Silt content of spawning beds . — Bottom sam- 

 ples obtained from upstream in Harris River 

 in August and October 1959 revealed that the 

 volume of silts and fine sands in spawning 

 beds increased significantly during periods 

 when tributary streams appeared to carry 

 increased amounts of inorganic sediments. 

 Similar increases did not occur in intertidal 

 Harris River, suggesting that settleable solids 

 were not transported downstream in signifi- 

 cant amounts, or in Indian Creek, which was 

 unlogged. Instead, the volume of fine particles 

 in intertidal Harris River and Indian Creek 

 declined significantly between late summer 

 and midautumn, possibly because of spawn- 

 ing activity. Mean percentages of materials 

 passing through an 0.833-mm. sieve were: 



Harris River (upstream): 



August 16, 1959—13.9 percent 

 October 27, 1959—17.4 percent 



Harris River (intertidal): 



August 13, 1959—16.7 percent 

 October 20, 1959—13.8 percent 



Indian Creek (intertidal): 



August 11, 1959—9.3 percent 

 November 10, 1959 — 7.4 percent 



For each area, differences between sampling 

 dates were tested with a t-test and were found 

 to be significant at the 5-percent level. 



The decrease in percentage of materials 

 passing through an 0.833-mm. sieve in inter- 

 tidal Harris River was thought to be caused 

 by redd digging (refer to table 3). Because the 

 density of female spawners in Indian Creek was 

 nearly twice that observed in intertidal Harris 

 River in 1959 (4.3 versus 2.4 per 100 square 

 feet), a decrease in the volume of fine particles 

 was to be expected here, too. By comparison, 

 spawning females were scarce in upstream 

 Harris River in 1959 (0.6 female per 100 

 square feet), and redd digging must have re- 

 duced the silt content of upstream Harris 

 River only slightly. Hence, logging silt prob- 

 ably did not replace a deficit created by redd 

 digging, but simply added an amount additional 

 to what was already present in August 1959. 



Effect of silt on permeability . — According 

 to the curve relating content of fine particles 



to permeability (fig. 6), permeability of spawn- 

 ing beds in upstream Harris River was re- 

 duced considerably by settleable solids added 

 in autumn 1959. This was confirmed by a 

 laboratory study. 



After size composition was determined, 

 gravel samples from intertidal Harris River 

 and Indian Creek were placed inconstant-head 

 permeameters (fig. 5). A mixture of fine 

 particles (5 parts retained by 0.208-mm. 

 sieve; 3, by 0.104-mm. sieve; and 12, by pan) 

 was added to each permeameter. The amount 

 of fine materials added varied from 2.4 to 

 2.8 percent of the total volume of each sample 

 tested. (The estimated increase in volume of 

 fine particles in upstream Harris River during 

 autumn 1959 was 3.5 percent.) Permeability 

 of each sample before and after addition of 

 fine particles is given in table 5. Addition of 

 fine particles reduced permeability an average 

 of 2.5 times (range was 1.4 to 4.3 times). 



Silt Removal by Flooding 



The deposition of fine materials in the up- 

 stream Harris River sampling area was found 

 to be temporary. Flooding occurred in the 

 periods November 5-7 (5.32 inches of rain 

 in 72 hours) and December 5-7 (7.02 inches 

 of rain in 72 hours). * In samples of bottom 

 materials obtained from upstream Harris 

 River after flooding (February 23, 1960). 

 the content of fine materials had returned 

 to the level observed in summer 1959 before 

 logging. 



Observations on individual size groups pass- 

 ing through an 0.833-mm. sieve revealed that 

 sizes retained by 0.208-mm. and 0.104-mm. 

 sieves and passing through a 0.104-mm. sieve 

 (pan) were most affected by logging and flood- 

 ing. Mean percentages of the total volume of 

 bottom material in upstream Harris River in 

 these four size groupings plus those retained 

 by a 0.417-mm. sieve show changes in volumes 

 of fine particles occurring in association with 

 siltation and flooding (table 6). 



^Rainfall was recorded at Mollis by the U.S. Forest 

 Service. 



12 



