of dams' detriments is long -- and controversial. Grossman writes about recent public demonstrations over the Glen Canyon dam, a 710-foot-high structure that began flooding huge amounts of the Colorado River less than 40 years ago, in 1963.

"There's an undercurrent of tree sitting and monkey-wrenching that is probably only bluster," she declares, "but the Utah and Arizona police are clearly though politely taking no chances. Yet behind the exuberance there is serious ecological science. Given what is known about the physics of delivering water through Glen Canyon's plumbing, this dam may not make that much sense. From the engineering perspective and the biological, there are sound reasons for reconsidering the wisdom of Glen Canyon Dam."

The most direct ill effect of damming has been the extinction or near-extinction of anadromous fish -- fish that spawn in small fresh-water streams and then go to the ocean, where they mature and then return to the water of their birth to spawn. Because of dams, many salmon, shad, steelhead and others are now cut off from their natural breeding cycle. Atlantic salmon of the U.S. are almost entirely extinct; many Western rivers have killed off their native Pacific salmon.

(All salmon you can buy in the East and most in the West today is artificially bred, jammed together in pens in which they can barely if ever swim, force-fed, genetically tinkered with and chemically dyed. To my senses, this manufactured faux fish is gustatatively less appealing than blotting paper, and may be less nourishing.)

Dams force accumulation of vast silt beds, robbing alluvial lands and waters and wetlands of natural nourishment. They concentrate toxic materials that otherwise would be diluted into impotence. They have drowned and buried the legacies of thousands of years of human and other natural activity. They may have caused more flooding than they have controlled. They have sucked up billions of dollars of public funds for very short-term economic impact -- often to generate electricity that because of construction costs is enormously more expensive than from other sources. They produce power struggles that reek with hypocrisy -- poisoning the political process and corrupting government.

Among the compelling ironies is that today repairing a neglected dam -- and vast numbers are leaking and slowly crumbling -- typically can cost three, four or five times more than removing it. And who's to pay? In most cases, taxpayers -- at federal, state or, in some cases, local levels.

The book has 10 chapters, each covering a dam, a state or a multi-state region. Some of the most fascinating material is traced back to the 1600s. A great deal of dam building activity occurred in the early and mid 1800s -- before other sources of power were available or practical for milling grain, wood and other vital products.

In state after state, river after river, stream after stream, Grossman reports the destruction of dams -- freeing water from stinking, often fetid impoundments, into fresh, clear moving streams or rivers. She writes of the recovered natural banks of a Wisconsin stream -- the Milwaukee River at West Bend. The Woolen Mills Dam had been removed in 1988, 12 years before she saw it: "We walk through tall stands of chicory, cattail, thistle and Queen Anne's lace. There are prairie cone flowers, hawkweed, bergamot 2 / 3 cup plant, angelica, cow parsnip, giant blue hyssop, blue bellflower, willows and oaks and, by the side of a pond area, pickerel weed. There are turtles here as well as perch and brook and rainbow trout."

This is not an exhaustive national survey. Writing something of that scope would require far more research and thousands of pages. Because of my personal interests -- many water-bound experiences in Maryland and Pennsylvania -- to me it a shame that Grossman has nothing on either of those states, which have enormous dam challenges.

But that could be said about virtually every state in the nation. Meanwhile, Watershed is a powerfully important contribution to understanding water itself. Other studies will follow, but for anyone seriously concerned with the present and future of the most fundamental of all natural resources and our complex dependence on it, this is a book that must be read.