by Michael Hart, Hart Environmental, Boulder

This summer the City of Boulder opened a new section of its South Boulder Creek Trail system. The new trail section runs east-west between Marshall Road and South Boulder Creek. Along the new trail is a complex of wetlands that includes wet meadows, willow shrubland, cattail marsh/meadow and open water. This diverse wetland ecosystem located in the South Boulder Creek floodplain is in part the result of the reclamation of land that was once mined for sand and gravel.
The nation’s aggregate mining industry, which includes sand, gravel, and crushed stone as well as industrial sand, is found in one form or another in all fifty states. Due to the diversity of the aggregate mining industry and the various factors that influence the location of mining operations, it is not uncommon for these mining operations to encounter wetlands. It is also not uncommon for aggregate mining and reclamation activities to result in the purposeful creation of wetlands. Here in Colorado sand and gravel is the primary source of aggregate and because these materials are alluvial in nature, they are most often excavated along stream corridors such as South Boulder Creek.
The wetlands that are located along the new trail section were purposely created through the reclamation process. The reclamation plan for this area, known as the Marshall Pit, involved the creation of a series of four settling ponds that were used to dispose of fine particles washed from the sand and gravel after it was excavated. The settling ponds acted to decant the gravel wash water by settling out fine particles. The four ponds were filled with wash fines over a period of four years. Because these fines have the ability to retain moisture, they serve as an excellent medium for the growth of wetland vegetation.
The ecological goal for the Marshall wetlands was to create a diverse hydrologic regime capable of supporting a variety of wetland plants that typically exist along the stream corridors and floodplains of the Colorado Front Range. To date, 13 acres of wetlands have been created at the Marshall site and 20 acres have been created at the Deepe Pit, which is contiguous to and north of the Marshall site. In order to establish hydrologic conditions capable of supporting diverse wetland plant species, surface water control structures have been installed between each settling pond/wetland basin. These are passive devices designed to maintain basin water elevations sufficient to establish various zones of saturation in each wetland basin. This self-sustaining hydrologic regime will rely on natural ground water fluctuations associated with the alluvium of South Boulder Creek and the surface waters that drain on to the site from lands to the west.
Vegetation monitoring at the Marshall site has shown that the dominant species in the wetland areas are primarily wetland species that contribute to the development of hydrophytic vegetation. The abundance of these species at the site indicates that the hydrologic conditions necessary for wetland development are present. In addition to the development of wetland structure, observations suggest that a variety of wetland functions is also present within the created wetland areas.
The wetland areas provide a variety of different habitat types that support a diverse assemblage of wildlife species. The open water areas provide resting areas for resident and migratory waterfowl as well as providing suitable conditions for fish, amphibians and aquatic reptiles. The shallow water provides hunting areas for large shorebirds like great blue herons. The meadow and mud flat areas provide habitats for small mammals and smaller shore birds, and the cattail marshes provide nesting sites for red-winged blackbirds. Willow shrubland areas provide nesting sites for a variety of songbirds as well as shelter for white-tailed deer. The open water and the diverse wetland types associated with the settling basins provide important habitat features that complement the riparian vegetation associated with the adjacent floodplain areas along South Boulder Creek.
The Marshall Wetlands project is an excellent example of innovative mined land reclamation. It illustrates that under the right circumstances sand and gravel mining can ultimately create valuable wetland ecosystems through the reclamation process.

Colorado Riparian Association