WSP09733

• File Number: 8443.400
• Description: Narrows Unit - Reports
• State: CO
• Basin: South Platte
• Water Division: 1
• Date: 2/1/1978
• Author: US DoI BoR
• Title: Supporting Data for Special Report Investigation of Review Issues Narrows Unit Colorado part 2 of 3
• Water Supply Pro - Doc Type: Report/Study

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Ice Jam

Ice jams have in the past and will in the future have a great influ-
ence on maintaining the shallow and open, unregulated sandbars on
the Platte River downstream from Overton, Nebraska. The formation'
of ice jams occurring from December through March forces the water
to flow under the ice with veiocities that scour the channel bottom.
It is this scouring action or movement and continually shifting of
the streambed that helps maintain the open channel of unvegetated
sandbars. Since the ice jams occur every year, there is little or
no chance for the vegetation to establish itself. No change in
water temperature is anticipated as a result of releases from
Narrows Dam through the 300 miles of channel.

The hydraulics associated with the 'scour of the streambed by ice
jam formation is best illustrated by the plot of gage height versus
stream discharge. Figure 2 is a plot of the recorded gage heights
during ice jams at the Overton gage from discharge measurements
taken from 1940 to 1970. The ice jams create an additional head of
from I to 2 feet that creates the higher stream velocities under
the ice cover. These gage heights during the ice jam shown on fig-
ure 2 show a water level equal to that of a much higher discharge.

Another closely related action caused by the ice jam is the movement
of the ice and water during the breakup of the ice jam. This may
occur one or more times during the winter and helps maintain the
shifting sandbars over a much wider channel.

Conclusions

There are at least three major factors occurring on the Platte River
in the 70-mile reach from Overton to Grand Island that will help
maintain the present channel width with the shallow, submerged unveg-
etated sandbars. One is the peak flood discharges that develop the
stream channel velocities for a continually shifting streambed. The
other two are associated with the ice jams. The second factor is
the scouring velocities developed under the ice that move the bed
material because of the additional head developed by the ice jam.
The third factor is the movement of streambed ~terial over a much
wider width of channel created by the breaking up of the ice jam.
This may occur more than one time each year.
.

All of the above factors aid'in the shifting of the sandbars and in
controlling the growth of vegetation. Narrows Dam would only affect
the first factor but should not affect the developmenc of ice jams.
No significant change in dominanc discharge is a~ected due to
Narrows Dam, because of the large uncontrolled drainage area ups cream
from the reach of the Platce River from Overton to Grand Island.