﻿1 
  62 
  Geological 
  Survey 
  of 
  Louisiana 
  [Sect. 
  

  

  slack 
  water 
  or 
  will 
  be 
  drawn 
  toward 
  the 
  narrow 
  outlet 
  channel. 
  

   The 
  timber 
  will 
  soon 
  obstruct 
  this 
  channel 
  and 
  the 
  raft 
  thus 
  

   started 
  will 
  extend 
  across 
  and 
  up 
  the 
  river. 
  This 
  raft 
  will 
  

   become 
  more 
  and 
  more 
  compact 
  with 
  the 
  accumulation 
  of 
  other 
  

   timber, 
  as 
  well 
  as 
  silt 
  and 
  leaves, 
  another 
  outlet 
  will 
  be 
  made 
  

   above 
  and 
  the 
  process 
  repeated 
  until 
  the 
  river 
  is 
  obstructed 
  for 
  

   many 
  miles. 
  

  

  It 
  will 
  be 
  seen 
  that 
  the 
  great 
  resultant 
  raft 
  is 
  not 
  a 
  single 
  pro- 
  

   longed 
  raft, 
  as 
  has 
  been 
  supposed 
  by 
  those 
  who 
  have 
  not 
  studied 
  

   the 
  river, 
  but 
  a 
  series 
  of 
  larger 
  and 
  smaller 
  rafts 
  with 
  open 
  

   spaces 
  between. 
  The 
  timber 
  portion 
  of 
  the 
  raft 
  occupied 
  from 
  

   a 
  third 
  to 
  a 
  half 
  of 
  the 
  whole 
  space.* 
  

  

  Rate 
  of 
  advance 
  of 
  the 
  head 
  of 
  the 
  rdft. 
  — 
  In 
  this 
  manner 
  the 
  head 
  

   of 
  the 
  raft 
  moves 
  up 
  stream 
  at 
  a 
  rate 
  varying 
  (i) 
  directly 
  with 
  

   the 
  amount 
  of 
  timber 
  brought 
  down 
  ; 
  (2) 
  with 
  the 
  amount 
  of 
  

   space 
  left 
  between 
  the 
  parts 
  of 
  the 
  raft 
  ; 
  and 
  (3) 
  inversely 
  with 
  

   the 
  width 
  and 
  depth 
  of 
  the 
  river. 
  For 
  a 
  short 
  space 
  of 
  time 
  the 
  

   growth 
  may 
  even 
  be 
  negative. 
  Thus 
  : 
  if 
  a 
  raft, 
  formed 
  above 
  an 
  

   inter-raft 
  space, 
  happens 
  to 
  give 
  way 
  during 
  very 
  high 
  water, 
  

   when 
  water 
  is 
  flowing 
  over 
  the 
  raft 
  below, 
  it 
  will 
  be 
  carried 
  

   down 
  till 
  stopped 
  by 
  the 
  lower 
  raft. 
  Thus 
  the 
  head 
  of 
  the 
  raft 
  

   will 
  actually 
  move 
  down 
  stream. 
  This 
  is, 
  however, 
  but 
  a 
  tem- 
  

   porary 
  interruption. 
  

  

  The 
  greatest 
  annual 
  raft 
  accumulation 
  recorded 
  is 
  five 
  miles. 
  

   In 
  a 
  letter 
  dated 
  Jan. 
  16, 
  1836, 
  Capt. 
  Shreve 
  says: 
  "Raft 
  has 
  

   accumulated 
  five 
  miles 
  since 
  last 
  May. 
  Unusually 
  high 
  freshets 
  

   having 
  brought 
  down 
  three 
  times 
  the 
  usual 
  amount 
  of 
  material. 
  ' 
  ' 
  f 
  

   The 
  same 
  thing 
  occurred 
  again 
  in 
  April, 
  1879, 
  when 
  a 
  single 
  

   freshet 
  formed 
  a 
  jam 
  whose 
  aggregate 
  length 
  was 
  five 
  miles. 
  J: 
  

   These 
  two 
  instances 
  are, 
  however, 
  very 
  much 
  above 
  the 
  average 
  

   rate 
  of 
  formation. 
  

  

  The 
  average 
  movement 
  of 
  the 
  raft 
  up 
  stream 
  from 
  1820 
  to 
  

   1872 
  was 
  a 
  trifle 
  over 
  four-fifths 
  of 
  a 
  mile 
  per 
  annum 
  ; 
  as 
  shown 
  

   by 
  the 
  records 
  of 
  the 
  U. 
  S. 
  Engineers 
  who 
  made 
  examinations 
  

  

  * 
  23d 
  Cong., 
  1st 
  Sess., 
  House 
  Ex. 
  Doc. 
  No. 
  98, 
  vol. 
  3, 
  p. 
  9. 
  

   f 
  24tli 
  Cong., 
  ist 
  Sess., 
  Senate 
  Doc, 
  vol. 
  3, 
  No. 
  197, 
  p. 
  2, 
  1836. 
  

   J46tli 
  Cong., 
  2d 
  Sess., 
  House 
  Ex. 
  Doc. 
  No. 
  i, 
  vol. 
  2, 
  part 
  2, 
  p. 
  95, 
  1879. 
  

   Also, 
  Annual 
  Rept. 
  Chief 
  of 
  Eng. 
  for 
  1879, 
  vol. 
  2, 
  p. 
  952. 
  

  

  