Urban watershed health and resilience, evaluated through land use history and eco-hydrology in Swan Lake watershed (Saanich, B.C.)




Townsend, Lise

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Swan Lake watershed, a sub-catchment of the salmonid bearing Colquitz Creek watershed located in the municipality of Saanich, on southern Vancouver Island, British Columbia, Canada, was studied to characterise the linkages between urbanisation and ecological health and resilience. Although rarely applied in watershed ecology, resilience (the ability to absorb disturbances without the loss of ecosystem identity) offers a useful construct in this case study to understand the effects of urban development over the past 150 years, and to outline some principles for integrated, watershed-scale management. Baseline landscape characteristics and processes of historical land-use were determined using paleoecology (pollen analysis) and historical records. Watershed health was assessed using: a Proper Functioning Condition assessment of riparian-wetland and stream channels; vegetation community mapping; vegetation plots; surface flow hydrology; and water quality analyses. Vegetation and lake hydrographs were compared with less disturbed reference ecosystems. Findings are discussed in terms of alternative stable state models and energy dissipation at the site and landscape scale. Analysis of the data revealed that over the past 150 years, forest clearing, agriculture, transportation infrastructure, and non-point source pollution have transformed the landscape and substantially altered the water and energy balance. Impervious surfaces and cleared land (covering 25% and 35% of the watershed, respectively) are inferred to have reduced latent heat dissipation of solar energy, an important landscape-scale process affecting resilience to climate change. Degraded stream channels represent reduced ecosystem services and lost social/economic value. The stream/lake hydrographs revealed a typical, urban flashy profile that exacerbates channel erosion and non-point source pollution, while excessive lake stage drawdown is also evident. Water quality is characterized by historic and ongoing excessive nutrient loading and associated cultural eutrophication, heavy metal pollution, and ecosystem “ageing” due to dissolved solids runoff. At the site level, invasive species, particularly reed canarygrass, dominate Swan Lake wetlands, whereas the pollen record shows abundant woody shrubs and associated species (some of which are now extirpated from the site) and an absence of grass; this helps to establish a rationale for vegetation management. Based on the findings of the above studies and according to a proposed conceptual model with assessment criteria in five categories (water, vegetation, energy, soil and nutrients), Swan Lake watershed has impaired ecological health and is not resilient to disturbances such as extreme climate/weather events. Future watershed management should therefore mimic the hydrological function and energy balance of the pre-development conditions.



watershed, urban ecology, resilience, hydrology, eco-hydrology