Such  a  checking  takes  place
when  a  stream,  during  stages
of  high  water,  overflows  its
banks  and  spreads  out  over  the
level  flood  plains  on  either
hand.  Each  year  or  each  sea-
son of  flood  adds  to  the  de-
posit, until  such  a  height  is
reached  that  the  river,  the
channel  of  which  is  being  constantly  deepened,  no
longer  overflows  it,  and  the  flood  plain  becomes
available  for  cultivation.  (Fig.  459.)  The  flood
plains  of  the  Mississippi  river  are  stated  by
Russell  to  be  50  to  60  miles  in  breadth  and  to
comprise  an  area  of  some  fifty  thousand  square
miles.
Another  phase  of  alluvial  deposition  is  that
shown  in  many  a  small  lake  throughout  the  gla-
ciated areas.  The  entering  streams  bring  from  the
adjacent  hills  sand,  mud  and  organic  matter  which
is  deposited  in  the  form  of  a  broad,  flat,  alluvial
fan  almost  as  soon  as  the  waters  commingle.  Each
year  the  shallowing  spreads  farther  and  farther
into  the  lake ;  reeds  and  other  aquatic  plants  lend
their  aid ;  and  in  a  comparatively  brief  period  of
time  there  is  formed  an  almost  perfectly  level  plain
traversed  by  a  sluggish  meandering  stream.  This
condition  is  shown  in  Figs.  460  and  461.  There  is
a  close  connection  between  soils  thus  formed  and

Prpsent  River  Bed


lat  Alluvium

Alluvcum

459.    Generalized  section  acroee  tbe  Mississippi  xiTer,  showing
aUuvlal  deposits.    (After  Le  Oonte.)

much  of  the  cumulose  deposits  just  described  and
of  the  loess  discussed  on  page  336.
