Crawford l'-'54). Water tiowiiig at tlif rale of about 

 50 cni-sec is considered swift- Howiiig ; velocities 

 greater tlian 300 cm-sec rarely occur. Fast currents 

 roll or slide pebbles and rocks along the bottom ; 

 move sand partly by rolling and partly by buoyant 

 transportation ; and carry line materials, such as silt 

 and organic matter, in suspension (Twenhofel 1939). 



In places where the topograjihic gradient is steep, 

 the stream bottom will be composed largely of cobble 

 and boulders too heavy to move, and smaller pebbles 

 which are trapped by obstructions. This habitat is 

 called a raf'ids. if extensive and turbulent ; riffles, if 

 of a lesser order. 



When the gradient is less steep and the water cur- 

 rent thus slower, gravel (particle size 2-64 mm) is 

 deposited first, then sand (0.06-2 mm), but the finer 

 materials are carried along. Only when the current 

 becomes negligible does the suspended material settle 

 so that silt (0.004-0.062 mm) or mud-bottom pools 

 or ponds are formed. Clay has a particle size even 

 smaller (Morgans 1956). These mud-bottomed pools 

 are the most fertile parts of the stream because of 

 the presence of organic matter entrained in the silt. 

 The rate at which o.xygen diffuses into water from the 

 atmosphere increases as the turbulence of the water 

 increases : rapids therefore have, often, the highest 

 oxygen content of a stream's waters. Ordinarily, 

 however, oxygen is near saturation in all parts of a 

 flowing, non-polluted stream. In a general way, rif- 

 fles, sand- and mud-bottom pools represent three 

 stages in the aging of a stream, and ecological study 

 of them gives a good idea of what the eosere would 

 be over a long period of time. 



Trout streams do not normally exceed 24 °C max- 

 imum summer temperature : streams with higher 

 summer temperatures are more characteristically oc- 

 cupied by species of Centrarchidae and Esocidae 

 (Ricker 1934). Streams have been classified into a 

 variety of different types, using the most character- 

 istic fish present as a basis (Van Deusen 1954). The 

 salt content of stream waters depends both in quan- 

 tity and in chemical nature on the fertility of the land 

 drained or the rock strata which produce the springs. 



STREAM BIOCIES 



When quantitative sampling is made of the 

 invertebrate populations of streams, one finds that 

 there is a sharp distinction of species found in riffles 

 and those found in mud-bottomed pools (Table 5-1 ). 

 The sand-bottom pool habitat has few characteristic in- 

 digenous invertebrate species, but it is occupied by 

 small numbers of individuals of species otherwise 

 occurring abundantly in the other two habitats. The 

 unstable bottom apparently prevents the development 

 of a characteristic community. The unionid clams 



are really the only invertebrate group to become es- 

 tablished in this habitat with any degree of success, 

 although they are not exclusive to it. There are, how- 

 ever, several fish species (Table 5-2) that find .s.iiidy 

 pools a favorite habitat, although they depend in 

 large part upon riffle organisms for their food. Many 

 lish overwinter in the deeper, more (|uiescent sand- 

 bottom pools, especially since low water temperature 

 makes them too sluggish to withstand rapid currents. 



Mud-bottom pools form in backwaters of the main 

 stream, behind natural or artificial dams in the main 

 channel, or where the current is sluggish. Very often, 

 a(|uatic vegetation fringes the edges of these pools. 

 These quiet pools are essentially young stages in the 

 developmeiU of ponds and support many animal spe- 

 cies indigenous to ponds. Such pond animals as 

 afiuatic annelids, dragonfly and damselfly naiads, and 

 burrowing mayfly naiads commonly occur also on the 

 muddy margins of streams in which the main channel 

 has a sand, gravel, or rocky bottom. 



The stream biocies consists most typically, there- 

 fore, of the inhabitants of the rifiies and sand-bottom 

 pools found throughout the course of the river. The 

 riffle and pool organisms make up two different fades 

 in this community. Mud-bottom pools and sluggish 

 streams are occupied by the pond-marsh biocies, to be 

 later described. 



Plants are not abundant in the stream biocies, 

 although the upper surfaces of rocks in a riffles may 

 be completely covered with branched filamentous 

 algae (particularly Cladophora) , and a few species 

 of water mosses (Fontinalaceae) may occur. Di- 

 atoms, mostly sessile forms, may be numerous in 

 early Spring and again in the Autumn. Dominance 

 in the true sense, .such as occurs in terrestrial com- 

 munities, does not exist, although the algae and 

 mosses passively provide food and shelter for active 

 forms. 



The most characteristic and abundant animal 

 forms of the stream biocies are the caddisfly larvae, 

 mayfly naiads, stonefly naiads, fly larvae, crayfish, 

 snails and clams, sponges and bryozoans, and fish, 

 each occupying its own particular niche (Berg 1948). 

 Plankton is mostly absent in swift-running water 

 (Carpenter 1928, Coker 1954), but may be abun- 

 dant in sluggish, pond-like stretches of large rivers. 

 The fishes listed in Table 5-2 are mostly warm-water 

 fishes. In the colder waters of mountain and northern 

 streams, the fish fauna changes. Trout, sculpins, and 

 sticklebacks become the most conspicuous species. 

 Streams that empty into the ocean may have a special 

 fauna of anadromous ("upstream") fish, such as sal- 

 n)on, shad, striped bass, and some trout, that spend 

 much of their lives in the sea but migrate into fresh- 

 water streams to spawn, and catadromous fish, such 

 as the eel, which migrate "downstream" into the sea 

 to reproduce. There are a few vertebrates other than 



Streams 43 



