Lessons From the Musselshell: The Flood

 

Editor's note: This is the third blog in a series by contributor Wendy Beye, chronicling a restoration effort on Montana's Musselshell River.

Montana's 2010-2011 winter was a skier's delight. Snow began piling up early, and continued to fall in record amounts through March. In April, when the expectation at this latitude is that snow will begin melting, it just kept coming down, even in the foothills. Disaster coordinators around the state began nervously checking on water content data from Snotel sites, and kept their fingers crossed that spring would be colder and drier than normal. No such luck.

In the Musselshell basin, water stored in mountain snow manteled thousands of acres with the equivalent of 26-36 inches of liquid, and the soil beneath the snow was still soggy from a similar bumper crop of snow and rain the previous year. The snowpack peaked in mid-May, and then warm spring rain began to fall. In Harlowton, near the upper reaches of the Musselshell River, May's total precipitation was more than 300 percent of normal. A storm during the third week of May poured nearly 9 inches of rain on top of the water poised for release from the snowpack. Gravity pulled the torrents downhill, out of the mountains and into the breaks where ranchers had spent their lives caring for their fields and livestock.

 As the biblical deluge continued, Teri Hice, Project Manager for the Deadman's Basin Water Users Association, started releasing stored water from the association's reservoir, hoping to create enough room to capture some of the flood water. Her efforts proved futile. She called Sheriff's Offices downstream and warned that she'd never before seen so much water coming out of the mountains. Water gages positioned in towns on the Musselshell River helped spread the alarm. 

On May 26, 15,000 cubic feet per second of water was roaring under and over the bridge at Lavina, while a hundred miles downstream at Mosby, the flow was 22,000 cubic feet per second. Previous average daily discharge records varied between 50 and several hundred cubic feet per second. In 32 hours, enough water to supply an entire summer's irrigation needs for all 110 members of the Deadman's Basin Water Users Association passed through Roundup. The water flowed at its record-breaking clip for almost a month, peaking two to three times. The historic flood stage when significant damage would begin to occur at Roundup was pegged at 10.0 feet. The 2011 flood surged to 14.45 feet, subsided slightly, then rose again two weeks later to 13.3 feet.

Restaurant at the south edge of Roundup, Montana, May 28, 2011. Image courtesy Wendy Beye.

 The precipitation that fell from the skies above Montana was undeniably a natural phenomenon. The catastrophic damage caused by water rushing down the Musselshell floodplain to its confluence with the Missouri River was partly the result of human activity along the river. In 1907, the Milwaukee Road built a railway along the top of a raised bed all along the Musselshell River. The railbed served as a dike, cutting off ancient river oxbows and straightening the river channel in many areas. Later, when US Highway 12 was constructed alongside the railbed, more of the river's channels were straightened. When a channel is straightened, the riverbed's grade is increased, and water flows more rapidly downhill. In addition, the floodplain was constricted by the incidental levies, while wetland areas that can store water like sponges were cut off from the river. Floodwaters gathered so much force that many more oxbows were bypassed by a new channels created through the process called avulsion, multiplying problems downstream.

While the flood continued, shell-shocked residents were isolated from their neighbors along the river and unable to travel to town to replenish essential supplies. “Road Closed” signs sprouted like yellow mushrooms on highways and rural roads. Ranchers couldn't reach their livestock, and the animals bunched up in field corners, up to their shoulders in cold muddy water. Irrigation pump stations were swept away, pipes turned to spaghetti, ditch banks breached, fences rolled into balls of tangled wire, ancient cottonwoods fell like bowling pins, and fields collected up to 4 feet of slimy clay sediment. Nothing could be done to slow or divert the river, though some tried, and lost their heavy equipment to the floodwaters.

Highway 12, one of the main east/west roads through Montana, under eight feet of floodwater in Roundup, Montana, May 28, 2011. Image courtesy Wendy Beye.

 The US Geological Survey calculates the magnitude of floods by using historical data plotted on a graph. The term “100-year flood” is the basis for development of many political jurisdictions' land use regulations. It does not mean that such a flood will necessarily only occur every 100 years in a particular watershed - rather that every year there is a 1 percent chance that such a flood will occur. The preliminary calculated magnitude for the Musselshell flood of 2011 was a “157-year flood event,” and there is a 0.63 percent chance each year that another such flood could impact the watershed. Governor Brian Schweitzer declared a state of emergency on June 17, and the letter that accompanies his request for federal assistance is a sobering summary of devastation suffered over much of the state.

While the 2011 flood on the Musselshell was a life-changing event for many, the aftermath and recovery have been every bit as traumatic. The next blog, Lessons From the Musselshell: The Aftermath, will follow the hard-hit ag producers in their efforts to assess damage.

Wendy Beye is a freelance writer living in Roundup, Montana.  She is assisting the Musselshell Watershed Coalition in its efforts to balance the needs of a healthy river system with those of the agricultural producers who rely on its water.

Steve Snyder
Steve Snyder
Feb 08, 2012 12:23 PM
Another issue is that, in most of America, we just have 100 years or so of weather records, so the definition of what a 100-year event is, is still a bit sketchy.
Wendy Beye
Wendy Beye Subscriber
Feb 08, 2012 08:03 PM
Very true, Steve. If one flies along the Musselshell, either in an aircraft or by using Google Earth, abandoned oxbows and channels abound, filled in with vegetation. Human activities in the floodplain also affect flood magnitude and damage. If the USGS discounts the 2011 flood in calculating the likely occurence of another event just like it happening again, they come up with a 258-year flood. But, it could happen again this year, although so far, snowpack is only average in the mountains.