Characterizing ice cover behaviour along the Slave River

Abstract

River ice is an important component of the traditional way of life for the communities along the Slave River both culturally and economically. During the winter, a stable ice cover provides local residents with safe access to their traditional hunting, trapping, and fishing grounds along the river. Periodic spring ice breakup flooding is required to maintain the ecological balance along the Slave River Delta. Recently, however, local observations have indicated changes in ice cover characteristics (e.g. air pocket formation, double layer ice, ice cover flooding) during the winter, which increase the risks of travelling on the ice. Also prolongs dry periods during the spring are leading to rapid growth of invasive vegetation that reduces the lake and channel areas of the Delta. Although some attempts have been made to understand the patterns of spring flood frequency in the Delta, very little is known about the Slave River’s ice cover characteristics and behaviour. Remote sensing techniques and field surveys were used in this study to understand the ice cover progression and to examine ice cover characteristics along the river during the winters of 2013-2014 and 2014-2015. RADARSAT-2 satellite imagery captured the changes in the ice cover and identified different types of ice during the winter seasons at two primary study sites – downstream of Fort Smith and the Slave River Delta. The mechanism of ice cover growth, with the formation of air pockets and layers underneath the ice cover was investigated. Steeper channels and several open water sections appear to be contributing to significant amounts of air entrainment into the water in winter. Changes in the hydraulic characteristics due to flow regulation and ice cover progression can also change the quantity and distribution of air pockets along the river ice cover. Additionally, the impact of flow fluctuations on the ice cover (e.g. ice cover flooding) was also observed. Increases in discharge cause the ice cover to crack or dislodge from the river banks, leading to water seeping onto the ice and flooding it, which has implications for the muskrat and beaver populations. A geospatial model was developed to determine the spatial patterns of ice cover breakup along the river from Fort Fitzgerald to the delta. This model successfully identified the areas of breakup initiation and persistence of ice until the end of the breakup. MODIS satellite imagery was used to describe the temporal patterns and evolution of breakup events between the years 2008 and 2011. In addition to geomorphological influences, air temperature and flow conditions also have strong impacts on the spatial and temporal patterns of the ice cover breakup.