River. The word is repeatedly thrust<br > into our consciousness. YVe read of worsening<br > pollution; we see pictures of~a river afire in<br > Cleveland， of dead fish in the Mississippi，<br > Hudson， and Columbia. If we listen care-<br > fully， we can hear the sound of political<br > promises breaking. Mercifully， there are<br > things we don t see or hear: factory wastes<br > spewed into rivers under cover of darkness;<br >deals made between industry and govern-<br > ment that assure loopholes in pollution<br > legislation.<br > Rivers bear the brunt of our industrial<br >society. A million years ago， man s impact<br >on river ecosystems was no greater than<br >that of the beaver. Roving bands of people<br >camped beside rivers and paddled these<br >watery roads. They speared fish， dug mus-<br >sels， and drank the clear water. Wastes<br >thrown into the water were quickly diluted<br >and digested; the rivers flowed sweetly on.<br > But there are limits to what a river can<br >digest. Today close to four billion people<br >crowd the earth， and many dump sewage<br >and other wastes into its waters. Rivers-<br >the most intensely used of all ecosystems-<br >are further changed by dams， channeliza-<br >tion， erosion， biocides， and thermal pollu-<br >tion. Today the sickest rivers are in those<br >nations considered to be the richest， most<br >