1. Basin Morphometry and Characteristics of Flowing Water

Although stream invertebrates are influenced directly by the physical features of the local habitat and only indirectly by geological forces shaping the morphometry of the entire ecosystem, ecologists and invertebrate zoologists must consider all those factors when attempting to understand what controls community composition. Some elements of stream morphometry that influence aquatic communities are stream order, channel patterns (e.g., meandering and braiding), erosion, deposition, and formation and characteristics of pools and riffles.

Classifying streams by their order, or pattern of tributary connections, is a simple technique that aids analysis of longitudinal (i.e., upstream–downstream) changes in stream characteristics within a single catchment; however, it is less useful, and indeed often misleading for comparing streams in different catchments. For example, communities within streams in the eastern coastal plain bear no close resemblance to communities within similar-order streams in the southwestern deserts (when flooded) (cf. Figs. 1C and 1D respectively). According to this scheme, as refined by Strahler (1952) and illustrated in Figure 2, a continuously running, headwater creek with no permanent tributaries is classified as a first-order stream (permanency is defined here as not being ephemeral on a seasonal or other regular basis). Two first-order streams combine to form a second-order segment; two second-order creeks join as a third-order stream, etc. Stream order increases only when two streams of equivalent rank come together; hence, a third-order stream that flows into a fourth-order system has no effect on the subsequent numbering scale of the larger stream. The Mississippi River, the largest river in North America, attains a stream order of 11–13 in its lower reaches (depending on which maps are used as the basis for analysis). Unfortunately, this scheme is not as useful for many streams in arid portions of the continent where a dry stream bed (Fig. 1D) may be replaced by a raging torrent in a matter of minutes or a few hours.

Viewed from the air, streams clearly do not flow in a simple, straight direction for significant distances (Fig. 3A), nor do stream beds maintain constant depths for long stretches. Instead, the channel traces a meandering pattern of gentle and sharp bends through the basin with alternating bars, pools, and riffles (Fig. 4) as the water travels downstream along a roughly helical path. The distance between meanders is a function of several factors, especially stream width. The river channel also has a tendency to undergo braiding (i.e., the division of the channel into a network of branches) in response to the presence of erodible banks, sediment transport, and large, rapid, and frequent variations in stream discharge (Gordon et al., 1992).

Water flows in a somewhat helical manner along the surface toward an undercutting bank where the deepest pool is usually located and then passes along the bottom toward a “point” or “alternate” sediment bar on the opposite bank from the pool. Sediment is continually being eroded from some areas (e.g., undercut banks and the head of islands) and deposited in other regions (such as on bars and the foot of islands). Successive deepwater pools are separated by shallow riffle regions near the midpoint of two pools. This sequence of bars, pools, and riffles is characteristic of both straight and meandering segments of most rivers, especially those with poorly sorted loads. (See Calow and Petts, 1992; Gordon et al., 1992; and references cited therein...