VARIABLE RATE 01? WATER WORK 203 



The negative mechanical work of moving water (grounding and sedi- 

 mentation) is beyond the reach of quantitative formulation — at least 

 pending the completion of Gilbert's determinations — but manifestly 

 varies inversely with the velocity and that differential internal movement 

 attending volume. The slackening current assorts its load, according 

 to its power, from boulders to pebbles and on down to sand and silt, 

 chiefly through the saltatory action; the overloaded stream builds bars 

 moving upstream to increase its own head and multiply its efficiency 

 below, and the underloaded stream builds bars moving downstream to in- 

 crease the head above, and in both cases the movement of the bar mate- 

 rial is saltatory and of such order as to initiate and accentuate the alter- 

 nation of flow from pool to rapid and vice versa. Practically each stream 

 tends to alternate between positive and negative mechanical work in suc- 

 cessive instants of time throughout every part of its course, though this 

 tendency may temporarily be interrupted by diastrophic or other derange- 

 ments of drainage systems. Practically, too, each stream tends to in- 

 crease also in bank-cutting and bar-building, and generally in the devel- 

 opment of curves. In some cases the latter tendency is so strong as to 

 suggest the widespread inference that a curved river can not be straight- 

 ened, since cutting across an oxbow merely produces extension of other 

 loops until the aggregate length of the river is restored, though Powell 

 established the counter-tendency of large rivers to straighten toward their 

 mouths, and Leighton finds a decided diminution of load within the 

 sinuous portions of the lower Mississippi. 



Whatever the ratio of factors in the mechanical work of running water, 

 and however complex and elusive the behavior of streams under natural 

 conditions, the essential fact remains that the internal work of streams 

 varies widely with external factors; and the no less essential fact also 

 remains that this internal work is constantly directed toward the modifi- 

 cation of the external factors : In a broad sense, the primary work of the 

 stream is the continuous adjustment of internal relations to external 

 relations (thus conforming to Spencer's characterization of life), with a 

 secondary yet no less continuous readjustment of external relations to 

 internal relations; in a strictly physical sense the stream functions, 

 strengthening through exercise and weakening through inaction, toward 

 the ends of (1) perpetuation of its own existence, (2) continuous growth, 

 and (3) increased facility of work. While all its properties are involved, 

 it seems clear that the distinctive function arises measurably in that 

 primary relation between particle movement and external conditions 



