major problem was sediment, which accounted for 47 percent of the nonpoint source pollutants in affected river waters.
Practices associated with forestry and farming not only increase the introduction of pollutants into streams, but also alter the physical structure and function of river-riparian ecosystems, as discussed in the sections below on overgrazing and on drainage and channelization.
The American Fisheries Society recently issued a position statement on the effects of livestock grazing on riparian and stream ecosystems (Armour et al., 1991) from which this summary is largely taken. Overgrazing of livestock in riparian areas is a major problem. Grazing is permitted on 91 percent of the federal land in the 11 contiguous western states, where federal land constitutes 48 percent of the total land area. Thirty-two percent of the land is private rangeland. The best information on the relationship between grazing and stream degradation apparently is available for land administered by the Bureau of Land Management (BLM), but the trends are probably similar for Forest Service and private lands. Fifty-eight percent of the 150 million acres of BLM rangeland is in fair to poor condition, and 19,000 miles of sport fishing streams, 100 million acres of small game and nongame habitat, and 52 million acres of big game habitat have declined in quality as a result of land use practices, including overgrazing.
Armour et al. (1991) point out that because riparian environments are lumped into much broader terrestrial classifications (e.g., ''rangeland," as in McElroy et al., 1975, classification), they become unidentifiable for land management purposes, and the problem is probably worse than the above figures indicate. For example, rangeland that is in fair to poor condition probably has river-riparian ecosystems that are in much worse condition because livestock (and wildlife) spend much more time and graze more heavily in the well-watered riparian area.
Overgrazing by livestock can eliminate streamside vegetation directly, or indirectly as a result of caving and trampling of banks, which can lead to channel widening, channel aggradation, lowering of the water table, and decline in water quality downstream because of turbidity, sedimentation, and animal waste. The water may become too turbid, warm, and shallow and the substrate too choked with fine sediment to support native fish and their food base.
Overgrazing on federal land might be reduced if it were not subsidized. The General Accounting Office (U.S. GAO, 1988) reported that the BLM recovered only about 37 percent of the cost of providing grazing on federal land and that the Forest Service recovered only