Seiche events at the Slave River Delta and Great Slave Lake, Northwest Territories: An assessment of process and impact

Seiche events are important hydrodynamic processes that impact the Slave River Delta, during the late summer and autumn. Given the scarcity of previous research on seiche events at river deltas and the physical dimensions and orientation of the Slave River Delta, this location at the southern shore of Great Slave Lake is ideal to study the occurrence and impacts of seiche events at a river delta. Seasonal changes to long-term water level trends and water level variability on Great Slave Lake, correspond to changes to the Slave River flow potentially associated with the impacts of regulation of the Peace River. However climate change in the Slave River basin has not been quantified and may have a role on the hydrologic changes noted at the lake. An assessment of lake level variability provides the hydrological context in which to address seiche events on the lake. Seiches are prevalent on the lake from July to ice freeze-up. however they are most prevalent during the autumn when lake levels approach the lowest open-water seasonal level. Seiches are defined by the magnitude of the fluctuation of water level. There have been some changes of the periodicity, frequency and typical magnitudes of seiches prior to and subsequent to river regulation. Water level records from Fort Resolution and Yellowknife Bay, NWT, provide a long record of seiche events. These are identified in the data record. Seiches that set-up against the south shore of the lake are forced by northwesterly winds predominantly. Wind speeds typically range from 15-30 km/h. Seiche set-down events are forced by southeasterly winds of the same velocity. It follows then that seiche set-up and set-down events at the north shore of the lake, are driven by the opposite wind directions. Southeasterly winds force seiche set-up, while seiche set-down events are forced by northwesterly winds at Yellowknife Bay. Data from the delta distributaries demonstrate the temporal and spatial dynamics of seiche events. The delta has a very low slope, which enhances the propagation of seiches up distributary channels. Seiches are theoretically both an erosive process and a deposition process. This is largely based on the morphology of a distributary. Sampled in-channel hydraulic data provide evidence of the fine-scale impacts of seiches to distributary hydrodynamics. Specifically, reduced flow velocity data provide some preliminary evidence that seiches may slow the transport of water through the delta distributaries during seiche events. Further research of the seiche impacts on suspended sediment concentrations is warranted given the large mass of sediment transported by the Slave River