2004 Mountain Snowpack Report for Missouri River Basin

Data provided by: U. S. Army Corps of Engineers - Civil Works - Missouri River Region - Water Management Information - Missouri River Basin - Runoff Forecast Information

he Missouri River runoff above Sioux City for CY 2003 was 17.6 MAF, only 70 percent of normal. As is usually the case, the runoff distribution was very uneven. Last year was the fourth consecutive year of drought which has taxed the main stem system storage leaving upstream reservoir levels very low, much like occurred in the drought of the mid 1980's and early 1990's. During the 2003 runoff season, Fort Peck and Oahe pools, as well as the Mainstem System storage, reached new record lows. The system releases have been reduced to low winter levels. Runoff for CY 2004 is forecast to be only 19.4 MAF, 77 percent of normal. Normally, by February 1st, 61% of the peak snow accumulation normally has occurred.

As of February 1 2004, mountain snowpack in the reach above Fort Peck is 99% of normal and the mountain snowpack in the reach between Fort Peck and Garrison is at 86% of normal. As of February 1, the snowpack in the North Platte River and South Platte River basins are 82% and 65% of normal, respectively.

The following tabulation is a summary of this year's mountain snowpack accumulations and the CY 2004 runoff forecast for the first of each month.

The main stem reservoirs are significantly below their base of the annual flood control zones due to three consecutive years of drought and the system stands poised to handle significant runoff if that were to occur during 2004.

**CY 2004 Mountain Snowpack Accumulations in Percent of Normal Peak**
Location	Jan	Feb	Mar	Apr	May	Jun	Jul
Above Fort Peck Dam	103%	99%	.	.	.	.	.
Fort Peck to Garrison	95%	86%	.	.	.	.	.
Percent of Normal Total Acc.	99%	92%	.	.	.	.	.
South Platte	65%	65%	.	.	.	.	.
North Platte	95%	82%	.	.	.	.	.

**Forecasted CY 2004 Missouri River Basin Annual Runoff in MAF**
Location	Jan	Feb	Mar	Apr	May	Jun	Jul
Above Sioux City, Iowa.	19.8	19.4	.	.	.	.	.
Percent of Normal 25.2 MAF	78%	77%	.	.	.	.	.

Normally by January 1, 42 percent of the peak accumulation has occurred. The mountain snow peaks usually occur near 15 April. By June 1, 50 percent snowmelt is expected. The January 2004 runoff above Sioux City was 0.44 MAF, 59 percent of normal. The January through May 2004 forecasted runoff above Sioux City is 7.3 MAF, 66 percent of normal. The 2004 Calendar Year runoff forecast for above Sioux City is 19.4 MAF, only 77 percent of normal. As stated earlier, the Missouri River basin endured its fourth consecutive year of drought. As per the National Weather Service drough severity index, as of January 27th, 2004 the current drought is considered "extreme" or "severe" in significant portions of the Missouri River basin.

The table above labeled CY 2004 Mountain Snowpack, gives information in percent of average for the two significant snowpack accumulation reaches of Fort Peck and Fort Peck to Garrison.

The snow melts during the May through July timeframe and provides significant main stem inflow which is stored to prevent downstream flooding and later used to meet main stem authorized project purposes.

Even knowing the amount of snow at the first of each month for selected mountain snowpack areas results in considerable runoff variability because the weather conditions during the melt period greatly influences the runoff yield.

The total percent of normal accumulation are shown for the first of each month through May. For the period of May through July the percentages shown are a percent of the peak accumulation for the year to indicate the remaining snow to melt in the mountains.

Summary of Winter 2002-2003

Like the previous winter, the winter of 2002 and 2003 was distinguished by the total lack of plains snow pack. The runoff in March was 102% of normal. However, the runoff in April and May was only 57% and 78% of normal, respectively. This is significant because, historically, runoff during the two months of April and May account for almost a quarter of the total yearly runoff. The mountain snowpack started out very low and then picked up significantly near the normal peak date. The mountain snowpack in the reach above Fort Peck peaked at 92% of the normal peak accumulation. The mountain snowpack in the reach between Fort Peck and Garrison peaked at 101% of the normal peak. However when the snowmelt occurred precipitation that normally accompanies the melt was very low and the result was a series of poor runoffs in May (78%), June (82%) and July (54%). The runoff in the summer and fall months was even more dismal: August (31%), September (42%), October (48%) and November (53%). This resulted in a CY runoff for 2003 of only 17.6 MAF, 70% of normal (25.2 MAF). During the 2003 runoff year, record low pool levels were observed at Fort Peck Lake and Oahe Lake. In addition, the System Storage reached a new record low. These records lows are expected to be lowered even further in the 2004 runoff year. Missouri River runoff above Sioux City, Iowa, for CY 2002, was 16.0 MAF, the 10th lowest runoff in over 100 years of record keeping. The decade of the 1990's was distinguished by several high runoff years, making this period the wettest period in over 100 years of record keeping in the reach of the Missouri River basin above Sioux City, Iowa. During 1999 and 2000 the Missouri River basin experienced above normal base flows in many tributary streams which augmented the runoff during 2001 and even slightly in early 2002. Since then, however, severe drought conditions throughout the Missouri River basin has impacted all tributaries.