and  the  Cause  and  Prevention  of  Floods.  407 
In  the  lower  or  tidal  portion  ample  capacity,  combined  with 
efficient  scour,  should  be  given,  and  every  facility  afforded  for 
the  free  flow  of  the  tidal  water,  and  the  low-water  level  of  the 
sea  brought  as  far  up  the  river  as  practicable.  By  this  means  a 
large  reservoir  is  secured  for  the  reception  of  the  downfall- 
water  in  times  of  floods,  which  then  fills  the  place  Avhich,  under 
ordinary  circumstances,  is  occupied  by  the  salt  water  carried  up 
and  down  by  the  tides. 
A river  should  not  only  be  of  sufficient  capacity  from  its 
outfall  to  its  source  to  carry  off  the  rainfall  of  the  district  with- 
out flooding  the  lands  through  which  it  passes,  but  the  water- 
level  should  be  kept  sufficiently  low  to  allow  of  the  thorough 
drainage  of  all  lands  within  the  basin.  Exceptions  to  this  rule 
may  be  made  in  the  case  of  areas  lying  at  so  low  a level,  that 
the  cost  of  adapting  the  outfall  to  their  efficient  drainage  would 
involve  works  of  such  magnitude,  that  it  would  be  more  econo- 
mical to  remove  the  water  off.  them  by  steam-power.  There  are 
also  occasional  floods  which  occur  only  at  such  long  intervals, 
and  are  then  of  such  short  duration,  that  in  such  cases  it  mav  be 
found  desirable  to  run  the  risk  of  flooding  rather  than  incur  an 
enormous  expense  in  protecting  the  land.  Again,  there  is  along 
the  valleys  of  all  rivers  a great  deal  of  low-lying  land  that,  as 
pasture  or  meadow,  is  not  damaged,  but  rather  benefited  by 
occasional  flooding  in  winter ; where  the  owners  have  con- 
verted such  areas  into  arable  land,  they  can  only  ask  to  be  pro- 
tected from  summer  floods,  and  carnot  expect  the  rest  of  the 
level  to  be  taxed  to  prevent  flooding  in  winter,  which  is  injurious 
owing  to  their  own  mistaken  action. 
The  space  occupied  by  the  water  in  a river  due  to  the  normal 
flow  is  so  small  as  compared  to  the  flood  area,  that  if  the 
channel  be  adapted  for.  the  latter,  it  would  be  so  large  as  to  be 
unable  to  maintain  itself  in  efficient  condition,  owing  to  the 
growth  of  weeds  and  the  accumulation  of  deposit  from  the  want 
ol  scour,  and  there  would  also  be  the  waste  of  a very  large  area  of 
land.  To  meet  this  difficulty,  the  channel  of  a river  flowing 
through  a cultivated  river-valley  should  therefore  be  divided 
into  two  sections ; the  lower  one  being  adapted  to  carry  the 
normal  flow,  with  side  slopes  as  steep  as  the  nature  of  the  soil 
through  which  it  passes  will  permit ; the  upper,  or  flood  section, 
having  the  sides  laid  at  a slope  sufficiently  flat  to  allow  grass 
to  grow,  and  of  the  grazing  of  sheep  and  cattle,  the  flood-banks 
being  set  back  sufficiently  far  to  allow  room  for  the  greatest 
floods  likely  to  occur.  Bridges  and  other  openings  must,  of 
course,  be  adapted  to  the  flood  discharge.  By  these  means,  the 
ordinary  flow  of  the  water  being  retained  in  as  small  a compass 
as  possible,  a deep  stream  would  be  maintained,  the  weeds 
