Table 4. Analysis of ice jam potential for the Missouri River near the BufordTrenton Irrigation District

Table 4. Analysis of ice jam potential for the Missouri River near the BufordTrenton Irrigation

District.

Calculated

ice

Snow-

Garrison

Known FDD thickness

fall

Year

(in.)

(J.D.)

(days)

(Kcfs) (in.) timing* (ft)

* E = early, L = late, VL = very late.

† + = high correlation with ice jamming, = lower correlation with ice jamming.

the total snowfall for the season and the timing of

greater, more rapid discharge increase to initiate

the snowfall, as an indicator. The timing of the

breakup.

snowfall is listed as being E for early, L for late or

As mentioned previously, it takes a certain

VL for very late. The climate of the Williston area is

magnitude of discharge and stage increase to re-

such that, in many years, the snow has completely

lease an ice cover and allow it to move down-

melted from the river ice surface prior to breakup.

stream. If the increase in discharge is rapid or the

In addition to potential thinning of the ice cover

ice is deteriorated, the required increase in stage

through thermal melting, this allows solar radia-

may be slight, but for gradually rising discharges

tion to penetrate and decay the internal structure

the required increase in stage may be equivalent

of the ice. In this manner, even a relatively thick ice

to three or four ice thicknesses. For the typical

cover can be weakened to reduce or eliminate the

freezeup and midwinter discharge of about 10,000

ice jam flooding potential.

cfs in the study area, a spring runoff event in ex-

A factor is also included in the table to reflect

cess of 25,000 cfs should be required to break up a

the elevation of Lake Sakakawea (Garrison Reser-

strong ice cover. The required breakup discharge

voir stage), since the most common location for ice

(Qb) varies, however, with the actual freezeup dis-

jam formation is the transition from a steeper to a

charge for a given year, as well as variations in the

milder energy slope such as that presented by a

other terms listed in Table 4. At very high dis-

river flowing into a lake or reservoir. Garrison Res-

charges, greater than about 90,000 cfs, no stable ice

ervoir reached its normal operating levels in 1965

jams would be able to form. The situation would

and since that time has been a potential factor in

then revert to an open-water flood scenario.

the occurrence and location of ice jam formation.

Two terms are included to account for snowfall

In addition to listing the magnitudes of these

characteristics. Ideally the snow-related portion of

various terms, an effort was made to discriminate

the prediction scheme would use the depth of

whether the values indicated a high, medium or

snow remaining on the ground prior to breakup

low potential for ice-related flooding. The criteria

as an index, but historic records of this parameter

for ranking the terms are provided in Table 5.

are not available. Instead we have used two terms,

15