Fluvial Ecosystem Diversity

Streams and rivers occur in almost bewildering variety. There is no real distinction between streams and rivers, except that the former are smaller. Some use the term "great river'' to dis tinguish rivers such as the Mekong, the Amazon, and the Mississippi from rivers of more usual size. Partly because the vast majority of river length is in the smaller headwater streams, and partly because these smaller systems have received considerably more study, many and perhaps most researchers consider themselves "stream ecologists.'' Attempting to understand how the principles of fluvial systems are manifested across scale is one of the primary themes of this book.

Fluvial ecosystems vary in many additional features. Some have the color of tea due to high concentrations of dissolved plant matter, while others have fewer chemical constituents and so remain clear; these are known as black-water and clearwater rivers, respectively. Rivers can tumble and cascade down steep slopes over large boulders, meander through gentle valleys, or flow majestically across broad flats as they approach the sea. Food webs in forested streams derive much of their food base from autumn leaf fall, whereas streams that are open, shallow, and stony typically develop a rich film of algae and microbes. Rivers that still have an intact flood-plain exchange organic matter and nutrients with the adjacent land, and all fluvial ecosystems exhibit high connectivity laterally, longitudinally, and vertically (Figure 1.1).

River science attempts to catalog this diversity, reveal the underlying processes that are responsible for the variety of patterns that we observe, and understand how those processes interact with different environmental settings and across scale from the smallest headwater streams to great rivers. Numerous classification systems for rivers have been developed to better comprehend natural patterns of variation, as well as guide management activities including restoration and assessments of river health. At the time of this writing it is not possible to describe one, overarching river classification. Indeed, this may not be an attainable goal: the variation of fluvial ecosystems is continuous, the variables often are too independent to form recognizable clusters, and different classifications have different purposes (Kondolf et al. 2003a, b). With these limitations in mind, however, a number of broad generalizations can be made that help to organize the variety and variability of fluvial ecosystems.

FIGURE 1.1 The fluvial ecosystem with its three major axes: upstream/downstream, channel/margins, and sur-ficial/underground environments. (Reproduced from Piegay and Schumm 2003 )

1.1.1 The fluvial hierarchy

Streams and the landscape units they drain form nested hierarchies. The smallest permanently flowing stream is referred to as first order. The union of two first-order streams results in a second-order stream, the union of two streams of second order results in a third-order stream, and so on (Figure 1.2). Stream order is an approximate measure of stream size, conceptually attractive, and correlates with a number of other, more precise size measures including the area drained, volume of water discharged, and channel dimensions. As a simple classification system it provides an informative tally of the numbers of small streams and large rivers (Table 1.1). The great majority of the total length of river systems is comprised of lower-order or headwater systems, each of short length and small drainage area. Rivers that we might consider to be of medium size, fourth through sixth order, are

Drawings Winnie Trhe Pooh

FIGURE 1.2 A drainage network illustrating stream channel order within a fourth-order catchment. The terminus may be a lake or the junction with a larger river. Intermittent streams occur upstream of the first-order tributaries, and often extend nearly to the catchment divide.

FIGURE 1.1 The fluvial ecosystem with its three major axes: upstream/downstream, channel/margins, and sur-ficial/underground environments. (Reproduced from Piegay and Schumm 2003 )

FIGURE 1.2 A drainage network illustrating stream channel order within a fourth-order catchment. The terminus may be a lake or the junction with a larger river. Intermittent streams occur upstream of the first-order tributaries, and often extend nearly to the catchment divide.

TABLE 1.1 Number and lengths of river channels of various sizes in the United States (excluding tributaries of smaller order). (From Leopold et al. 1964.)

Order

Number

Average

Total

Mean drainage

0 0

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