Hydrology and Stream Stability

The Blue Earth River begins its journey towards Mankato as an artificial drainage ditch 6.5 miles southeast of Rake, Iowa at an approximate elevation of 1,204 feet above sea level. From there, the river travels 146 miles to reach the Minnesota River at an approximate elevation of 761 feet.

This is nearly 40 percent of the elevation change required to meet sea level via the Minnesota River, Mississippi River, and Gulf of Mexico. Towards the lower end of the watershed, the Blue Earth River collects water from the Le Sueur and Watonwan Rivers. Combined, these three watersheds are referred to as the Greater Blue Earth River Basin. 

Research shows average annual precipitation has increased since the 1990s. Average annual precipitation from 1895-1990 was 28.54 inches, and from 1991-2019 was 32.78 inches. 1991 represents a significant breakpoint in the relationship between precipitation and streamflow in the watershed (see figure below).

The chart at right shows average annual precipitation patterns for the Blue Earth River watershed from 1895 to 2019. There is an overall increasing trend in precipitation (shown in blue, 7 year moving average). The average annual precipitation during that period for the Blue Earth River watershed was 29.52 inches. The purple line shows the average higher precipitation values of 32.75 inches (75th percentile values) and the red line shows lower (25th percentile values) which was 25.66 inches.

Since the 1991 breakpoint, only seven years have recorded below average annual rainfall totals. In recent decades, increasing precipitation trends coupled with land use changes that often expedite the timing and increase the magnitude of runoff have led to increasing streamflow trends. 

The substantial increase in spring precipitation—particularly early in the growing season in April and May—has important implications for streamflow in the watershed. For well over a half century the watershed’s landscape, nearly completely devoid of water-storing wetlands due to ditching and drain tile with surface intakes, has been dominated by corn and soybean monocultures—crops whose daily water use, evapotranspiration rates, and canopy cover are quite low early in the growing season. These changes have likely conspired to increase the total volume of streamflow and, in many cases, expedite its timing.  The photo at right shows flooding on the East Branch, Blue Earth River.

Since the 1990s, research indicates that streamflow has increased significantly. The chart at right shows this documented change in discharge. Mean annual discharge from 1950-2019 for the Blue Earth River watershed was 856.36 cubic feet per second (cfs). Mean annual discharge for 1950-1990 was 634 cfs and 1991-2019 was 1,171 cfs—an increase of nearly 85%.

The chart below indicates all but eight years were below average streamflow in the 1950-1990 time period, while 16 years from 1991-2019 were above average; most notably 1993, the highest average discharge year at 3,187 cfs. 

Elm Creek is the headwaters in the western portion of the watershed. The Elm Creek subwatershed is delineated in black.

The East Branch Blue Earth River is the largest tributary to the Blue Earth River. The photo at right shows the geomorphology survey site on East Branch Blue Earth River. It is located five miles northwest of Walters, MN within the Walnut Lake Wildlife Management Area (WMA).

The East Branch subwatershed is one of the most altered with 80 percent of streams being channelized or drainage ditches and 91 percent of the watershed is either cultivated or developed. While this degree of alteration creates difficulty in restoring watershed health, it also provides great opportunity for changes in landscape management.

The lower portion of the East Branch

Another geomorphology survey site on the lower East Branch is located 6.5 miles east of Blue Earth, MN (photo below). The Lower East Branch Blue Earth River site has a natural pattern with banks well protected by roots from old trees and has connection with its floodplain. Adjacent to this site, floodwaters have risen to be a few feet deep on the floodplain multiple times in recent years.

The aerial photo at right is located near Vernon Center. This shows the changes that have occurred throughout time in this portion of the Blue Earth River. The numbers indicate the amount of feet that the river has widened from 1938 to present. The Blue Earth River mainstem is in the middle of a successional stage as it adapts to drastic changes in its watershed since the mid-1800s.

You can see the changes in river width by exploring the historic aerial image compared to a recent LiDAR imagery.

Roughly 56 percent of all watercourses in the watershed have been altered or modified for drainage improvements (shown in red). Natural watercourses are shown in blue, impoundments are shown in green. When water courses are modified, it changes the movement, duration, and flow paths of water, accelerating flows so they reach the major rivers sooner.

The lower sections (reaches) of the Blue Earth River are known for the scenic river valley and gorge. Explore the "Paddling" story map to learn more about this beautiful section of river.

The lowest reach of the Blue Earth River is relatively flat and excess sediment is clogging the stream channel. High flows have widened the streambed and formed sand and gravel bars, burying habitat for fish and aquatic organisms.

The MN DNR findings suggest taking advantage of systems solutions –those that address the root cause of the problem and result in multiple benefits – to protect and restore ecosystem functions and increase resiliency. Productive agricultural landscapes with a diversity of crops, grasslands, riparian forests, and wetlands can serve as a foundation for creating and maintaining healthy watersheds. 

Water Storage: Keeping the water close to where it falls, both on and in the ground, can help reduce runoff and lower the overall amount of water flowing through our waterways. Promote practices to increase infiltration, storage, and evapotranspiration, and restore and protect areas that naturally store water.

Invest in Soil Health: Establishing cover crops, reducing tillage, adding perennial and winter-annual crops to crop rotations, and establishing buffers throughout the watershed are economical ways to build soil organic matter and increase water retention and infiltration. Taking marginal land out of production through public conservation programs is also an effective strategy.

Build Resiliency: Reconnecting floodplains, re-meandering streams, managing stormwater, and restoring wetlands and lakes can make the watershed more resilient to the impacts of increased runoff and frequent high intensity rain storms.

The Blue Earth River watershed has many historic lake basins that have been drained and are now in agricultural production. The largest drained basin in the East Branch Blue Earth River watershed was known as Lake Ozah Tanka. This approximately 1,350 acre shallow lake, located north of Frost, MN, is now recognized as farm land with numerous drainage ditches flowing through.

The East Branch Blue Earth River watershed has vast opportunities for wetland restorations, especially in the western half of the watershed. Wetland restorations are an important route to store water on the landscape.

On agricultural fields, there is already momentum for soil health practices. Farmers are increasingly using minimum tillage practices and cover crops to increase soil organic matter. These practices help store more water and nutrients protecting valuable soils and slowing the flow into streams.