﻿HYDROLOGY 
  OF 
  NEW 
  YORK 
  437 
  

  

  what 
  further 
  into 
  the 
  detail 
  than 
  this 
  statement 
  implies. 
  The 
  

   clearing 
  up 
  of 
  lands 
  and 
  drainage 
  of 
  towns, 
  together 
  with 
  farm 
  

   drainage, 
  are 
  all 
  efficient 
  causes 
  for 
  floods. 
  In 
  a 
  general 
  way. 
  the 
  

   water 
  falling 
  in 
  the 
  form 
  of 
  rain 
  or 
  snow 
  runs 
  off 
  by 
  these 
  various 
  

   means 
  quicker 
  than 
  formerly. 
  In 
  the 
  meantime 
  the 
  channels 
  of 
  

   rivers 
  have 
  not 
  been 
  correspondingly 
  enlarged 
  to 
  meet 
  the 
  in- 
  

   creased 
  demands 
  upon 
  them, 
  with 
  the 
  result 
  that 
  overflows 
  fre- 
  

   quently 
  ensue. 
  An 
  examination 
  of 
  rainfall 
  statistics 
  shows 
  that 
  

   as 
  a 
  whole 
  the 
  rainfall 
  is 
  no 
  greater 
  now 
  than 
  formerly, 
  and 
  the 
  

   increased 
  frequency 
  of 
  floods 
  must 
  therefore 
  be 
  ascribed 
  to 
  not 
  

   only 
  irregularity 
  in 
  the 
  rainfall, 
  but 
  to 
  the 
  greater 
  rapidity 
  with 
  

   which 
  water 
  runs 
  off. 
  Broadly, 
  we 
  may 
  say 
  that 
  the 
  higher 
  the 
  

   degree 
  of 
  civilization, 
  the 
  more 
  quickly 
  will 
  surface 
  water 
  be 
  

   discharged, 
  and 
  hence, 
  without 
  there 
  is 
  a 
  corresponding 
  increase 
  

   in 
  the 
  discharging 
  capacity 
  of 
  streams, 
  floods 
  will 
  become 
  more 
  

   frequent, 
  with 
  their 
  attendant 
  evils. 
  

  

  Frequency 
  of 
  floods. 
  This 
  matter 
  has 
  been 
  referred 
  to 
  in 
  a 
  

   preceding 
  chapter, 
  discussing 
  floods 
  on 
  the 
  Tiber 
  and 
  Seine. 
  

   Very 
  serious 
  floods 
  have 
  occurred 
  on 
  these 
  streams 
  as 
  often 
  as 
  

   about 
  once 
  in 
  thirty 
  to 
  fifty 
  years. 
  In 
  the 
  United 
  States 
  there 
  

   are 
  no 
  records 
  long 
  enough 
  to 
  show 
  certainly 
  how 
  often 
  floods 
  may 
  

   be 
  expected, 
  but 
  probably 
  their 
  periods 
  will 
  not 
  be 
  materially 
  

   different 
  from 
  these 
  streams 
  — 
  in 
  some 
  cases, 
  heavy 
  floods 
  have 
  

   occurred 
  on 
  the 
  Tiber 
  only 
  a 
  few 
  years 
  apart 
  and 
  it 
  is 
  intended 
  to 
  

   indicate 
  here 
  only 
  very 
  general 
  averages. 
  

  

  Are 
  storage 
  reservoirs 
  effectual 
  in 
  preventing 
  floods? 
  There 
  

   is 
  considerable 
  doubt 
  about 
  storage 
  reservoirs 
  being 
  a 
  universal 
  

   remedy 
  for 
  floods. 
  On 
  some 
  streams 
  excessive 
  cost 
  would 
  pre- 
  

   clude 
  their 
  use 
  — 
  that 
  is 
  to 
  say, 
  the 
  benefits 
  received 
  would 
  not 
  

   be 
  commensurate 
  with 
  the 
  expense 
  1 
  . 
  On 
  other 
  streams 
  their 
  use 
  

   may 
  be 
  of 
  the 
  greatest 
  value. 
  On 
  the 
  Genesee 
  river, 
  41 
  per 
  cent 
  of 
  

   the 
  total 
  catchment 
  area 
  can 
  be 
  controlled 
  by 
  a 
  reservoir; 
  on 
  

   Salmon 
  river, 
  55 
  per 
  cent; 
  on 
  Black 
  river, 
  90 
  per 
  cent, 
  and 
  on 
  the 
  

   upper 
  Hudson 
  river, 
  50 
  per 
  cent. 
  On 
  all 
  these 
  streams 
  the 
  percent- 
  

   age 
  of 
  control 
  is 
  large 
  enough 
  to 
  prevent 
  destructive 
  overflows. 
  

  

  Views 
  of 
  French 
  engineers. 
  The 
  utility 
  of 
  storage 
  reservoirs 
  

   in 
  diminishing 
  damage 
  from 
  floods 
  was 
  taken 
  under 
  considera- 
  

   tion 
  in 
  France 
  in 
  1856. 
  Investigations 
  made 
  in 
  the 
  valleys 
  of 
  

  

  