Unionid Mussel Surveys, Relocation, and Monitoring for the Interstate 74 Bridge Replacement Project

Project located within the Rock Island District, Mississippi River Pool 15 

2014 - Present

  • Mussel sampling in large turbid rivers 

  • Large riverine biological survey and assessment 

  • Statistical analysis 

  • Impact analysis 

  • Surface supplied air diving 

EcoAnalysts, Inc. (formerly Ecological Specialists, Inc.) was subcontracted by Stantec Consulting Services, Inc. (Stantec) to assist with endangered unionid mussel issues associated with the replacement of the Interstate 74 Bridge over the Mississippi River. In 2014, EcoAnalysts conducted a quantitative mussel survey to determine the density, distribution, and species composition within the new bridge construction area and existing bridge demolition area. A significant mussel community, including federal and Illinois endangered species, was located in bridge construction and demolition areas within Sylvan Slough on the Illinois side of the river. Although density was lower, federal and Iowa endangered species were also found near the Iowa bank. EcoAnalysts assisted Stantec and Iowa DOT with agency coordination (USFWS, Iowa DNR, Illinois DNR), impact assessment, and minimization and mitigation development for the Biological Assessment (prepared by Stantec).

In 2015 EcoAnalysts conducted surveys of mussel beds in Pools 15 and 16 to find suitable recipient sites for relocated mussels. Seven mussel beds were delineated and characterized using qualitative and quantitative methods to determine density and species composition. Five of these sites harbored similar species to the I-74 bridge and could serve as suitable recipient sites. Cumberlandia monodonta (a federally endangered species) is typically found within large rock in higher current velocity areas. Three sites within Sylvan Slough were found that could accommodate this species. Due to concerns with relocating C. monodonta, a pilot study was conducted in the fall of 2015. Ten individuals were collected from the existing pier, tagged with PIT tags, and relocated to the three identified areas. These mussels were monitored twice (in 2015 and 2016, respectively), and will be monitored periodically for the next 10 years.

In 2016, EcoAnalysts assisted with agency coordination, developing take estimates, and developing monitoring and mitigation studies. In one of the largest known mussel relocations in the country, EcoAnalysts then relocated approximately 153,000 mussels from the new bridge corridor in 2016 and 2017. Thirty-two species were relocated, including three federally endangered species and four additional state threatened or endangered species. Relocated mussels were placed in recipient areas at varying densities. EcoAnalysts will monitor these areas periodically over the next 10 years to evaluate how large increases in mussel density affect both resident and relocated mussels. Baseline data for monitoring was collected prior to the 2016 relocation, and the first monitoring events were conducted in 2017 and 2018. In addition, EcoAnalysts will conduct monitoring in the new bridge construction area to examine unionid relocation.

For each phase of the project (initial survey, recipient site investigation, relocation, annual monitoring), EcoAnalysts prepared technical reports detailing survey methods, results, and recommendations; monitoring reports also include statistical comparisons of data over time.