Farm Horizons, May 2011
Can the Crow River flooding of today really be compared to that of 1965?
By Jen Kotila
When the Crow River flooded this spring, it was again compared to the historic flood levels of April 14, 1965, when the river crested in Delano at 23.25 feet. However, since 1965, there have been numerous changes in the landscape within the Crow River Watershed (CRW), meaning a comparison simply isn’t possible.
Some of the changes that have occurred within the watershed are agricultural drainage, the building of levies, and reduction in permeable land surface due to development.
All of these changes have affected the hydrology, or flow patterns, of the Crow River. The runoff during the spring thaw and heavy rain events reaches the river more quickly today, causing it to rise faster, because the water has nowhere to accumulate and soak into the soil before it reaches the river.
The data suggests these changes will continue to take place in the coming years, and there will continue to be record crests of the Crow River in places such as Delano, Mayer, and Rockford.
Looking at records dating back to 1950, of the top 20 crests reported in Delano, 13 have occurred in the last 20 years. Those crests range from 15. 75 feet June 7, 1991 to 20.30 feet March 21, 2010. The third- and fourth-highest crests happened in 2010 and 2011, respectively.
Characteristics of the Crow River
Portions of 10 counties make up the Crow River’s 1.76-million-acre watershed, which is located in south-central Minnesota, reaching the Mississippi River near Dayton, in Wright County, according to a 2005 diagnostic study of the CRW conducted by the Crow River Organization of Waters (CROW).
There are two major subwatersheds within the CRW, the North Fork, which is 178.3 miles long, and the South Fork, which is 111.6 miles long. Buffalo Creek, which meets the Crow River near Lester Prairie and New Germany, is another subwatershed.
The watershed for the Crow River is composed of two different ecoregions, which are geographical boundaries based on soil types, topography, vegetative cover, and underlying geology.
The majority of the North Fork of the Crow River, or 61 percent of the CRW, is located in the north central hardwood forest ecoregion. The majority of the South Fork, or 39 percent of the CRW, is located in the western corn belt plains ecoregion.
Of the land that makes up the CRW, 92 percent of the South Fork subwatershed is used for agriculture, and 83 percent of the North Fork subwatershed is used for agriculture. The North Fork has more forest, water, and wetlands.
Average annual precipitation for the CRW is 25 inches per year.
How is flooding predicted
Although one can never straight-out compare the flooding of today with that of 1965, experts still do because they know how much water is out there, and all the variables they are dealing with that cause the flooding, said Rob Collett, a Minnesota Department of Natural Resources (DNR) hydrologist. Because the 1965 crest is the highest crest on record, it provides a common measurement to reference.
There are many different variables that are taken into account when predicting the amount of flooding that will take place for the CRW each spring, including how saturated the ground is before it freezes, how much snow there is throughout the winter, how fast the snow is predicted to melt, and how much precipitation there will be during the spring thaw.
“There was tremendous rain last September,” Collett said, noting it made the river higher than normal already last fall.
This winter, snow cores taken by the DNR showed there were 6 to 9 inches of water throughout the CRW. “That’s a ton of snow. Think about it as 8 to 9 inches of snowmelt, then, in the spring, add 2 inches of rain. All you can say is, ‘Look out below,’” Collett explained.
The DNR also gauges the river’s depth and flow through flow gauges it has at points along the Crow River.
The National Weather Service (NWS) plugs all the known variables from the DNR, as well as the predicted temperatures for the next 10 days, and the predicted precipitation for the next two days, into a model to predict the level of flooding for the watershed.
Due to the rain last fall, the amount of snow this winter, and the predicted warm weather and rain, the NWS predicted record flood levels, possibly surpassing those of 1965.
Why the flooding wasn’t as bad as predicted
Along with the known variables, there are other variables within the CRW that affect its hydrology, such as the changes that have occurred to its landscape in the form of agricultural tiling in the western portion, and development in the eastern portion.
Although the landscape has changed over the last 20 years, it is becoming better understood how changes in the landscape affect hydrology, Collett said.
For instance, although agricultural tiling causes the water from snow melt and rainfall to drain from the surface and reach the river faster, it probably also helped to get rid of a lot of the water that fell last September, Collett said.
One of the main factors for less flooding than predicted this year, though, is that the temperatures stayed a little lower than predicted, and the precipitation came in the form of snow, rather than rain.
“When you’re flooding, snow’s your friend,” Collett said.
Implications for future flooding
It is almost certain that there will continue to be record floods of the Crow River at places such as Delano and Rockford, as suggested by data.
Although agricultural tiling may exacerbate the flooding, the facts tend to show that the flooding is intensified even more by the development of the eastern portion of the CRW.
For example, at Mayer, only five of the top historical crests for the Crow River have taken place in the last 20 years, and the springs of 2010 and 2011 are the second- and fifth-highest crests, respectively. At Rockford, nine of the historical crests have occurred in the last 20 years, and in 2010 and 2011 the ninth-, 10th-, and 11th-highest crests, respectively, occurred. (There were two different historical crests in 2011).
The South Fork of the Crow River and Buffalo Creek flow through mostly rural, agricultural land before it gets to Mayer. After Mayer and through Delano, more development has taken place within the CRW.
The same is true for the North Fork of the Crow River. Before it reaches Rockford, it passes mostly through agricultural land.
Even though it can be argued that all the snow this year led to the near-record-setting flooding throughout the CRW, this winter and the winter of 2000-2001 are the only winters within the last 20 years that had record amounts of snow coinciding with the top 20 historical crests of the Crow River at Mayer, Delano, and Rockford.
Although the facts point to the bigger culprit being urban development, as more and more agricultural tiling is added throughout the CRW, it will continue to add to the problems being seen with flooding along the Crow River. In recent years, the amount of pattern tiling has dramatically increased, according to McLeod County’s Water Plan (Chapter Four of McLeod County’s Land Uses and Conditions).
Our ditches, rivers, and streams were built for the old drainage, Collett explained. “We are putting in more miles of pattern tiling than we have township or county roads without a plan,” Collett said. “We wouldn’t build roads without a plan; it’s not sustainable.”
More than 18,000 miles of tile, or three-fourths of the earth’s circumference, are added to agricultural land in Minnesota every year, according to the DNR.
The water flowing more quickly through the CRW could result in numerous negative impacts, including increased downstream flooding, increased rise and fall of water levels in lakes and wetlands, and greater erosion of the banks and channels of rivers, streams, and ditches, according to McLeod County’s Water Plan.
Flowing water carries more than 60 million tons of Minnesota’s upland topsoil every year, enough to fill the Metrodome 21 times, according to the DNR.
The purpose of pattern tiling is to drain soil in order for it to be more productive as crop land. “High commodity prices make it much harder to make land competitive for conservation programs,” Collett explained.
Wetlands are being destroyed, reducing the buffers needed to hold water in order to allow sediment to settle and pollutants to be filtered before the water reaches streams and rivers. Minnesota’s topsoil, along with the pollutants that are carried off its land by water runoff, are ending up down the Mississippi River.
To try to reduce or reverse the negative effects the draining of wetlands has caused, “We strategically want to find wetlands to restore, probably in the headwaters of watersheds, where you get the biggest bang for your buck,” Collett said.
It is also important for those living in urban areas to do what they can to reduce runoff and flow of water to the Crow River. For instance, rather than having an impermeable paved surface for parking, a permeable surface that directs the water to a rain garden or holding pond would allow pollutants in the water to be filtered before reaching the river. It also slows the flow of water towards and into the river.
“We need a good farm bill, and we need good stewardship of the land,” Collett said.