| Abstract/Description or Keywords |
The development of Kenney Dam and the Skins Lake Spillway as part of the Kemano Project in the early 1950’s caused changes in flow and sediment supply to the Nechako River. In conjunction with these changes there has been a reduction in juvenile White Sturgeon production, with the low number of juvenile sturgeon being attributed to changes in spawning habitat. The Vanderhoof Reach has been identified as a critically important spawning reach, prompting several investigations to assess the historical and contemporary characteristics of the reach. As part of the ongoing sturgeon recovery effort for the Nechako River, Northwest Hydraulic Consultants Ltd. (NHC) developed a suspended sediment and bedload sampling program in the spring of 2014. This program was subsequently implemented with the help of hatchery and MFLNRO staff. The bedload data demonstrated that a bedload-discharge rating curve can be established for both the Upper Site and Lower Patch, and summary data are provided below. As shown in the above table, in 2013 and 2014, about 2000 m3/year more sediment was moved past the Lower Patch than the Upper Site. The 2013-2014 annual load data suggest that each year, 2 to 8 percent of the total amount of bedload introduced by the Cheslatta fan avulsion (> 0.5 mm, 44,000 m3) moves past the sites. This is a much smaller percentage than estimated in previous reports (NHC, 2013, 2014). The previous analysis were based on BC watershed yield data (NHC, 2013) and a single bedload sample (NHC, 2014), and as such, were less reliable. For example, the 2011 predicted bedload at the Upper Site and the Lower Patch is 10,600 and 28,400 m3, respectively. Also, the 2012 predicted bedload at the Upper Site and the Lower Patch is 5,700 and 16,100 m3, respectively. These volumes are significantly higher than that measured in 2013 and 2014 as shown in the above table. Collection of bedload samples during future high flows will be required to confirm the applicability of the rating curve during high flow years like 2011 and 2012. Going forward, the bedload sampling program should concentrate on collecting samples immediately after ice clears, as well as at the beginning, midpoint and end of the cold water release period while flows are high and bedload transport rates would be maximum. The cold water release samples are particularly important to collect if transport rates become supply-limited, which would help guide if and how sturgeon habitat sediment mitigation efforts could be implemented, and if a single bedload-discharge rating curve can be used during periods of prolonged high flows. The suspended sediment data suggest that the majority of the suspended sediments are supplied during the freshet, rather than the cold water release period which has a much larger discharge than freshet. This could be explained by several processes. First, the tributaries are a significant source of fine sediment to the mainstem during freshet and not during the release. Second more bed and/or bank material is mobilized during freshet and not during the release. This suggests that possibly the regulated and greater release flows may be less effective in eroding the bed and banks or mobilizing the previously deposited bed material compared to freshet. Collection of additional data will assist in identifying if this is the case. As shown in the above table, the measured 2014 total sediment load (bedload and suspended) for the Upper Site and the Lower Patch is 18,400 and 20,400 m3, respectively. This suggests that the bedload/total sediment load ratio was 5% for the Upper Site and 14% for the Lower Site. These values straddle the 10/90 percent split between bed and total loads that is commonly expected in gravel-bed rivers such as the Nechako River, and suggest that the bedload transport rates are not out of step from the suspended sediment load. The outlet of Murray Creek and Stoney Creek are located between the Upper Site and the Lower Site. Suspended sediment may therefore be higher at the Lower Site due to the input from these creeks. A turbidity sensor was installed on the center pier of the Burrard Avenue Bridge in October 2014 to collect data representative of the main Nechako River. This sensor is in addition to an existing one located on the left bank of the river which was more directly influenced by the suspended sediment from Murray Creek. Collection of bedload samples and turbidity data from the 2015 freshet and cold water release, especially during break up, will greatly improve our understanding of the overall sediment regime and influence of the tributaries on sediment inputs to the Nechako River. This information will then help determine when sediment is passing the sturgeon spawning locations, and if upland sediment production prevention programs may assist the White Sturgeon Recovery Initiative. |
