HYDROLOGY & RIVER STABILITY
Hydrology & River Stability
The Le Sueur River is a leading contributor of sediment and nutrients to the Minnesota River. With the goal of improving watershed health and building resiliency in the system, Minnesota Department of Natural Resources specialists have assessed watershed hydrology trends and stream channel stability. Stream stability assessments examine physical characteristics of streams and rivers, based on how they formed over time and how they are responding to changes. Results can be used to understand current conditions, changes and trends, and to develop targeted strategies to restore and protect watershed health.
Higher Flows and Flooding
Since 1990, stream channel-forming flows have almost doubled, causing massive stream bank and streambed erosion. Recent flooding has severely impacted the communities of St Clair, New Richland, and areas near Mankato. High flows impact homes, roads, private property, and recreational activities. Aquatic life and instream habitat changes from seasonal flow alterations. Fishing, paddling, and other outdoor activities are popular, but opportunities decrease due to dramatic flow alterations.
Evidence in DNR’s hydrologic change assessment show 1990 as a significant breakpoint. Changes in precipitation and flow throughout the watershed are evident when comparing pre-1990 to post-1990 data (see graph below):
Annual precipitation is up 21%
All seasonal flows have intensified, with the greatest increases in summer and fall
High flows in the Le Sueur River are up 92%
1.5-year recurrence interval flows have almost doubled; this is the channel-forming bankfullflow
Changes in Land Use
In the 1940-50s, agricultural land in the watershed began shifting from a mix of crops, including early season varieties, to predominantly corn and soybeans, which use water mainly in the summer. Agricultural and urban land use alterations, drainage projects, and past wetland loss have reduced water holding capacity on the land and in the soil.
The lowest reach of the Le Sueur is relatively flat and has an excess amount of sediment clogging the stream channel. High flows have widened the streambed and formed sand and gravel bars, burying habitat for fish and aquatic organisms.
Steep eroding bluffs are common in the lower portions of the watershed within the Cobb, Maple, and Le Sueur river channels.
Around 60 percent of all watercourses in the watershed have been modified for drainage improvements. This changes the movement, duration, and flow paths of water, accelerating flows so they reach the major rivers sooner. The majority of headwaters streams have been channelized or ditched.
The chart above shows the volume of precipitation and runoff, as well as the ratio of runoff to precipitation, increased notably after 1990.
Rice Creek is still a well-functioning river channel with a connected, natural floodplain in its mid-to-lower reaches to absorb flooding, although most of the headwaters have been artificially straightened and ditched.
Stream Stability Assessment Sites
Surveys sites for channel stability and streambank erosion reflect widespread changes, instability, and evidence of rivers evolving to changing conditions. Most channels are incised through downcutting or channel straightening. Accelerated bank erosion and stream instability is a factor leading to sediment impairments and aquatic habitat loss. On the map, Stream Stability Sites are indicated (yellow dots) and River Flow Gage (black triangles). The subwatershed areas (in green) are areas that the Minnesota Department of Natural Resources have identified as potential candidates for detailed stream stability analysis.
Excerpt from Minnesota Department of Natural Resources
Dimensions of Hydrology