carried away industrial and human waste. During early settlement days in the United States, human communities and factories were widely spaced, and waste discharges relatively minor and nonpersistent, especially when compared to those of today's industrial society. As a consequence of the spacing, volume, and degradability of early wastes, rivers were able to cleanse themselves through natural processes before the water reached the next downstream user. As settlements expanded in size and became more closely spaced, the wastes began to contain a larger percentage of persistent toxicants, the ecological damage became more severe, and the possibility of self-cleansing was more limited. At the same time, agricultural, mining, and timber harvesting activities accelerated, resulting in widespread alteration of watersheds, floodplains, and riparian zones that in turn altered water and sediment regimes in rivers and streams, adversely affecting plant and animal communities. Flow regimes and dilution capacity were reduced or altered by dams, irrigation, and interbasin transfer of water. The cumulative impact of all these changes was frequently missed because of the incremental nature of the changes. Even when their effects became impossible to ignore, the automobile made it easier for a more mobile population to escape to pristine aquatic sites with aesthetic and recreational appeal than to set about repairing those sites damaged by anthropogenic activities.
The changes that have stressed flowing water systems have impaired their value for both human use and environmental services. Stresses arise from (1) water quantity or flow mistiming, (2) morphological modifications of the channel and riparian zone, (3) excessive erosion and sedimentation, (4) deterioration of substrate quality, (5) deterioration of water quality, (6) decline of native species, and (7) introduction of alien species. The locus of the problem can be in the watershed, along the riparian or floodplain zone, or in the channels and pools.
The most extreme form of stress, common in the arid West, is the complete appropriation of water flowing on the surface, either by direct withdrawal or by pumping from the riparian zone (see Box 5.1). Only slightly less extreme is the conversion of reaches of free-flowing rivers to a series of lakelike impoundments (e.g., the Willamette River; see Box 5.2 and Appendix A). In these cases, the free-flowing river no longer exists, and restoration of some semblance of the natural system would require drastic measures such as reduction of water withdrawals or removal of dams. In some cases (the Willamette and Columbia rivers), a few species of migratory sport fish (salmon) are maintained on dammed rivers by using hatcheries and fish ladders, but this is aquaculture, not restoration.