From Headwater Streams to Rivers

Abstract

THE MENTION OF STREAMS and rivers usually conjures up distinctly different visions: small streams as tumbling, turbulent ribbons of crystal clear water ruled by trout; and rivers as the slow, muddy giants of Mark Twain. There is also a general, popular insight that streams belong to nature-albeit for man's enjoymentand rivers belong to man-to carry his wastes and commerce and to cool his industry. The facts are that rivers belong to streams. The headwater networks and the entire water courses are inseparably interrelated with the landscapes through which they flow, despite massive efforts by the Corps of Engineers and Bureau of Reclamation to repeal these basic laws of nature. As implied in the title, the fundamental structural relationship is a large network of tiny streams which coalesce to larger and larger streams that are in turn collected by rivers. This hierarchical arrangement can be described by a numerical ordering system (fig. 1): the smallest streams are order 1; those receiving first order headwater tributaries are order 2 and so on to the largest river of any given drainage system which is the highest order. The Mississippi River at its mouth is order 12. Our present understanding of how running water systems change physically, chemically and, especially, biologically, with stream order-that is, from headwaters to their mouths-is the theme of the discussion that follows. Undoubtedly there are exceptions to the generalizations presented below and some portions, particularly those dealing with large rivers, are more speculative than others. For those who study streams, the exceptions and unknowns are exciting and hold the promise of new and ever more encompassing generalizations.