154 



ANALYSIS OF THE ENVIRONMENT 



As a rule, subsurface water masses orig- 

 inate at the surface; they sink and spread 

 in accordance with differential density rela- 

 tions. Two major exceptions to this rule are 

 furnished by the equatorial masses of the 

 Pacific and Indian Oceans that are formed 

 by subsurface mixing. The place of origin 

 of a given water mass is indicated by the 

 area in which, at least for a part of the 

 year, the vertical temperature-salinity rela- 

 tions of the given mass are present as hori- 

 zontal surface characteristics. 



Another general rule is that subsurface 

 water masses are not regularly fomied at 

 the surface of the oceans in low latitudes. 

 Again two important exceptions are known: 

 the intermediate masses originating in the 

 Mediterranean and in the Red Sea. Both 

 are composed of dense water with high 

 salinity that underUes the polar intermediate 

 water masses in the Atlantic and Indian 

 Oceans, respectively. 



FLOWING FRESH WATER 



The modification of the physical environ- 

 ment by running water on land consists 

 primarily in dissection and degradation of 

 physiographically young areas. These proc- 

 esses continue until peneplain formation 

 is reached. The shift of masses of conti- 

 nental material into the great delta re- 

 gions of the world helps produce stresses 

 that in time lead to crustal readjustments. 

 These in turn lead to re-elevation of the 

 continental blocks, or of some parts of them, 

 and the cycle continues. Aggradation, as 

 well as degradation, may occur by direct 

 action of running water. When the bed of a 

 river comes to lie below base level for that 

 stream, the current slackens, mud is depos- 

 ited, and aggradation occurs. 



Running water carries fine particles in 

 suspension and sweeps coarse matter along 

 the channel bed as a so-called tractional or 

 bed load. The stream often acts as a sort- 

 ing agency. When the current is reduced 

 in velocity and turbulence, a part of the 

 bed load is deposited on the bottom, and 

 some of the suspended material settles into 

 the bed load. The heavier objects are de- 

 posited first and, other things being equal, 

 are found upstream from deposits of lighter 

 stuffs. 



The suspended load consists in part of 

 colloidal particles that carry an electrical 

 charge; fine clays may be so carried. These 

 charged particles are less affected by cur- 



rent velocity than if they were mechanicall)' 

 suspended. They aggregate and become 

 flocculent when brought into contact with 

 electrolytes in solution or with colloidal 

 particles of the opposite electrical sign. Con- 

 tacts of this kind may occur below the 

 junction of two streams or where fresh 

 water flows into the sea (Meinzer, 1942; 

 Twenhofel, 1942). 



The capacity of a stream to carry a bed 

 or traction load varies as the third or fourth 

 power of the velocity of its current. The 

 variation is in relation to such factors as 

 slope and total discharge of the stream and 

 with the form and fineness of the trans- 

 ported material. The erosive power of a 

 stream is related to the forces that deter- 

 mine its load-carrying capacity (Gilbert, 

 1914). 



Flowing fresh waters are sometimes re- 

 ferred to as composing a lotic environment 

 for plants and animals, as contrasted with 

 the so-called lentic environment in lakes 

 and ponds. Lotic waters have greater geo- 

 graphical continuity, both because of con- 

 nections through the oceans into which they 

 flow and because of the not infrequent 

 pirating of the head waters of one stream 

 by another. Because the current flows in one 

 direction only, streams tend to carry plank- 

 ton organisms out to sea or into lakes or 

 larger streams and so retard the develop- 

 ment of an autochthonic plankton in the 

 main current of the given stream. River 

 plankton develops rather in lakes, bogs, and 

 marshes, which are drained by the streams, 

 or in the stagnant backwaters of river 

 bayous. The more rapid the current of the 

 river, the sparser is the plankton, both be- 

 cause such organisms are more speedily 

 swept from the river and because of the 

 active destruction of plankton in the quick- 

 ened flow of river rapids. Here, as else- 

 where, the rule holds that, other things be- 

 ing equal, the more plankton, the more 

 fish. Hence the clearing currents of rivers 

 act to decrease the size of the fish popula- 

 tion without necessarily carrying healthy 

 fish downstream. 



Nonplanktonic stream animals, such as 

 fresh- water isopods and amphipods, may fly 

 nymphs, various beetles, and dipterous 

 larvae, to name no more, are also carried 

 downstream. Under almost ideal field con- 

 ditions, such animals settle to the bottom of 

 a lake near the outer Hmit of the slackened 

 inflowring current and may live there for 



