﻿Related 
  Sedimentary 
  Surface 
  Forms. 
  169 
  

  

  3. 
  Data 
  concerning 
  sand-waves 
  in 
  rivers. 
  

  

  1. 
  Ground-plan. 
  — 
  Hider 
  describes 
  these 
  sand-waves 
  

   as 
  "a 
  series 
  of 
  ridges 
  irregular 
  in 
  shape, 
  transverse 
  to 
  

   the 
  direction 
  of 
  the 
  current, 
  which 
  in 
  deeper 
  water 
  and 
  

   the 
  most 
  rapid 
  current 
  under 
  favorable 
  conditions 
  become 
  

   more 
  regular 
  in 
  shape 
  and 
  size 
  approaching 
  the 
  form 
  of 
  

   waves" 
  (ib., 
  p. 
  2199). 
  

  

  Waves 
  of 
  dimensions 
  large 
  enough 
  to 
  be 
  accurately 
  

   measured 
  by 
  sounding 
  extend 
  beyond 
  the 
  line 
  of 
  the 
  main 
  

   current 
  and 
  deep 
  water 
  (ib., 
  p. 
  2196). 
  At 
  Helena 
  (Ark.), 
  

   Johnson 
  found 
  them 
  extending 
  from 
  one-half 
  to 
  three- 
  

   fourths 
  the 
  distance 
  across 
  the 
  river 
  (1885, 
  p. 
  65). 
  

  

  2. 
  Profile. 
  — 
  The 
  numerous 
  profiles 
  given 
  in 
  Hider 
  's 
  

   report 
  show 
  that 
  the 
  profile 
  of 
  such 
  sand- 
  waves 
  is 
  

   extremely 
  irregular. 
  21 
  In 
  the 
  upper 
  part 
  of 
  fig. 
  3, 
  I 
  

   reproduce 
  two 
  series 
  of 
  waves, 
  as 
  given 
  by 
  Hider. 
  In 
  

   the 
  lower 
  part, 
  the 
  same 
  two 
  series 
  are 
  shown 
  without 
  

   exaggeration 
  of 
  height. 
  That 
  we 
  are 
  not 
  dealing 
  with 
  

   the 
  typical 
  profile 
  of 
  wind-dunes 
  as 
  seen 
  in 
  current 
  rip- 
  

   ples, 
  with 
  a 
  sharp 
  crest, 
  gentle 
  windward 
  and 
  steep 
  lee- 
  

   sides, 
  is 
  obvious. 
  Gently 
  rounded, 
  broad 
  crests, 
  often 
  

   nearly 
  symmetrical, 
  characterize 
  these 
  sand-waves. 
  

  

  3. 
  Lines 
  of 
  flow. 
  — 
  No 
  direct 
  observations 
  on 
  the 
  lines 
  

   of 
  flow 
  of 
  the 
  water 
  within 
  the 
  neighborhood 
  of 
  these 
  

   sand-waves 
  are 
  on 
  record. 
  There 
  is 
  some 
  indirect 
  evi- 
  

   dence, 
  however, 
  which 
  seems 
  to 
  have 
  considerable 
  weight. 
  

  

  A 
  necessary 
  prerequisite 
  for 
  the 
  formation 
  of 
  vortices 
  

   is 
  that 
  the 
  obstacle 
  behind 
  which 
  they 
  form 
  be 
  rigid. 
  

   The 
  sediment 
  forming 
  the 
  bodies 
  of 
  these 
  sand-waves 
  is, 
  

   however, 
  by 
  no 
  means 
  rigid. 
  At 
  a 
  point 
  where 
  the 
  Mis- 
  

   sissippi 
  was 
  65 
  feet 
  deep, 
  McMath, 
  having 
  descended 
  in 
  

   a 
  diving 
  bell, 
  in 
  stepping 
  to 
  the 
  sand 
  bed, 
  ' 
  ' 
  sank 
  into 
  it 
  

   about 
  3 
  feet, 
  and 
  then 
  thrusting 
  his 
  arm 
  into 
  the 
  yielding 
  

   mass, 
  could 
  feel 
  its 
  flowing 
  motion 
  to 
  a 
  depth 
  of 
  two 
  feet, 
  

   the 
  velocity 
  diminishing 
  downward" 
  (Gilbert, 
  1914, 
  p. 
  

   156). 
  

  

  Similar 
  observations 
  were 
  made 
  on 
  the 
  gravelly 
  beds 
  

   of 
  the 
  upper 
  part 
  of 
  the 
  Rhine 
  and 
  of 
  its 
  small 
  tributary 
  

   Birsig. 
  The 
  bed 
  of 
  these 
  mountain 
  streams 
  was 
  found 
  

  

  21 
  A 
  part 
  of 
  these 
  irregularities, 
  if 
  not 
  most, 
  may 
  be 
  only 
  apparent, 
  due 
  

   to 
  the 
  great 
  difficulties 
  encountered 
  in 
  sounding 
  from 
  a 
  boat, 
  at 
  high 
  stages 
  

   of 
  the 
  river 
  especially. 
  

  

  