| Abstract/Description or Keywords |
The onset of flow regulation in the early 1950’s, as well as major tributary avulsions, have altered the flow and sediment regimes of the Nechako River causing notable geomorphic change. Juvenile white sturgeon production has declined in conjunction with these changes and has been attributed to the infilling of spawning beds with fine sediment (McAdam et al., 2005). A critically important spawning reach was identified at Vanderhoof and a series of investigations have been conducted to assess the historical and contemporary characteristics of the reach. As part of the ongoing Nechako sturgeon recovery effort, the sediment sampling program developed by Northwest Hydraulic Consultants Ltd. (NHC) in the spring of 2014 was once again implemented in 2015. The exceptionally high flow experienced in 2015, representing the third largest annual maximum daily discharge since 1952, presented an excellent opportunity to refine our understanding of sediment dynamics within the reach. Bedload and suspended sediment was sampled intensively from March to October with the help of Carrier Sekani Tribal Council, Nechako White Sturgeon Conservation Center and MFLNRO staff. To supplement this data, underwater imagery was taken to assess the availability of suitable larval habitat within the spawning area. In addition, we conducted a reach-scale survey to collect velocity, bathymetry and bankline topography data. The 2015 suspended load was used to back-calculate a basin sediment yield of 0.05 Mg/km2/day. This input rate plots below the lower limit of the normal trend for BC watersheds, similarly to other lacustrine landscapes. Data suggests bedload is roughly 12% (Ī5%) of the total load at Vanderhoof, and average bedload transport rate from the last 3 years was 3,400 m3/annum. Our revised estimate translates to a total basin yield of about 170,000 m3 of bedload sediment over the past 50 years, an order of magnitude less than previous estimates obtained by assuming basin yield is 0.7 Mg/km2/day and bedload is 10% of total load. The later was simply based on regional sediment yield maps from historic observations on unregulated rivers, and the discrepancy emphasizes the benefit of direct site observations. Bedload transport was higher at the Upper Site than the Lower Patch resulting in the storage of 6,200 m3 of sediment within the reach. This imbalance suggests high flow years may input a pulse of sediment that becomes stored within side-channels and transported downstream at a much slower rate. This dynamic is logical because backwatering during high flow extends about 1.5 km upstream of the Burrard Avenue Bridge, causing the deposition of bedload midway through the reach. Bedload transport is more variable at the Upper Site than at the Lower Patch because Upper Site transport rates increase with discharge rather than being moderated by backwatered flow. However, when averaged over time, our estimates indicate that bedload yield through the reach is between 2,000 m3 and 4,000 m3 per year. It is unclear what this dynamic looked like in the pre-regulation period as peak flows would have been higher and even more material would be expected to deposit in the upstream area. During the summer of 2015 the bedload transport rate at the upper site behaved in a hysteretic manner indicating that the availability of bedload sediment within the channel became limited during the high flow period. Bedload material was almost entirely sand finer than 2 mm and no trend was observed between grain size and discharge or transport rate. On account of the magnitude of the 2015 flood, and the minimum amount of coarse material that moved, it is unlikely that coarse material will be mobilized and provide suitable interstitial habitat during any post-regulation flood. The 2015 suspended sediment load was at least double that of 2014. Three periods of high transport occurred corresponding to freshet, bankfull and overbank conditions. Most of the annual load was transported during the period of peak flow. Cross-channel variation in sediment concentration confirms that Murray Creek significantly influences mainstem concentration during freshet by increasing the load by approximately 20%. Geomorphic changes detected by digital elevation model (DEM) differencing are highly uncertain at this point, however they do suggest a depositional trend in the downstream portion of the reach. Considerable uncertainty arises as a product of differencing two interpolated elevation models and additional uncertainty analysis based on survey point density would be needed to assess significance of change. Consequently, at this time, reliable change detection is limited to areas that were repeatedly surveyed with high point density. Two such areas include the confluence of Murray Creek and the northern side-channel upstream of the Lower Patch, where the deposition detected by DEM differencing is supported by good data coverage and field observations. Underwater images show that the bed has remained generally consistent with previous observations and trends. The Lower Patch continues to infill especially within the width of the channel actively conveying bedload downstream, while the Middle Patch remains in good condition. The gravel bed towards the right bank of the Lower Patch was relatively free of fines, as were several pools further upstream. These locations would have provided suitable substrate habitat during the spawning period. However, spawning activity was mostly detected downstream of the island complex within the sand-bedded channel. This spawning area contains the deepest pools of the entire reach, which would have been over 6.5 m deep during spawn. A sediment cleaning plan is currently being developed for the Nechako to immediately improve the quality of larval habitat at the Lower Patch and to determine the feasibly of this approach towards remediation. The task will need to be performed in April, between ice-off and the onset of sturgeon movement towards the spawning reach in May. Given the site-specific context and goals of the operation, we have identified mechanical cleaning and hydraulic cleaning as two potential techniques which may be used. Of these alternatives, we believe mechanical cleaning is most appropriate. We recommend the following actions take place in 2016: Mechanically remediate the Lower Patch using a 4x4 Walking Excavator; Sample bedload at the Lower Patch regularly to monitor output of stored sediment and transport over the remediated surface; No longer sample bedload at the Upper Site unless exceptional flows occur; Only sample suspended sediment during key periods; Collect underwater images of the Lower Patch regularly to determine changes in the quality of remediated substrate; Sample bedload along a new transect downstream of the Burrard Ave. Bridge during the remedial work to help assess the impacts of the work; Continue to monitor turbidity using the Center Pier sensor and consider temporarily installing a sensor during the remedial work to assess turbidity caused by the construction activity. The primary purpose will be to demonstrate the effect of the cleaning for future permitting activities. |
