77 i  page  126.)  The  distribution  of  the  rain-fall  of  these  periods
has  a  greater  effect  on  the  low  water  flow  than  the  total  rain-fall
for  the  year.
The  relations  between  rain-fall  and  run-off  for  the  period  for
which  complete  data  is  available  should  be  investigated  and  such
relations  established  as  clearly  as  possible  for  the  drainage  areas
under  conideration.  (See  Chapter  VIII.)  With  the  information
concerning  run-off  commonly  available  and  the  rainfall  records  for
a  considerable  term  of  years,  it  will  be  possible  to  draw  fairly  accu-
rate conclusions  as  to  the  probable  variation  and  average  flow  of
the  stream.  The  probability  of  a  larger  maximum  or  a  smaller  min-
imum than  the  stream  flow  observations  themselves  indicate  can
also  be  determined  from  such  an  investigation.
340.  Study  of  Topog^phical  and  Geological  Conditions. — ^The
topographical  and  geological  conditions  may  ordinarily  be  inves-
tigated from  data  available  in  the  publications  of  the  United  States
Geological  Survey,  or  of  the  Geological  Surveys  of  the  state  in
which  the  drainage  area  may  lie.  The  information  sought  from  this
investigation  is  a  kno^vledge  of  the  conditions  that  will  effect  run-
off, consequently,  such  a  study  is  not  of  particular  importance  pro-
vided sufficient  rain-fall  and  run-off  data  is  available  for  the  purpose
of  the  investigation.
If,  however,  the  hydrographical  condition  of  the  areas  under
consideration,  or  of  other  adjacent  and  similarly  located  areas  have
not  been  previously  investigated,  and  if  few  or  no  local  observa-


Digitized  by  LjOOQ IC


678  The  Investigation  of  Water  Power  Projects.
