The Des Moines River near Camp Mitigwa was a distal reach of meltwater
on the Des Moines Lobe. During the Late-Wisconsin age, large fluctuations
in discharge and sediment load resulted in significant erosion and
deposition events. Down cutting was caused by high magnitude glacial
flood events from the headwater regions of the Algona Moraine. The
high discharge peaks with lower sediment load caused the Des Moines
River to erode. Between Wisconsin flood events, deposition occurred
on the new lower valley floor. The materials being carried by the
river during that time were larger rocks and sediments.
In contrast, the modern, Holocene period has
had relatively minor floods and discharge fluctuations. This is primarily
due to the fact that glacial ice is no longer present in the Des Moines
River basin. The sediment load is derived from tributary and riverbank
erosion, which is finer textured than the coarse glacial debris carried
by meltwater.
The Des Moines River eroded its bed severely
during the high discharge fluctuations of the Late-Wisconsin. It has
eroded less during the milder Holocene. Terraces of the two time periods
can be distinguished by the character of the material making up the
terraces. The glacial meltwater of the Late-Wisconsin carried larger
debris, versus the milder flood events of the Wisconsin which carried
finer sediments. Digging in these terraces reveal several layers of
different debris sizes that can be seen in the lower increment. They
show different discharge events and reflect the magnitude of the discharge.
The Des Moines River terraces are also effected
by the meandering movement of the river channel. The Des Moines river
is a graded river and thus will seek equilibrium of neither eroding
nor aggrading its bed. By definition, a graded river will seek equilibrium
of neither eroding nor aggrading its bed. To get to this point it
will adjust its slope and sediment load for the given discharge. Bettis
et al hypothesize that the Des Moines River has migrated across the
valley floor forming horizontal depositional sequences. This is consistent
with a graded river in that graded rivers often meander. Discharge
fluctuations causing changing sediment loads will often develop bends
that will continue to exaggerate themselves. Slope adjustment may
have also been necessary. Slope could be reduced by increasing the
length of the stream without changing elevations, which would produce
a meander.. Meandering would result in a lateral migration of the
river channel.
The depth of the ravines running from the Des
Moines River into camp is also controlled by the elevation of the
Des Moines River. The ephemeral streams in the ravines follow the
principle of a graded river. As glacial discharges and sediments loads
varied in the Des Moines River, the elevation of theses ephemeral
streams' outlets changed. With every downcutting event of the Des
Moines River, the smaller tributaries would downcut to match.
