/ 



Handle 



Water surface 



(Silt in suspension) 



Cap 



t ^ ^ Gasket 



>^Basin 



Figure 1. --Sampler for collecting bottom materials. 



into the basin. Because the water was con- 

 tinuously agitated, the finest materials passed 

 into and remained in suspension as the sample 

 was collected. After solids were removed from 

 the tube and placed in the basin, a watertight 

 cap was inserted in the tube to retain water 

 and suspended solids in the basin. 



Sorting and Measuring Samples 



In Southeastern Alaska, where it rains 

 frequently, the problem of drying samples 

 before sorting is almost insuperable unless 

 covered facilities are provided. For sorting 

 samples in the field, it was necessary there- 

 fore to adopt "wet" techniques, and the samples 

 were not dried for analysis. 



Bottom samples were separated into 10 size 

 classes by washing and shaking them through 

 nine standard Tyler sieves having the follow- 

 ing square mesh openings (in mm.): 26.26, 

 13,33, 6.68, 3.33, 1.65, 0.833, 0.417, 0,208, 

 and 0.104. Silt passing the finest screen was 

 collected in a vessel. 



Washing samples through sieves is efficient 

 and affords nearly complete separation of size 

 groups. One problem with very fine sieves, 

 however, is their tendency to become clogged 

 and to retain water. The finest sieve used 

 (0,104 mm.) was found to be the smallest that 

 would allow water and suspended materials to 

 pass through without seriously clogging the 

 sieve. 



This technique possibly introduced some 

 bias because of the loss of silt in suspension 

 within the tube as the sampler was lifted from 

 the stream. Samples collected during low 

 stream discharge no doubt contained a greater 

 absolute quantity of solids in suspension within 

 the tube, on the average, than samples col- 

 lected during periods of high stream discharge. 

 To minimize this source of error, sampling 

 was restricted to periods of low discharge. 



The diameter of the tube used at a particular 

 location depended on the size of the gravel, 

 A 4-inch-diameter tube was used in most 

 streams sampled. In a stream with coarse 

 gravel it was necessary to use a 6-inch- 

 diameter tube. After a sample was removed 

 from the streambed, it was transferred to 

 10-quart plastic buckets to facilitate handling. 

 After at least 10 minutes had elapsed to allow 

 suspended materials to settle, excess water 

 was decanted from the buckets. 



After passing through the finest screen, 

 water and suspended materials were placed 

 in a large funnel (fig. 2) and allowed to settle 

 for 10 minutes. The volume of settled solids 

 was then measured. Solids remaining in sus- 

 pension were discarded. More than 90 percent 

 of the suspended solids settled within 10 

 minutes. 



The volume of solids retained by each sieve 

 was measured after the excess water drained 

 off. The contents of each of the larger sieves 

 (26.26 through 0,417 mm,) were placed in the 

 device shown in figure 3, and the water dis- 

 placed by solids was collected in a graduated 

 cylinder and measured. Solids retained by the 

 small diameter sieves (0.208 and 0,104 mm.) 

 were sometimes carefully washed into a gradu- 

 ate with a measured volume of water, and the 

 increase in volume was read directly. In these 

 instances, the fines were transferred to the 

 graduate through a 6-inch plastic funnel. 



