Modelling aquatic vegetation and dissolved oxygen after a flood event in the Bow River, Alberta, Canada

Abstract

Traditionally, macrophyte density has been considered the primary factor affecting the large dissolved oxygen fluctuations in the Bow River. After a major flood event scoured macrophytes in 2005, and subsequently changed river dynamics, the City of Calgary needed to update their predictive computer model for water quality to reflect the new conditions, which led to this study. A 2006 aquatic vegetation survey was also completed to assess post-flood conditions. The survey found that the average macrophyte dry weight was much lower (28 g/m2 ± 100 (p = 0.05)) than the historic average of 241 g/m2 ± 29, while the average periphyton chlorophyll-a concentration was higher (343 mg/m2 ± 71) than the historic average (158 mg/m2 ± 17)). Dissolved oxygen (DO) fluctuations were similar to pre-flood levels despite changes in the dominant vegetation. Using the results of this survey, the significant and previously unrecognized effects of periphyton diurnal processes on DO concentrations in the Bow River were identified and the Bow River water quality model (BRWQM) was recalibrated to reflect these findings. Adjustments were made to the BRWQM’s periphyton submodel to account for the more dominant role played by these organisms in river processes, and a competitive shading factor between macrophytes and periphyton was also introduced to more accurately model the species' competition for available sunlight. This newly calibrated and validated version of BRWQM was tested and found capable of predicting the occurrence of low DO concentrations in the Bow River and can provide a useful tool for forecasting the water quality effects of the city's planned wastewater infrastructure expansion.