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IT is hard to think of a major natural resource or pollution issue in North America today that does not affect rivers.

Farm chemical runoff, industrial waste, urban storm sewers, sewage treatment, mining, logging, grazing, military bases, residential and business development, hydropower, loss of wetlands. The list goes on.

Legislation like the Clean Water Act and Wild and Scenic Rivers Act have provided some protection, but threats continue.

The Environmental Protection Agency (EPA) reported yesterday that an assessment of 642,000 miles of rivers and streams showed 34 percent in less than good condition. In a major study of the Clean Water Act, the Natural Resources Defense Council last fall reported that poison runoff impairs more than 125,000 miles of rivers.

More recently, the NRDC and Izaak Walton League warned that pollution and loss of wetlands - made worse by last year's flooding - is degrading the Mississippi River ecosystem.

On Tuesday, the conservation group American Rivers issued its annual list of 10 ``endangered'' and 20 ``threatened'' rivers in 32 states, the District of Columbia, and Canada.

At the top of the list is the Clarks Fork of the Yellowstone River, where Canadian mining firms plan to build a 74-acre reservoir as part of a gold mine less than three miles from Yellowstone National Park. The reservoir would hold the runoff from the sulfuric acid used to extract gold from crushed rock.

``In the event this tailings pond failed, the impact to the greater Yellowstone ecosystem would be cataclysmic and the damage irreversible,'' Sen. Max Baucus (D) of Montana, chairman of the Environment and Public Works Committee, wrote to Noranda Minerals Inc., an owner of the ``New World Mine.''

Last fall, an EPA official expressed concern about the mine and its potential impact, especially the plastic-lined storage reservoir. ``I am unaware of any studies evaluating how a tailings pond could be maintained to ensure its structural integrity forever,'' said Stephen Hoffman, chief of the EPA's Mining Waste Section. ``It is my opinion that [underwater] disposal of tailings at New World may present a potentially significant threat to human health and the environment.''

The results of an environmental-impact statement, now being drafted by the Forest Service and Montana Department of State Lands, could determine the mine's future.

The Anacostia River in D.C. is on the endangered list. ``This watershed is being paved over with parking lots, roads, malls, and other development projects,'' states the American Rivers report. ``At the same time, toxic chemicals, untreated sewage, and other poisons are being flushed into a waterway that once welcomed tall ships but is now so choked with mud that it can only be navigated by canoe.''

Also on the list is Maine's Penobscot River, in which nearly extinct Atlantic salmon are threatened by a proposed dam. The state has approved the dam; the Federal Energy Regulatory Commission still must pass judgment.

Other ``endangered rivers'' include: the Clavey River in California, the Columbia-Snake system, the Mississippi, the Missouri (where 10 native fish face extinction due to flow regulation by the Army Corps of Engineers), the Rio Grande, the Thorne River in Alaska, and the Southwest's Virgin River.

``Our rivers, contrary to popular belief, are in terrible shape,'' says American Rivers president Kevin Coyle. ``They are poisoned, dredged, dammed, channelized, and overdeveloped.''

In its recent proposal to reauthorize the Clean Water Act, the Clinton administration noted ``dramatically improved water quality since 1972,'' when the act was passed. But it also reported that 30 percent of rivers continue to be degraded, mainly by silt and nutrients from farm and urban runoff, combined sewer overflows, and municipal sewage. Bottom sediments are contaminated in more than 1,000 waterways, the administration reported in releasing its proposal in January. Between 60 and 80 percent of riparian corridors (riverbank lands) have been degraded.

As with endangered species and their habitats in forests and deserts, the complexity of ecosystems is seen in rivers and the effects of development - beyond the obvious threats of industrial pollution, municipal waste, and in-stream diversions to slake the thirst of new communities in dry regions like the Southwest.

Managing waterflows in Nebraska's Platte River to aid farmers can adversely impact migrating sandhill cranes and endangered whooping cranes. Diverting water for snowmaking in Colorado threatens Snowmass Creek. Logging increases soil erosion in the Rogue-Illinois river system of Oregon. Proposed tidal floodgates could destroy waterfowl habitat in the Saugus River near Boston.

While such threats continue, efforts are under way to reverse them and repair the damage.

Planners in the Pacific Northwest are beginning to regulate dam operations so salmon might recover. The recent North American Free Trade Agreement promises, in principle, at least, to improve environmental conditions along the Rio Grande. Government officials say the days of massive, earthmoving water projects are over. The Army Corps of Engineers is taking on a ``greener'' hue in its operations.

While there are many political hurdles ahead, reauthorization of the Clean Water Act this year holds promise for US rivers. Rep. Norm Mineta (D) of California, who chairs the House committee overseeing the bill, calls it ``probably the most important environmental legislation this Congress will enact.''