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31 CHAPTER .2 PHYSICAl. CHARACTERISTICS Itltroduct~on The physical characteristics of a stream ecosystem reflect the character of prior geologic proce9ses within the dral,nage basin. The flowing water of drainage basins is the JIOst importat,t of all ge()logical processes and plays an important part, nOl: only in shaping the ellrth' s surface but also in providing the abiotic conditions for ecosyst... development. From headwaters to Illouth, streams comlll(mly pass through a gradual transition from higher to lower elevation (gradient>, and the stream valley becomes less confined. The steepness o,f the gradilmt is related to the drainage topography. The ~I1I1ys1cal composition of the streambed is a product of that gra.dient, ILiS well as the erosional force of high discharge and the susceptibility of the underlying materials to erosion. With this elevational transition, stream bottom ~terials become smaller, banks become lower, channel width and the occurrence of side channels tend to increase", and poolS OCCUI' at less frequent intervals. In streams where gradient or discharge is high, boulder, rubble, and gravel may predollinate in the streambed. Finer particles tend to be swe,pt away at high discharges. Where gradient or discharge is relatively low, fine gravel, sand, and silt tend to accumulate. The entire coalescing network of channels within a drainage basin creates a physical structure that interacts with the '"ater cycle to form a templet (Southwood 1977) for biological responses that results in consistent patterns of community structure and functi.on (Vannote et al. 1980, Minshall et al. 1983). Worldwide sillilarities .ln stream ecosystems probably result from geographical similarities in the phYSical structure of stre&lll8, since specific climates rarely produce uniqu,. landforms or channel morphology (Leopold et al. 1964). Thus, knowledge of the physical characteristics of streams is a prerequisite to undentanding the stream ecosystem concepts discussed in Chapter 4. Stream Chann~~ Stream channels may be restricted or limited in :Lateral movement by relic tenaces or valley walls. Therefore, channels llre said to be entrenched, confined, or unconfined ,(Chamberlin 1980)" , An entrenched channel is one where the stream bank is in continuous contact with the valley walls or terraces. A confined chaw2el is one j~n continuous or repeated contact with the outside of major meander bends and an unconfined channel is one not touching the valley wall or terraCE'S. (Figure 1)" Streaa channels have been grOl,l,\>,!!d. l>12J:II~ basis, 9f'".f,"~H~~'- ,'lnto three major categories: (l)'bedrock-conU'oIUcI, (2) alluvld "liild :(3) , sellli-controlled channels (Shumm arid Meyer 1979). "Becll'o~-cbl'[tt'oInd"'"