CURRENTS OF AIR AND OF WATER 



155 



weeks. Directly or indirectly, wave action 

 is the usual cause of death (Dendy, 1944). 



The current produces other effects on 

 river systems. The chemical content of the 

 water tends to become generalized in the 

 lower reaches, even though the different 

 tributaries receive drainage from diverse 

 types of soils. Currents of fresh water, with 

 their dissolved salts, suspended matter and 

 warmth, produce profound changes in the 

 marine environments into which they even- 

 tually flow. There is usually an absence of 

 a distinct, deep-water stratum even in the 

 largest and deepest rivers. 



Floods introduce special complications in 

 the way of an increased rate of flow, in in- 

 creased volume of water that overflows 

 the usual channel to cover the flood plain, 

 and in the increased turbidity of the river 

 water. Many rivers are always turbid; the 

 mud-carrying Missouri is such a stream. 

 Among other effects of turbidity, the less- 

 ened penetration of light is important. 



The velocity of the current of a stream 

 seems to be one of its most significant eco- 

 logical features. Velocity is more important 

 than the division based on size into perma- 

 nent brooks, creeks, and rivers proper, and 

 more important than the distinction be- 

 tween upper, middle, and lower river. 

 Rapid water tends to contain a somewhat 

 similar community of animals, whether in 

 the upper reaches of a stream, in mid- 

 course, or near the mouth. The velocity of 

 flow depends mainly on three factors: (1) 

 the steepness of the basic gradient; (2) the 

 roughness of the stream bed; and (3) the 

 hydraulic radius. The hydraulic radius is 

 found by dividing the area of the cross sec- 

 tion of a stream by its wetted perimeter, 

 and stream velocity itself is determined by 

 the following formula: 



V = c VWs 



V represents velocity; c is a coefficient 

 based on type of bottom; R is the hydraulic 

 radius; and s gives the slope or gradient 

 (Galtsoff, 1924). 



The more rapid the flow, other conditions 

 being equal, the higher the oxygen content 

 of river water. The rate of flow also affects 

 the temperature relations. The daily and 

 yearlv range of temperature is less in rap- 

 idly flowing streams, even though they are 

 often shallow, than in the sluggish portions 

 of a stream in the same latitude. Temper- 



ature tends to be uniform at all depths, 

 even in a large river such as the Mississippi. 

 Source waters often excepted, small streams 

 tend to fluctuate with the temperature of 

 the air more than do larger ones, and the 

 latter are more sensitive to changes in air 

 temperatures than are large ponds or lakes. 

 Thermal stratification, especially that asso- 

 ciated with thermocline formation, occurs 

 but rarely in streams, and then for limited 

 periods and only in deep, slow-flowing 

 rivers. 



As a stream erodes its way back into 

 hitherto ungullied land, it first flows only 

 when there is a run-off of rainwater. As the 

 gullies cut by such occasional currents be- 

 come deeper, the duration of flow increases. 

 At length the level of fairly permanent 

 ground water is reached, and durable pools 

 occur in the more deeply eroded pockets. 

 With further erosion, these finally become 

 connected by permanent rapids, and the 

 stream enters a pool and rapids phase that 

 is frequently of long duration. With further 

 erosion, the stream bed becomes eroded to 

 more nearly uniform level and finally 

 reaches the sluggish stage of a meandering, 

 base-leveled river. 



Each of these phases in the physiographi- 

 cal history of a stream is typically reached 

 first in the region near the mouth, and each 

 tends to move farther and farther upcountry 

 as the stream lengthens. This is a schematic, 

 oversimplified history of stream develop- 

 ment. Local variations of gradient or sub- 

 strate may hasten or retard the aging of a 

 given stream or of portions of it. Each char- 

 acteristic part tends to be inhabited by an 

 appropriate community of organisms, and 

 these, too, move with the change in posi- 

 tion of their typical habitat. This whole set 

 of processes is known as physiographical 

 succession in streams. Its ecological signifi- 

 cance was perceived by Woodworth (1894) 

 and developed by Adams (1901) and Shel- 

 ford (1911). Terrestrial aspects from the 

 point of view of plant ecology were outlined 

 by Cowles (1901). The standard descrip- 

 tion of this phenomenon is based on the 

 work of Shelford (1913); some of the dis- 

 tribution relations foimd by him are shown 

 in Figure 30. 



An entirely different stream history is 

 presented by mountain streams, especially 

 when the mountains extend above the 

 snowline. The headwaters of such streams 

 may be composed of rivulets in tundra- 



