Silt persistence, and not solely silt quantity, however, determines the 

 amount of damage to salmon spawn. The quantity of suspended sediment in 

 the stream, though an index to the erosion behavior of the watershed, is 

 not an indication of the potential damage which the stream might receive. 

 The important consideration appears to be how much of the erosion material 

 which enters a stream remains to silt the stream bottom, and how much of 

 it is carried out of the stream by normal floods. The greatest damage to 

 spawning beds occurs when silt enters a stream at times other than storm 

 periods. At such times the water velocity is insiifficient to carry the 

 sediment in suspensiono Water velocities necessary to dislodge deposited 

 particles are far greater than the velocities reqiiLred to carry the same 

 particles in s\ispensiono The introduction of silt into streams during 

 non-erosion periods results in a considerable amount of bottom deposition 

 which may reduce the total amount of stream suitable for spawning. The 

 quiet water of deep holes, and the slower parts of streams, may allow the 

 fine material to settle and form a layer of silt on the stream bottcm. 



The objective of this phase of the study is to determine the degree of 

 stream siltation which occurs imder natural conditions, and to what 

 extent, if any, it is accelerated as a result of logging on the study 

 watersheds. 



Suspended sediment . Suspended sediment samples were collected from the 

 study streams in 1950 and 1955; 6 samples in 1950 and 12 samples in 1955. 

 The sediment content of samples collected in 1950 are shown in table 8 

 below. 



Table 8,— Sediment content ^ 1950 



Date Stream Gage ht,(ft,) % silt by wt, p.p.m. 



10/7/50 



Harris 



4.10 



0,013 



132 



10/9/50 



Harris 



Not recorded 



0.015 



148 



10/8/50 



Kiaybeso 



2.77 



0,005 



48 



10/11/50 



Harris 



4.80 



0,003 



34 



10/11/50 



Harris 



4.70 



0,008 



78 



10/7/50 



Indian 



2,70 



0,011 



101 



The hygroscopic nature of the filter paper reqioires that the laboratory 

 analysis be made under controlled conditions where temperature and 

 relative humidity remain constant. The 1950 laboratory analysis was not 

 made -under controlled conditions and the sxospended sediment values shown 

 above may only be approximate. The results indicate, however, that the 

 suspended sediment content of these samples was relatively low. 



The suspended sediment content of samples collected in 1955, determined 

 by a U, S, Geological Sxarvey laboratory, are shown below. 



The number of samples collected in the past has not been adequate. An 

 intensified program of sediment sampling of all study streams at all 

 water stages will be started in 1956 for analysis by the U, S, Geological 

 Survey laboratory. 



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