erii erosion control in cultivated areas. With slower 

 runoff, more rainwater soaks into the ground, and 

 the water table is raised. It is also desirable to main- 

 tain vegetation on the immediate stream banks to 

 slow up undercutting. Streambank vegetation is also 

 beneficial for shading the water and keeping it cool 

 enough for such fish as trout. If artificial dams are 

 necessary, they should be small, and located where 

 the drainage begins in the numerous headwaters of 

 the streams. Contour plowing, strip planting, and sod 

 ditches also slow up water movement in hilly areas 

 and should be practiced. 



Pollution 



Pollution occurs wlien foreign substances are 

 introduced into a body of water in amounts sufficient 

 to change its character and chemical composition. 

 This type of pollution is of two forms : industrial 

 wastes, such as those from lead and zinc works, tan- 

 neries, breweries, paper mills, gas plants, mines, 

 atomic energy plants, etc. : and organic sewage. In- 

 dustrial and mine wastes are often acid, and extreme 

 acidity will kill fish and other organisms. Clams are 

 greatly reduced or disappear altogether in acid 

 waters. Industrial wastes contain a great variety of 

 chemical compounds, including salts of the heavy 

 metals, and many of them are very toxic to fish. 

 Young fish and species of small fish appear especially 

 sensitive, and the polluting materials may cause physi- 

 cal or chemical injury to the gills without actually 

 being absorbed into the body (Doudoroff and Katz 

 1953). 



The control of radioactive wastes from uranium 

 mills and other atomic energy plants has become an 

 especially serious problem in modern times. No 

 stream can purify itself of these wastes. However, 

 they become diluted downstream, undergo natural 

 decay, settle out in the mud bottom, and are taken 

 up by organisms. Organisms take up elements at 

 equal rates whether they are radioactive or not. 

 Radioactive elements may thus accumulate and be- 

 come concentrated in organisms to an extent many 



thousands of times greater than their concentration 

 in water. This is of ])otential harm to man (Tsivo- 

 glou (7 al. 19S7). Fortunately, streams are but little 

 used at the present time for the dis|X)sal of radioac- 

 tive wastes. The ecological significance of radioactive 

 wastes and fallout from atomic explosions has been 

 summarized by Odum (19.^9). 



The introduction of small quantities of organic 

 wastes may increase the size and productivity of ani- 

 mal [copulations by adding to the basic nitrogen sup- 

 ply. The limit of the sewage load that a stream can 

 carry without harm is, however, low and soon 

 reached. As fresh organic material oxidizes, carbon 

 dioxide and toxic gases are released into the stream, 

 and there is a drastic reduction in the oxygen content. 

 Fermentation is more rapid in summer than in win- 

 ter, and may begin in wastes before they are dis- 

 charged into the stream. The decomposing organic 

 material continues to be oxidized as it is carried 

 downstream, and when this action is completed the 

 stream is again pure (Coker \954). 



There have been many attempts to determine the 

 degree to which a stream is polluted, by means of 

 chemical analyses of the water. There is difficulty, 

 however, in evaluating the extent to which each of 

 the many chemical compounds to be found is harm- 

 ful to the various kinds of organisms. There is con- 

 siderable variation in this respect, even between dif- 

 ferent stages in the life-cycle of the same species. 

 Furthermore, the sewage load may vary from time to 

 time, and infrequent heavy loads may wipe out the 

 animal life in localities where chemical measurements 

 made at other times do not indicate harmful pollu- 

 tion. 



Various investigators (Richardson 1928, Ellis 

 1937, Paine and Gaufin 1956, Gaufin and Tarzwell 

 1956) have attempted to use invertebrate animals as 

 indicators of pollution. The presence of midge fly lar- 

 vae Tendipes ripariiis. Glyptotcndipcs. mosquito 

 larva Cule.v pipiens. rattail maggot, and sludge fly 

 delimit zones of septic pollution. There are relatively 

 few species that can tolerate septic conditions, but 

 those that do may become very abundant. The oligo- 

 chaete worms Tubifex and Lininodrilus, and certain 

 midge fly larvae, such as Tendipes plumosiis. indi- 

 cate low o.xygen. In general, pond invertebrates are 

 much more tolerant of low oxygen concentration than 

 are those belonging to stream habitats. Species espe- 

 cially tolerant of pollution are those that have adapta- 

 tions for obtaining oxygen at the water surface, such 

 as the dipteran larvae of Culicidae, Syrphidae, and 

 .Stratiomyidae, aquatic Coleoptera and Hemiptera, 

 and pulmonate snails. Gill-bearing species generally 

 require clean water of high oxygen content. Among 

 fish, pond species such as carp, bullhead, perch, and 

 crappie are relatively more tolerant than stream 

 species. 



Streams 57 



