The  cross  section  of  the  river  bed,  its  physical  character  and
longitudinal  slope,  are  the  factors  which  determine  the  hydraulic
gradient  of  a  stream  under  different  flows.  They  are  so  variable
in  character  and  their  detail  condition  is  so  difficult  of  determina-
tion that  sufficient  knowledge  is  seldom  available,  except  possibly
in  the  case  of  some  artificial  channels,  to  determine,  with  reason-
able accuracy,  the  change  of  the  surface  gradient  and  cross  section
of  the  water  under  various  conditions  of  flow.  Where  a  power  plant
IS  to  be  installed,  it  is  important  to  ascertain  the  relation  of  flow
to  head  in  order  that  the  available  power  may  be  accurately  deter-
mined. Where  a  river  is  in  such  condition  as  to  make  the  de-
termination of  a  discharge  rating  curve  possible,  either  by  direct
river  measurement  at  the  point  in  question  or  by  a  comparison  with
the  flow  over  weirs  at  some  other  point,  such  determination  should
be  carefully  made,  as  such  knowledge  is  of  the  utmost  importance
in  considering  the  problem  of  continuous  power.
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The  Rating  Curve.
