False Bay Creek Water Flow

Abstract

The False Bay Creek watershed on San Juan Island, WA, USA supplies freshwater for wells used by residents, drinking water for the city of Friday Harbor and water for agriculture and ranching in the lower watershed. On a small island, such as San Juan Island in the San Juan Archipelago, fresh water is precious and must be used efficiently to serve many people and competing interests. There is a strong community desire to conserve and recover ecological conditions in the False Bay Watershed, including improving habitat conditions for associated fish and wildlife. The community is interested in better characterizing and understanding the role for freshwater flows in the watershed, especially as it relates to the system’s capacity to recover lost aquatic habitats and species. Water usage and flow in the creek, which is in the largest watershed on the island, is unknown as there has been no long-term monitoring of freshwater flow levels or water volume in the creek. This study was designed to initiate a long-term stream flow data collection and monitoring plan for False Bay Creek. This was accomplished by field sampling at two locations in the watershed, capturing precipitation and flow transitions from the end of the dry season (May-October) and the beginning of the wet season (October-May). Stream flow data was collected from 10/20/2021 to 11/24/2021 at two locations on False Bay Creek, one in the upper watershed downstream of a city pumping station “Aug-2” and one in the lower watershed in a culvert under Bailer Hill Road. The water flow at the Aug2 location ranged from 0.13 cubic feet per second on 10/20/2021 to a high of 13.98 cubic feet per second on 11/22/2021. The water flow in the culvert under Bailer Hill Road ranged from -0.22 cfs on 11/8/2021 to a high of 30.91 cfs on 11/24/2021. While the shift from summer to fall brought expected increases in precipitation, San Juan Island also received an unprecedented amount of rain fall (3.9 inches in 48 hours) from 11/12 to 11/14, causing flooding events and resulting in higher than expected flows, such as between 11/16-11/21 where sampling was impossible. The existing infrastructure was unable to handle the extreme flooding events. Given climate models predicting more intense storms with greater frequency, this flooding event provided a window into future stream flow collection needs and challenges. Here we report on initial flow monitoring and sampling methodology. Then we highlight the vulnerability of existing infrastructure on Bailer Hill Road and describe how drainage infrastructure and sampling design may need to be updated and more data collected prior to initiating any modifications to the dams in the watershed. I also recommend adding a future testing location below Trout Lake in the upper watershed and a future testing location below Lower Zylstra dam in the lower watershed to expand on the data collected in this study.