Lemon Creek Watershed Geomorphic Assessment and Sediment Management Alternatives Analysis - Juneau, Alaska
Lemon Creek is located in the City and Borough of Juneau (CBJ), Alaska, five miles northwest of downtown Juneau. The creek flows approximately seven miles east to west, from its headwaters at Lemon Glacier to the outlet at Gastineau Channel. Several small, clear water tributaries are included in the Lemon Creek watershed, which encompasses approximately 25 square miles of alpine and forested uplands, wetlands, and urban areas.
The Alaska Department of Environmental Conservation (ADEC) has identified Lemon Creek as an impaired water body, due to sediment, turbidity, and habitat modification. Designated, protected uses for Lemon Creek water include drinking source water, industrial and agricultural source water, contact and non-contact recreation uses, and growth and propagation of aquatic life and wildlife (Alaska’s Water Quality Standard Regulations, 18 AAC 70). ADEC lists urban runoff and gravel mining as primary pollutant sources to Lemon Creek (ADEC, 1995).
In the fall of 2002, the CBJ expressed concern about potential flooding in the lower, urbanized portion of Lemon Creek. Past gravel mining operations have provided a much-needed source of gravel for local construction projects, with the added consequence of lowering the streambed and increasing flood conveyance. Since no in stream gravel mining has occurred during the past 20 years, the streambed has aggraded over time. The channel’s capacity has been reduced, effectively increasing the likelihood of flooding of adjacent structures during major storm events.
Prior studies of the affects of gravel deposition on channel capacity were conducted. In 2002, the USGS surveyed 42 cross sections and developed a provisional HEC-RAS river hydraulic model along Lemon Creek from above Egan Drive to above the gorge. In 2003 and 2004, the USGS repeated the survey of a number of these cross sections. The USGS calibrated the provisional HEC-RAS model to a 2004 high flow event. In the fall of 2002, the CBJ hired Inter- Fluve, Inc. to conduct a reconnaissance level sediment transport study, documented in a report submitted to the CBJ in the spring of 2003. Findings indicated that gravel deposition would continue, possibly at problematic levels in response to individual flood events. A number of recommendations were made including further study of the Hidden Valley area to better understand the sediment supply reach and an analysis of sediment management alternatives.
Several state and federal agencies responded to the City’s request for assistance in developing sediment management alternatives that reduce flooding potential while conserving fish and wildlife habitat. Subsequently, CBJ, Alaska Department of Natural Resources (ADNR) – Office of Habitat Management and Permitting, Alaska Department of Fish and Game (ADF&G) – Sport Fish Division, U.S. Fish and Wildlife Service (USFWS), and the U.S. Department of Agriculture – Natural Resources Conservation Service (NRCS), signed a Memorandum of Agreement in March, 2004 (Appendix A) as a cooperative commitment to this project. The purposes of this study are to: · Assess the geomorphology and sediment transport regime of Lemon Creek, · Identify potential sediment management alternatives, · Identify fish and wildlife habitat protection and rehabilitation opportunities, and · Educate property owners within the watershed about the findings of this study.
Data and Resources
End Date: 2004/11/29