The continuing and increasing need for stream mitigation to offset impacts from highway construction has prompted MoDOT to seek alternative stream restoration projects that provide greater benefits to stream habitat, thereby generating a greater amount of mitigation credits available for future use. MoDOT sought advice from MDC on needs regarding stream work in areas where large corridor projects are going to be constructed. Missouri fisheries biologists suggested the treatment of low water crossings within federally threatened fish (Niangua Darter) designated critical habitat as a priority project providing long-lasting stream benefits.
Low water crossings on Missouri county roads can limit aquatic organism movement because they create jump and velocity barriers. Removal and modification of these structures enables fish populations to reconnect, increasing opportunity for genetic diversity and boosting population growth. The new open-bottom spans will also improve sediment transport and should act to minimize continuing maintenance costs.
Collaboration with resource agencies, regulatory agencies and the County, was necessary to gain approval of the project. This is the first MoDOT Stream Mitigation Bank approved by the regulatory/resource agencies. It is an innovative approach to mitigation that demonstrates MoDOT’s commitment to quality stream mitigation projects. The project purpose enables MoDOT to use its greatest asset, engineering, to couple with natural design concepts to make an excellent mitigation project. Additional information is available in the handout “Low Water Crossing Modification as Stream Mitigation Technique.”
Pre-cast spans have been used recently in projects done by others in Niangua darter habitat. This is an emerging technique that continues to develop with each structure addressed. Three low-water crossings (Green’s Slab, Griswald’s Slab, and Howard’s Ford) will be modified using open-bottom, pre-cast spans to enable the natural stream bottom to adjust with flow events and enable more natural sediment transport through the structure. A brief article, “Low-Water Crossings Receive ‘Green’ Improvements,” was featured in MoDOT’s statewide newsletter, Connections.
In 2002, the Federal Highway Administration (FHWA) identified ecosystem conservation as one of three performance objectives under the agency's Vital Few Goal of Environmental Streamlining and Stewardship. Ecosystems are interconnected communities of living things and the physical environment within which they interact. Preserving and enhancing ecosystems is critical to protecting our diverse biological resources, and sustaining our communities and economies that rely on their products. As a demonstration of its commitment to this goal, FHWA agreed to annually identify exemplary ecosystem initiatives in States or Federal Highway divisions. In 2009, the Missouri Department of Transportation’s “Low-Water Crossing Modification as a Stream Mitigation Technique” was selected by FHWA for an Exemplary Ecosystem Imitative award.
Green’s Slab
Inter-agency partnerships were formed between MDC, Missouri State University (MSU) and MoDOT to monitor the Niangua darter. MDC and MSU are conducting long-term in-stream biological monitoring to determine if these low water crossing improvements will have a positive demonstrable affect on Niangua darter populations. This data collection will prove valuable to both state and federal regulatory agencies in their future management decisions involving the darter throughout its range in Missouri. The data gathered from the monitoring will be used as a part of the criteria to determine project success.
Griswald’s Slab
Low water crossings are common across the state and the partnerships developed during this project continue to provide great mitigation projects that stem from watershed and habitat needs. A second low water crossing mitigation project in Niangua darter habitat is being pursued for another road project. This additional crossing was identified as a priority by resource agencies since the other three in the same vicinity are being addressed.
