| Citation |
Polzin, M.L., and S.B. Rood. 2013. DDMMON #8-1 Lower Duncan River Riparian Cottonwood Monitoring Program. Year 3 Report – 2012. Vast Resource Solutions and University of Lethbridge, Alberta. Unpublished report by Vast Resource Solutions, Cranbrook, B.C., for BC Hydro Generations, Water License Requirements, Castlegar, BC. 76 pp. + Apps. |
| Abstract/Description or Keywords |
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) that was recommended by the WUP Consultative Committee. This study is intended to evaluate the impacts of operating Alternative S73 (Alt S73) on black cottonwoods (Populus trichocarpa) and other riparian vegetation along the lower Duncan River. The study provides site-specific data to guide the river flow regulation and to improve the understanding of the relationships between flow regime, physical environmental conditions, and riparian vegetation. This report describes Year 4 (2012) of the monitoring project for the study area, which includes the lower Duncan River and the adjacent free-flowing lower Lardeau River that serves as a comparative reference reach. To address management questions and associated hypotheses (table following), the floodplain zones, riparian vegetation, and black cottonwood recruitment are being assessed. The performance of Alt S73 on the lower Duncan River riparian community combines all years of the study and this 2012 report represents some combined data from 2009, 2010, and 2012. Vegetation monitoring comparisons showed similar spatial patterns for Year 1 (2009) versus Year 4. There was a trend for increasing cover by vegetation and continuing growth of the woody vegetation. At this time, no impact was detected that was interpreted to be directly related to the new Alt S73 flow regime. The cottonwood seedling establishment and recruitment had greatly reduced density and spatial patterns as compared to results in Years 1 and 2. The high river stage through the growing season, along with extensive deposition, effectively eliminated seedling recruitment for the Duncan Reach in 2012. Cottonwood establishment and recruitment patterns deviated from the pattern that was emerging following the 2010 assessments. Conversely, the 2012 data adds a flood response dimension to the data set. The combined data from Years 1, 2, and 4, along with further monitoring in subsequent years should enable the development of a quantitative model that will characterize colonization requirements for cottonwoods. Mapping of the two reaches using orthorectified air photos showed changes due to channel migration. Following above-average stage for two consecutive years along the lower Duncan River and two consecutive flood events (Q5 and Q10) along the lower Lardeau River, changes to vegetation community size, channel position, and width were detected for both rivers. The Lardeau River had the largest magnitude of channel migration compared to the lower Duncan River. This resulted in erosion of some vegetation communities adjacent to the channel edge, including the erosion of some mature riparian forest along the Lardeau River. It is too early to formally assess the performance of Alt S73. However, important factors and trends are emerging. Years 1 and 2 indicated that the colonization requirements appeared to be tied particularly to elevational position with reference to stream stage pattern, geomorphic context, sediment substrate, longitudinal position (upstream-to-downstream), influences of tributary inflows, channel morphology, inundation duration and timing, and sediment deposition and scour. The greatly reduced seedling establishment and recruitment in Year 4 confirms the importance of inundation duration and timing, and sediment deposition and scour, as two main factors affecting seedling recruitment. These two factors obscured all other factors in 2012. We anticipate that continued monitoring of cottonwood seedlings, and full vegetation monitoring and mapping in years 2015 and 2018 will provide important data for hypothesis testing and addressing the management questions. lower Duncan River; black cottonwood (Populus trichocarpa); seedling recruitment; riparian vegetation monitoring; flow regime |
