the calculated values is straight and shows that 

 the conductivity- solids relationship applies also 

 in these soft waters (fig. 1). 



In confirmation, the measured values 

 of TDS in 75 additional analyses made in North 

 Carolina soft water str( ims at high and low 

 levels of flow and at warm and cold tempera- 

 tures were compared with values calculated 

 from the equation. The range of measured con- 

 ductivity was 14.1 to 101.1 microhmos and 

 range of measured TDS was 14 to 82 ppm. The 

 average difference between measured and cal- 

 culated values of TDS was 3.54 ppm which is 

 within the standard error of + 2 .0 ppm . The 

 differences ranged from to 17 ppm, but inspec- 

 tion of the analyses showed that the greatest 

 disparities occurred where the water samples 

 contained a consi derable amount of organic 

 matter which does not ionize . 



The estimation of TDS in soft water, non- 

 turbid, trout streams in the Appalachian Moun- 

 tains has been expedited by the preparation of a 

 reference table in which TDS contents in ppm 

 have been calculated for resistivities of 10,000 

 to 115,000 ohms and conductivities of 90.9 to 

 8.7 micromhos (table 2). A measurement of 

 either resistivity or conductivity is corrected 

 to 77' F. (25° C.) and then referred to the table 

 to obtain an estimate of TDS. 



It is advisable to accumulate a series of 

 resistivity measurements for a given stream 

 under a variety of water conditions in order to 

 compute an average TDS content. In general, 

 the concentration of TDS varies inversely with 

 water levels in streams, as shown in a list of 

 22 observations made at a gauging station on 

 Little River in Great Smoky Mountains National 

 Park (table 3). The average of the TDS estimates 

 was 17.1 ppm and they ranged from 15.3 to 19.1 

 ppm. The estimates were greatest when water 

 levels were low in August 1956, and were small- 

 er when near flood levels occurred in early 

 February 1957. Larger differences may occur, 

 however, in other streams at various water 

 levels. 



16 ppm. The resistivity of the sample at 77° F. 

 was 74,000 ohms which by reference to table 2 

 indicated a TDS estimate of 16.7 ppm. This 

 close agreement is typical of those obtained in 

 checks made of measured and estimated TDS 

 contents of southern Appalachian streams . 



The ranges of resistivities and estimates 

 of TDS contents were examined in selected trout 

 streams in the southern and northern Appalachian 

 Mountains (table 4). In Great Smoky Mountains 

 National Park, resistivities at 77° F. in 24 streams 

 ranged from 13,700 ohms in Abrams Creek tc 

 111,900 ohms in Indian Creek and TDS ranged 

 from 59.8 to 13.4 ppm respectively. That the 

 difference in TDS contents of Abrams and Indian 

 Creeks is biologically significant has been shown 

 in observations of their fauna. For example, the 

 growth rate of trout in Abrams Creek is greater 

 than in other streams listed whereas it is poorest 

 in Indian Creek. (King 1942). Abrams contains 

 not only a greater density of fish and bottom organ- 

 isms but also a greater variety of species than 

 does Indian Creek. 



Among the streams of Shenandoah National 

 Park, Big Run, Rapidan River, and Staunton River 

 have estimated TDS contents of 20 ppm and Cham- 

 berlain (1950) rated their food grade as 1 (413 to 

 586 pounds per acre). Piney Run has 27.5 ppm 

 of TDS and a food grade of lA (826 pounds per acre). 

 Gooney Run has 28 ppm TDS and a food grade of 

 lAA (1,037 pounds per acre). Other factors such 

 as temperature, gradient, scouring, and stability 

 of flow also affect stream productivity but the im- 

 portance of total dissolved solids is unmistakable 

 and an estimate of same facilitates ranking the 

 stream , 



Corrected resistivities and estimates of 

 TDS in 10 streams of White Mountains National 

 Forest ranged from 30,000 ohms and 31 ppm in 

 Ellis River to 90,400 ohms and 15 ppm in Cutler 

 Branch. No data are available, however, to demon- 

 strate the biological significance of the difference 

 in the TDS contents. The list does show thatflie 

 high resistivity-low dissolved solids condition pre- 

 vails also in the northern Appalachians . 



One of the water samples taken from the 

 gauging station on Little River was analyzed by 

 a U.S.G.S. laboratory and the TDS, measured 

 as residue after evaporation at 180° C, was 



