﻿168 
  

  

  Bucher 
  — 
  Origin 
  of 
  Ripples, 
  and 
  

  

  be 
  about 
  100 
  cm., 
  in 
  water 
  hardly 
  more 
  than 
  30-40 
  cm. 
  

   deep. 
  

  

  In 
  San 
  Juan 
  River, 
  Pierce 
  observed 
  regressive 
  sand- 
  

   waves 
  measuring 
  450-600 
  cm. 
  from 
  crest 
  to 
  crest 
  with 
  an 
  

   amplitude 
  of 
  about 
  100 
  cm. 
  In 
  the 
  deeper 
  sections 
  they 
  

   appeared 
  at 
  their 
  best 
  development 
  on 
  rapidly 
  rising 
  

   stages 
  of 
  the 
  river 
  up 
  to 
  10 
  feet 
  gage 
  height 
  With 
  

   further 
  rise 
  of 
  the 
  river 
  the 
  movement 
  was 
  "drowned 
  

   out" 
  (1916, 
  p. 
  42). 
  

  

  Fig. 
  3. 
  

  

  I 
  ' 
  ? 
  !K 
  S 
  1 
  SOO 
  ft 
  

  

  Fig. 
  3. 
  Two 
  profiles 
  of 
  sand-waves 
  observed 
  and 
  measured 
  in 
  Mississippi 
  

   Eiver 
  near 
  Lake 
  Providence, 
  La. 
  (After 
  Hider, 
  A., 
  1883, 
  pi. 
  5.) 
  Vertical 
  

   scale 
  = 
  10 
  times 
  horizontal. 
  B. 
  The 
  same, 
  not 
  exaggerated. 
  

  

  B. 
  Progressive 
  Sand-waves 
  in 
  Rivers. 
  

  

  1. 
  Definition. 
  

  

  In 
  the 
  deeper 
  parts 
  of 
  large 
  streams 
  and 
  rivers 
  the 
  

   sediment 
  moves 
  in 
  ripples 
  of 
  truly 
  gigantic 
  proportions. 
  

   Their 
  shape 
  justifies 
  the 
  name 
  "sand-waves," 
  while 
  

   their 
  size, 
  a 
  serious 
  menace 
  often 
  to 
  navigation, 
  sug- 
  

   gested 
  the 
  name 
  "reefs," 
  both 
  of 
  which 
  are 
  used 
  by 
  

   English 
  engineers. 
  

  

  2. 
  The 
  first 
  appearance 
  of 
  (< 
  sand-waves. 
  ,} 
  

  

  These 
  sand-waves 
  exist 
  only 
  in 
  water 
  of 
  high 
  velocity, 
  

   above 
  the 
  third 
  critical 
  point, 
  carrying 
  great 
  quantities 
  

   of 
  sediment 
  in 
  suspension. 
  Direct 
  observation 
  of 
  their 
  

   mode 
  of 
  appearance, 
  therefore, 
  is 
  impossible. 
  It 
  is 
  of 
  

   importance, 
  however, 
  that 
  Hider, 
  who 
  has 
  made 
  a 
  most 
  

   careful 
  study 
  of 
  these 
  sand-waves 
  in 
  the 
  lower 
  Missis- 
  

   sippi 
  River, 
  Found 
  that 
  regularity 
  of 
  shape 
  and 
  constancy 
  

   of 
  wave-length 
  are 
  not 
  attained 
  until 
  the 
  river 
  has 
  

   remained 
  stationary 
  for 
  some 
  time 
  (1883, 
  p. 
  2196). 
  

  

  