When the Tides Don’t Turn: Floodgates and Hypoxic Zones in the Lower Fraser River, British Columbia, Canada

Floodgates are common flood control structures in coastal river systems, which allow tributary drainage into river main stems and decrease flooding risk of land upstream of diking systems. Floodgates have been shown to impact upstream aquatic habitats and alter organismal community structures in some systems by impounding water and acting as a physical barrier to migratory species; their impacts on water quality have been less well described. This study investigated water quality in tidal creeks with and without floodgates on the lower Fraser River, British Columbia, Canada. There are an estimated 500 floodgates in this region. Water quality measurements were taken upstream and downstream at three floodgate sites and three reference sites across a 10-day period in July/August. The average dissolved oxygen (DO) concentration upstream of floodgates was 2.47 mg/L and fell as low as 0.08 mg/L, which was significantly lower than the comparable region of reference sites (8.41 mg/L) during this sampling period. In contrast, the average DO concentration downstream of floodgates was 7.38 mg/L and in reference sites 8.35 mg/L. All DO concentration measurements upstream of floodgates in July and August fell below the 6-mg/L minimum set by the Canadian Council of Ministers of the Environment. These hypoxic zones extended at least 100 m upstream of floodgates. Thus, floodgates may be facilitating the occurrence of local hypoxic zones in summer months in these locations. Floodgate-induced hypoxia may not only cause local exclusion of sensitive native fishes but may also act as a chemical barrier that decreases connectivity among aquatic systems. Understanding these environmental impacts associated with floodgates can inform floodgate design and post-installation management, which is an increasingly important issue as coastal municipalities across the world deal with aging floodgate infrastructure and sea level rise.