Summary of 2011 Spring Flooding Upstream from Lake Sakakawea

Overview- The spring and summer of 2011 saw one of the most significant flooding events along the Missouri River in recorded history. Numerous record high water marks were reached on the Missouri River and many of its tributaries upstream from Lake Sakakawea, as well as record water releases from reservoirs along the basin. The causes of such large amounts of water flow began well before the winter season, and were only worsened by record snowfalls and precipitation events leading up to the event.

Basins- The Missouri River drainage basin above Lake Sakakawea encompasses nearly all of Montana east of the continental divide, as well as much of north central Wyoming. The headwaters of many of the Missouri’s tributaries lie in the high terrain of the eastern Rocky Mountains, where heavy snowfalls and significant winter snowpack’s are common. Fort Peck Dam, located just west of Wolf Point, Montana, is a large flood control and hydro-electric facility which creates the Fort Peck Reservoir, the fifth largest man-made lake in the United States. The reservoir lies within the Missouri River and Musselshell River valleys, and for flood control purposes, is used primarily to store spring snowmelt floodwater for later release. The Yellowstone River joins the Missouri River just east of the Montana/North Dakota state line and just west of Williston, ND. A free-flowing river, the Yellowstone has no dams for flood control, and at its confluence with the Missouri, is actually the larger river. All runoff occurring in north central Wyoming, and east of the Continental Divide in Montana, flows down the Missouri through Williston and into Lake Sakakawea.




Set-up- During the past few years, the Northern High Plains has experienced a “wet period,” as evidenced by the rising lake levels from the recent dry years. Both Fort Peck Reservoir and Lake Sakakawea marked low water records in 2007, but by the fall of 2010 were at above normal pool. The result of the wet seasons was a large area of well saturated soils and reservoirs at near capacity. Following this, the winter of 2010/2011 produced near-record to record snowfalls, and by proxy, near-record to record snowpack and near-record to record amounts of liquid water equivalent. A large area of the Missouri basin, stretching from Williston, ND through Glasgow and Havre, MT broke snowfall records, and in fact received up to two to three times the normal amount of seasonal snowfall. Other points within the basin experienced snowfall amounts well above normal, most within the top 10 since records began over 100 years ago. The wet winter proceeded into a wet spring as well, and in conjunction with the seasonal melt, produced enormous amounts of runoff. Before the Missouri River ice even broke up, the river had already overflowed its banks and spilled into the flood plain near Williston, mostly as a result of precipitation and melt from the Yellowstone River basin. Overall, from October 1, 2010 through June 21, 2011, most points within the Missouri basin upstream from Lake Sakakawea received 125-250% of normal precipitation, and in some cases exceeded normal yearly precipitation by significant amounts. Finally, a heavy rainfall event occurred between May 18 and 26 over central Montana, resulting in many locations receiving rainfall amounts of 5 to 7 inches. May turned out to be the wettest month on record for at least Billings and Miles City, and other sites received more rain in May than what normally falls in an entire year. According to the U.S. Army Corps of Engineers, this was the final piece of the puzzle that pushed the basin beyond capacity.

Seasonal Snowfall

City

2010/2011

Normal

% of normal

Billings, MT
72.2
59.0
122.37
Glasgow, MT
108.6*
30.0
362.00
Great Falls, MT
108.6
60.9
178.33
Havre, MT
73.3*
45.4
161.45
Williston, ND
107.2*
43.4
247.00

* Record seasonal snowfall.

Total Precipitation from Oct. 1, 2010 to June 23, 2011

City

Precipitation

Normal

% of Normal

Billings, MT

17.60
10.92
161.17

Bozeman, MT

10.32
10.49
98.38

Butte, MT

10.52
8.42
124.94

Glasgow, MT

16.97
6.73
252.15

Great Falls, MT

15.84
10.12
156.52

Havre, MT

11.93
7.30
163.42

Helena, MT

11.60
7.25
160.00

Lewistown, MT

19.40
11.87
163.44

Miles City, MT

16.21
9.01
179.91

Sheridan, WY

13.48
11.04
122.10

Williston, ND

17.26
8.33
207.20
 

 




Meteorology- The meteorological reason for the high amounts of precipitation stems from the formation of a strong La Nina event. The cooler than normal water temperatures of the western Pacific is known to result in wetter and cooler than normal conditions across much of the northern United States. What made this season different, and much more potent, was a near constant preferred storm track that stayed in place over western Montana and eastern North Dakota. On average, there were twice as many >1 inch snow events during the winter season as normal. The preferred storm track remained in place through the spring, and was enhanced by near persistent upslope flow into the higher elevations of Montana. As the soils were already saturated leading into the snowy season and soil moisture remained frozen through the winter, most snowmelt and nearly all liquid precipitation within the basin became runoff, with very little soil infiltration during the spring and early summer. Eventually, the reservoirs and other flood control could no longer contain the massive amounts of water within the system. The U.S. Army Corps of Engineers has stated that the 2011 flood season was a 1 in 500 year event, and that the record snowmelt and wetter than normal spring could have been easily handled by the flood control system, but the extremely heavy May precipitation pushed flood control beyond its limit. Resulting Crests- Record crests were recorded at many gauge locations within the basin. The Musselshell River reached record heights along much of its course, while the Milk River near Glasgow, the Big Horn River near Big Horn, Little Big Horn River near Hardin, and the Tongue River near Miles City all reached preliminary record crests. Due to the massive amounts of water behind Fort Peck Dam, record releases were needed to maintain the facility and reservoir. With these releases flowing downstream, the Missouri River near Williston reached a record height of just below 31 feet, nearly three feet higher than the previous record of 28 feet set in 1912, well before flood control efforts began. It should be noted that a portion of the higher stage near Williston was due to the elevated headwaters of Lake Sakakawea; however flows through the Missouri River near Williston were measured at higher values than any previously observed. From there the flow entered Lake Sakakawea, where once again record releases were required, and flooding of parts of several municipalities occurred. This scenario was repeated for the length of the Missouri River. Other Thoughts- Public reaction to the flooding was mostly negative, with the U.S. Army Corps of Engineers receiving much of the blame, although forecasters also received some ire for failing to predict the heavy precipitation at long enough range. Mother Nature remains highly unpredictable, despite science’s best efforts, and such a record event generally remains beyond the reach of modern long range forecasting.






Figure 1- Missouri River flowing over floodplain and service road near Williston, ND.


Figure 2- Overview of flooding within the Missouri River floodplain near Williston, ND.



Figure 3- Oil pump within flood area at Indian Hill near Williston, ND. Picture taken at stage of just over 24 feet.



Figure 4- Lewis and Clark Recreation Area near Williston, ND. River gauge location near bridge in background.



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