As a part of the planned expansion for the existing Borrego Landfill facility in the desert region of San Diego County, HELIX Environmental Planning performed biological studies, constraints analyses, and a preliminary environmental assessment for this project. The biological studies included comprehensive survey and mapping of vegetation communities, sensitive species habitat, potential jurisdictional waters and wetlands, and other sensitive biological resources. Specific evaluation was required during both desktop and field review to determine downstream connectivity and nexus to waters of the U.S., or lack thereof, pertaining to several dry desert wash and erosion features in the study area. It was concluded that some of the features continued further to the south of the study area, eventually connecting with the Borrego Sink Wash, which discharges into San Felipe Creek. San Felipe Creek is believed to have a significant nexus to the Salton Sea, which is a traditional navigable water (TNW). Therefore, some of the unnamed drainage features were determined to potentially qualify as waters of the U.S. under the regulatory jurisdiction of the U.S. Army Corps of Engineers (USACE), RWQCB, and/or California Department of Fish and Wildlife (CDFW) based on the presence of a defined and consistent OHWM and bed and bank; downstream connectivity to the Salton Sea, which is a TNW; and, presence of other streambed characteristics.