Wading through ankle-deep water in High Island Creek near Henderson in September 2014, I found it difficult to imagine the rampage of flowing water that heavy rain had brought three months prior. The record flood of June 19-20, 2014, had left its mark on this tributary of the Minnesota River: stream banks eroded more than 40 feet in spots, the 58-year-old dam washed out, and nearly 2 feet of sediment deposited on the floodplain. Such changes are symptomatic of an unstable river system.
Watershed specialists for the Department of Natural Resources have been documenting river changes like these since 2008, when passage of the Clean Water, Land and Legacy Amendment provided funding for monitoring and assessment of river stability statewide. After many field seasons of river-stability assessments, one message is clear—most southern Minnesota rivers are unstable. Land-use changes and loss of water storage in southern Minnesota, coupled with climatic changes, have caused many rivers to take on an increased diet of water from runoff and sediment from erosion.
Dramatically altered watershed
DNR watershed specialists used land surveys from the 1800s and a database of restorable wetlands to generate an illustration of what the 238-square-mile High Island Creek watershed would have looked like in 1860. Like much of southern and western Minnesota, the watershed was pocked with shallow wetlands known as prairie potholes. These potholes provided important habitat for waterfowl and helped store and filter rainwater. But these soggy geographic features created conditions that were not conducive to modern agriculture, so ditches and drainage tile were used in the late 1800s and early 1900s to empty the wetlands. Today, because of these dramatic alterations to the High Island Creek watershed, rain events send a greater volume of water more quickly to High Island Creek. Increased runoff leads to more erosion of the stream channel. This impairs water quality and diminishes habitat for fish and macroinvertebrates.
In other parts of Minnesota where watersheds have had fewer land-use changes, rivers have remained more stable, resilient, and able to withstand the impacts of climatic changes. Since no one has control over the timing and magnitude of rainfall events, we must take precautions to lessen the impact of these events to attain stability in our southern rivers.
By definition, a stable river can transport the water and sediment of its watershed while maintaining its essential shape and character. Channel size, pattern, gradient, and riparian corridor are all critical components to reduce bank erosion and allow streams to maintain quality habitats for biotic communities. Stable rivers also have immediate access to their floodplains so extra water can readily flow onto surrounding lowland, resulting in minimal bank erosion in the channel.
Prior to European settlement in the 1800s, rivers in southern Minnesota had much different-looking watersheds. Lakes, wetlands, wet meadows, and perennial vegetation covered the land, collecting a majority of precipitation that fell in the watershed. Since European settlement, most of this region has been turned into agricultural fields with underground tile networks, drainage ditches, channelized rivers, roads and other impervious surfaces, and urban areas—all speeding the movement of water toward local rivers. Climatic changes have amplified the effects of landscape conversion.
DNR hydrologists have been studying precipitation and flow changes at long-term stream-gauge locations, and trend analyses have shown shifts in hydrology dating back to the early 1900s at multiple locations. One climate trend that has been apparent at most gauge sites is the frequency of extreme years. Because some years are extremely wet while the next may be extremely dry, the trends stay "average" even though it is apparent that conditions are not average anymore.
When relating hydrology to the livelihood of a river's biological communities—fish and macroinvertebrates, such as caddisflies and midges—we must look at flow rates these communities experience throughout the year. For example, High Island Creek had a record-breaking flow around 4,000 cubic feet per second June 20, 2014, after intense rainfall covered its watershed. (That's roughly equal to 4,000 basketballs flowing past a stream gauge every second.) By Aug. 1, the creek had dropped below 100 cfs. Prior to ice-up in November, it was flowing at only 6.5 cfs. Fish and other aquatic organisms had experienced record-high discharge and nearly zero-flow conditions within five months.
When assessing a river's health, DNR specialists look for barriers to fish migration, the ability for a channel to regularly access its floodplain, and the impact of improperly sized road crossings.
A stable river allows organisms to use the whole stream without barriers. Dams, culverts that are too high, and waterfalls limit the ability of riverine fish to re-establish their communities upstream of the barrier. In a section of the Minnesota River watershed near Mankato, DNR specialists found that natural and artificial barriers inhibited fish movement at nearly half of the biological sample sites monitored by the Minnesota Pollution Control Agency. A graduate research study by Minnesota State University, Mankato showed that a low-head dam on High Island Creek prevented 27 of the watershed's 42 fish species from moving upstream. After the 2014 flood breached the creek's dam, essentially eliminating the barrier, DNR crews found nine more species that were not previously collected upstream.
A river's floodplain provides refuge and spawning habitat for some fish and other biota during times of extreme flows. Sediment and nutrients settle in the floodplain, providing clean water. By storing water, an accessible floodplain—without buildings or other development—also reduces the severity of flooding downstream. Floodplains are designed by nature for nutrient and sediment removal for the river in high flows. However, if a row-crop field in the floodplain is inundated, the river could pick up contaminants such as pesticides and excess nutrients and transport them downstream.
Some southern Minnesota rivers still have connections with their floodplain, but often they do not have the proper floodplain vegetation. Buffer strips of perennial vegetation along the stream corridor are important for a variety of reasons. Most important, vegetation can help hold soil on the stream banks. A 2014 buffer study done in 37 southern Minnesota counties by the nonprofit Environmental Working Group found many streams and ditches lacked buffers, in spite of existing laws and ordinances that required them. In response, Gov. Dayton proposed and the Legislature approved a new buffer law in 2015. It requires vegetated buffers of up to 50 feet to be installed along lakes, rivers, streams, and ditches by 2018.
Road crossings on rivers create a lot of stress due to improper sizing. Many times, bridges and culverts may be sized large enough for the river channel but not for the floodplain.
"Floodplains are commonly overlooked in the design of our road crossings," says Kevin Zytkovicz, DNR hydrographer. The typical design of a road crossing forces all of the water on the floodplain to funnel through an undersized opening. This accelerates water velocities and increases bank and channel erosion downstream. And it causes more sediment to be deposited on the floodplain upstream. This could be remedied by designing road crossings to better accommodate floodwaters.
Understanding the natural tendencies of rivers is an essential skill for predicting future conditions of local rivers. Draining wetlands, channelizing rivers, building levees, expanding drainage networks, removing buffer strips, and other human activities in many southern Minnesota watersheds have led to a succession of changes.
When a channel experiences changes in its watersheds, especially detrimental changes, it becomes unstable. The streambed may erode downward to a point where the stream no longer has access to the floodplain. Without frequent floodplain access, a stream holds too much energy within its channel, forcing it to widen. Once the river starts widening out, bank erosion accelerates even more. This increases turbidity and adds to the total suspended solids during high-flow events.
Pat Baskfield, MPCA hydrologist and coordinator for the Watershed Pollutant Load Monitoring Network, has witnessed these changes firsthand at his home on the Watonwan River. "When I first moved here 17 years ago, a tree falling from the bank would bridge across the river," Baskfield recalls. "Now it wouldn't come close, due to the channel being much wider."
Channel characteristics are important to assess when studying aquatic biological communities. For example, when rivers become too wide, fine sediments commonly fill pool habitat, removing refuges that fish rely on throughout the year. It is also common for fine sediment to inundate the river's riffles where fish spawn and feed. These changes make it difficult for fish and macroinvertebrate communities to live and flourish.
Keeping Water on Land.
Until stable hydrology is achieved, rivers in southern Minnesota will continue to change their dimension, pattern, and profile. Stability depends on multiple natural connections that a river should have. A free-flowing river without barriers has annual access to its floodplain, riparian buffer strips, and input from groundwater. And a stable river has a healthy diet, without too much runoff and sediment.
There are multiple ways of retaining water. Likely the most effective way to reduce impacts of storm events is to restore historical wetlands and lakes that can catch, filter, and store water. Landowners can install grassed waterways and terraces. To reduce soil erosion and improve moisture retention, farmers can increase organic matter of soil, leave residue (corn/bean stalks) on the land, and plant cover crops between growing seasons. Water-retention practices will also help agricultural producers in periods where rainfall is limited. In urban settings, surge basins, storm-water ponds, rain barrels, rain gardens, and other practices can help reduce the peak flows of rainfall events, so rivers do not have to endure such high discharges.
It is time that we put our rivers in southern Minnesota on a healthier diet. In order to realize our aim of attaining stable rivers, everyone needs to play a part in holding some water on the landscape to allow for groundwater aquifer recharge, flood reduction, and water-quality protection.
So when the rains come in spring, ask yourself, "How can I keep this water from getting to nearby rivers?" Water running off the land feeds a river nutrients and sediment it does not need. By reducing runoff, you can help make local waterways cleaner, healthier, and more stable.