Designing and constructing large-diameter gravity sewer systems in environmentally sensitive corridors presents significant technical and regulatory challenges. This presentation examines the permitting and design considerations for a major sewer trunk replacement involving multiple crossings of Mill Creek in Nolensville, Tennessee—a protected stream and critical habitat for the endangered Nashville crayfish. The project required extensive coordination with the U.S. Army Corps of Engineers and the Tennessee Department of Environment and Conservation (TDEC) to satisfy federal and state environmental requirements, including Section 404 permitting. The existing 24-inch sewer was undersized and required upsizing to 36-inches to accommodate current and future flows. Increasing pipe diameter introduced cover constraints at Mill Creek crossings, necessitating an iterative design process to balance added hydraulic capacity with preservation of the existing streambed and aquatic habitat. Regulators were concerned with any change to the existing stream that could inhibit crayfish and fish navigation; modification to the normal pool elevation; and stream temperature increases. Key topics include strategies for minimizing ecological impacts, navigating complex permitting processes, and selecting construction methods that protect water quality and sensitive species while maintaining project schedules. Attendees will gain practical insight into balancing critical infrastructure improvements with environmental stewardship and lessons learned applicable to future gravity sewer projects involving protected species and challenging stream crossings.