| Abstract/Description or Keywords |
The Kenney Dam and Nechako Reservoir have regulated flows in the Nechako River since 1952. A number of studies have examined the consequences of reduced annual flows, reduced annual peak flows, and increased sediment supply from the 1961 Cheslatta River avulsion on the morphology of the Nechako River. These previous studies have usually focused on the upper Nechako River – from Cheslatta Falls to Vanderhoof – and have emphasized chinook salmon as the species of concern. White sturgeon in the Nechako River are endangered and attention has recently been focused on planning for their recovery. White sturgeon are now primarily found in the river between Vanderhoof and the Stuart River confluence, but are occasionally found as far upstream as Fort Fraser. Subsequent analysis has clearly identified that this population is undergoing a recruitment failure which began in the mid-1960’s, or about a decade after the closure of the Kenney Dam. In this report, we extend the assessment of geomorphic changes to the lower Nechako River – from Vanderhoof to the Isle de Pierre Rapids – emphasizing changes that may have affected white sturgeon life stages and contributed to the observed recruitment failure. Based on historical aerial photography and field reconnaissance, we found evidence that sand bedload deposition started in the Hulatt/Finmoore area in the 1964 freshet – the first significant flood event following the Cheslatta avulsion – and is ongoing to the present day. The sand deposition probably covered coarser gravel substrate, possibly reducing sturgeon spawning success. We infer that the 1964 freshet may also have started the process of pool infilling in the meandering river reach between Vanderhoof and Hulatt Rapids, possibly reducing sturgeon overwintering success. The timing of these changes coincides closely with the sturgeon recruitment failure. Water Survey of Canada gauging records and field reconnaissance suggest that a wave of sand and granule bedload passed the Vanderhoof gauge site between the late 1960’s and the early 1990’s. The bedload wave probably consisted of Cheslatta-derived material slightly coarser than the material that passed through in 1964 without affecting channel morphology. The wave appears to have passed now, and the ongoing sand deposition in the Hulatt/Finmoore area will likely cease within the coming decade or so. The riverbed will then downcut somewhat in response to reduced sediment supply, but the substrate will not return to its pre-regulation character because of the reduced flow conditions. Similarly, infilled pools in the meandering reach between Vanderhoof and Hulatt Rapids will not reform to their pre-regulation depths because the reduced flows no longer produce the same scour velocities that they once did. We found no evidence of geomorphic changes downstream of the Stuart River confluence, apart from localized growth of bars within the first 2.5 km caused by bed adjustments in the lower Stuart River as Nechako water levels dropped. Stuart River flows moderate the influence of regulation on Nechako River flows downstream of the confluence, and coarse sediment inputs from the Stuart River control substrate character in this section. The wave of sand and granule bedload travelling downriver may have altered the substrate composition by filling cobble- gravel voids, as opposed to covering the existing substrate, but we had no way of examining this. |
