River systems are very complex, constantly changing and evolving. The diagram to the below depicts a channel evolution model that generalizes how rivers change over time. Currently, much of the Le Sueur River and major tributaries are incising and widening due to an increase in flow. Beyond water quality impacts, channel widening can also result in the loss of productive land or property.
River stability is the ability of a stream to transport the sediment and flows produced by its watershed, while maintaining a consistent dimension, pattern, and profile without aggrading or degrading (Rosgen, 1996). Current incision and widening in the Le Sueur River and tributaries is a result of the rivers adjusting to changes in hydrology and climate. Measured incision and widening in the Le Sueur River and its tributaries suggest these rivers are not stable. This is likely due to recent changes in hydrology, land use, and other impacts.
Over decades, the river channel has widened and deepened, putting more pressure on its banks and bluffs, which collapse under the pressure (MPCA, 2012). County Soil and Water Conservation Districts in the watershed have identified the following resource concerns as top priorities for conservation and cost sharing efforts. This section is excerpted from the Le Sueur River: Rapid Watershed Assessment (NRCS, 2009).
Trends in Local Channel Changes
Studies have indicated that the main channel of the Minnesota River has widened in places over the past 70 years by about 50 percent contributing 100,000's of tons of gross sediment per year (Lenhart, 2011). On the Minnesota River mainstem, researchers have noted a loss of sinuosity, floodplain disconnection, and increased
stream flow. Minnesota River Basin streams are actively adjusting to changes in flow leading to higher rates of channel erosion and less floodplain deposition (Lenhart, 2011).
The channel is widening throughout the system. The entire cross section of the river channel is changing. A study at the Minnesota River (near Jordan) researchers examined air photos and noted an annual rate of increase of 1.41 feet (0.43 meters) per year from the 1940's to the present (Gran, 2011).
Le Sueur River
MNDNR researchers have been discovering similar channel changes in the Le Sueur River. Air photo analysis from 1939 to 2010 indicates river widening (photos right). With hydrological changes (e.e. tiling, loss of water storage, and intense rainfall events) in the Le Sueur River Watershed, we are seeing channel evolution occur in all areas of the watershed.
This stage is a stable system at equilibrium, typically a meandering or straight channel that is well vegetated. The system is in frequent contact with its floodplain.
Something pushes the system out of equilibrium. Flow increases can push the system out of equilibrium due to the increased stream power.
As stream flow increases, the river begins to incise to adjust to the lower channel base height.
Degradation & Widening
During the down cutting of the channel, bank stability decreases as bank height and the steepness of the bank increases. This leads to bank destabilization, mass wasting of the bank, and channel widening.
Aggradation & Widening
Aggradation of the channel is the dominant feature and channel widening continues. (Reduction of stream power and high sediment loads cause portions of the system to begin aggrading).
If the system remains disturbance free a new quasi equilibrium will be reached (Simon and Hupp, 1986). While it is true that there eventually will be a new equilibrium, scientists don’t know how much more channel degradation or widening will occur before a new level of stability is reached or how long it will take to get there.
Degradation = a lowering of base level over time due to channel incision processes (lowering, incising).
Aggradation = raising of local base level due to sediment depostional process over time (adding material).
Geologic History References
Simon & Hupp 1986
Increasing River Flows
River flows have increased over recent decades. Many researchers have been trying to understand what is driving increased flows since the 1940's in area watersheds. Many researchers are attributing increased flows to three primary factors:
An upward trend of precipitation amounts and intensities.
Reduced spring evapotranspiration and increased spring runoff due to the gradual shift from perennial plant cover (prairie, pasture) and winter annuals to row crops.
Increasing density of tile and ditch drainage to remove water for row crop production.
The increase in streamflow was shown to be correlated to widening of the river channels over the past 70 years. Rivers that had significant increase in annual flow volume experienced channel widening of 10-40%, whereas rivers with no flow increase had no change in channel width.
A recent trend analysis for flow also found results skewed by season: a large and significant increase in the spring (May-June) with much smaller changes in the fall (Sept-Oct) (Schottler, 2013).
“Changes in streamflow can have important water-quality consequences such as increased river erosion, including stream channel widening, resulting in greater sediment loading and increased river turbidity.” Researchers noted that efforts
to mitigate excessive sediment loads and turbidity must include strategies to manage watershed hydrology and reverse conditions contributing to higher river flows (Schottler, 2013).
WATER YIELD = was calculated for each watershed by dividing flow by the respective watershed area.
RUNOFF RATIO = is water yield divided by precipitation
The chart above illustrates the increase in runoff ratio for the Blue Earth, Le Sueur and Cottonwood River Watersheds from 1940's to the present (Schottler, 2013). Water yield normalized to precipitation.
Geologic History References
The 1855 map illustrates the likely extent of wetlands during the public land survey from 1853-1870.
By 1938, the county drainage network (shown in yellow) was in place and the majority of wetlands had been drained.
By 1991, more subsurface tile (shown in red) was recorded.
Additional expansion of subsurface tile was recorded. Researchers noted a slight increase in wetland areas in 2003-2011 likely due to the implementation of the Conservation Reserve Program (CRP), Reinvest in Minnesota (RIM), and the Wetland Reserve Program (WRP).
The maps (left) depict the changes in hydrology in a small watershed in the Le Sueur River Watershed - Beauford (location map above). Researchers examined historic aerial photos over time to assess changes in the extent of wetlands, open and subsurface drainage systems. The study found that Beauford watershed lost most of its wetlands from 1855 - 1938 when the county drainage network was installed in Blue Earth County. The study found a direct correlation between wetland loss and installation of open surface drainage systems and subsurface drainage systems. The amount of wetland loss correlates with other scientific research that estimates that 90 percent of wetlands have been lost in this part of Minnesota (Kuehner, 2004).
The movement of water across the broader Minnesota River Basin before Euro-American settlement would have been different from today. The landscape consisted of a vast prairie pockmarked with wetlands. The prairie sod allowed rapid infiltration of precipitation. The wetlands were connected to subsurface hydrology. The flows of the rivers were likely sustained by ground water inputs for most of the year. As prairies were plowed precipitation followed surface water runoff paths into lakes and wetlands which were ditched and drained in many areas to remove water rapidly from the landscape thus enabling large-scale farming (MPCA, 1997).
The map above shows current wetlands and lakes in the Le Sueur River Watershed (National Wetlands Inventory, 1982).
The map below shows percent potential tile installation across the Le Sueur River Watershed based on the following criteria: 2009 USDA Crop data for row crops; 30 meter DEM and slope ranging from 0-3%; and SSURGO soil drainage classes of very poorly drained or poorly drained soils.
Geologic History References
Tile installation USDA 2009