| Citation |
Polzin, M.L.1, B. Herbison and S.B. Rood2. 2016. DDMMON #8-1 Lower Duncan River Riparian Cottonwood Monitoring Program. Year 6 Report – 2015. 1VAST Resource Solutions and 2University of Lethbridge, Alberta. Unpublished report by VAST Resource Solutions, Cranbrook, B.C., for BC Hydro Generations, Water License Requirements, Castlegar, BC. 56 pp. + Apps. |
| Abstract/Description or Keywords |
This study evaluated the impacts of operating Alternative S73 (Alt S73) on black cottonwoods (Populus trichocarpa) and other riparian vegetation along the lower Duncan River. A ten-year riparian vegetation monitoring program along the lower Duncan River was initiated in 2009 as part of the implementation of the Duncan Dam Project Water Use Plan (WUP). The alternative flow regime selected by the Consultative Committee was the following: peak flows of 400 m3/s from May 16 to July 31 with declining flows to 250 m3/s August through September, and then further declines to 73 m3/s for October, then the flows gradually increase to mid-May peak flow rate. The study provided site-specific data to guide the river flow regulation and to improve the understanding of the relationships between flow regimes, physical environmental conditions, and riparian vegetation. This report describes study Year 6 (2015) of the monitoring project, which includes the lower Duncan River and the adjacent free-flowing lower Lardeau River, as a comparative reference reach. The river floodplain zones, riparian vegetation, and black cottonwood recruitment are being assessed in order to address the integral management questions and their associated hypotheses: 1) Are there changes in black cottonwood recruitment or riparian habitat communities? 2) What are the drivers of black cottonwood success (following table, page iv) since Alt S73 was implemented? The performance assessment of Alt S73 on the lower Duncan River riparian community will combine all years of the study. The 2015 data adds to that existing data set. Riparian vegetation monitoring comparisons showed similar spatial patterns from study Year 1 (2009) to study Year 6 (2015). The lower Duncan reach species richness has displayed some variation across years, particularly for the herbaceous vegetation due to seasonal variations between years. Conversely, tree and shrub species have remained more consistent since 2009 which is characteristic of woody species. The field data, the prior years’ results, and the associated analyses support our expectation that the river flow regime is the primary driver affecting black cottonwood seedling establishment and recruitment along the Duncan River. However, there is an apparent confounding effect from weather, which was shown through the monitoring of the Lardeau River reach. It is anticipated that that the seasonal weather effects can be resolved from the hydrologic effects due to the change in the Duncan River flow regime. Building upon the results over multiple years, the 2015 results indicate that the river flow regime is the primary factor for sediment deposition and erosion. However, new recruitment sites from this process has not occurred since 2009. Within this study system, cottonwood colonization appears to be tied to elevational position with reference to stream stage pattern, geomorphic context, sediment deposition, influences of tributary inflows, and for the final segment, lake level. Cottonwood establishment and recruitment occurs at the higher elevation of the active channel zone, this has continued the channel narrowing process observed by Miles (2002). Full riparian vegetation monitoring and mapping will occur again in 2018 and that will provide the last data set for deliberate testing of the hypotheses and to address the key management questions. Duncan River; black cottonwood (Populus trichocarpa); seedling recruitment; flow regime |
