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Missouri: Low-Water Crossing Modification as a Stream Mitigation Technique

Pre-construction photo of Missouri's Niangua darter low-water crossing modification.
Pre-construction

Once the Missouri Department of Transportation (MoDOT) recognized that it had five transportation projects in its portfolio requiring stream mitigation, the agency sought advice from the Missouri Department of Conservation (MDC) on restoration needs in these project areas. MDC fisheries biologists suggested a comprehensive approach to improving low water crossings within the designated critical habitat of the Niangua darter (Etheostoma nianguae), a Federally listed threatened, State-listed endangered fish endemic to the Osage River Basin.

The existing range of the Niangua darter includes stream systems impaired by a number of low water road crossings. These crossings are bridges that are dry and passable roadways during low-flow conditions but that can overflow during high-flow conditions, precluding vehicle traffic. If not properly designed, these crossings create significant habitat barriers for aquatic species. Modifying these structures with open bottom precast spans enables aquatic organism populations to reconnect, increasing opportunities for genetic diversity and boosting population growth. Open bottom spans also restore stream function by improving channel stability and improve habitat by facilitating sediment transport through the system. In addition, open bottom spans minimize ongoing maintenance costs throughout the life of the structures and reduce the duration and frequency of flood events, thus increasing safety for vehicular traffic.

Post-construction photo of Missouri's Niangua darter low-water crossing modification.
Post-construction

MDC and U.S. Fish and Wildlife Service representatives prepared a priority assessment of low water stream crossings within the range of the Niangua darter and identified 32 low water crossing structures as priority structures affecting the species. MoDOT chose to consolidate its mitigation approach to upgrade four of these 32 crossings with open bottom spans rather than pursue four separate smaller mitigation projects spread out over a large geographic area.

MDC and Missouri State University are presently monitoring the performance of these improved low water crossings to determine if they enable Niangua darter populations to rebound as predicted. A Memorandum of Understanding between MoDOT and the county, which owns the crossings, outlines the appropriate maintenance practices to preserve aquatic organism passage and sediment transport functions in the future.

In this project, MoDOT coupled its greatest asset—engineering—with natural design principles to develop a comprehensive and ecological approach to stream mitigation. Following completion of the last crossing, the lower reach of the Little Niangua River will be free from barriers that have for decades fragmented the habitat of a vulnerable aquatic species and limited sediment transport. MoDOT believes that this project will promote habitat diversity and thus increase the odds of survival for imperiled species such as the Niangua darter.

For more information, contact Melissa A. Scheperle at Melissa.Scheperle@modot.mo.gov.

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