Duncan Watershed Riparian and black cottonwood Monitoring, Implementation Year 8, DDMMON8-1

Author: M.L. Polzin, S.B. Rood

This report summarizes years 6 to 8 of a 10-year study initiated to evaluate the impacts of the Alt S73 flow regime on black cottonwood and other vegetation. Seedling information was collected for those germinated in 2017 as well as previously measured seedlings from 2015 and 2016. Transect-specific stages were collected at locations with gradually sloping point bars. Statistical comparisons were made between seedling establishment and recruitment across 2015, 2016 and 2017 data sets. Within and between comparisons were completed for representative reaches along the lower Duncan River and the free-flowing Lardeau River.

– The 2017 results showed that seedling establishment, survival, and recruitment continue to display variability. There has been a trend of continual decline in black cottonwood establishment levels since the implementation of Alt S73. The reference reach shows a similar decline.
– black cottonwood establishment and survival are linked to water inundation duration, river stage during the growth season, sediment erosion and deposition, establishment elevation, and the growth season weather. These factors, except the weather, are influenced by river regulation. The past 8 years results show strong trends indicating that river flow regime does influence establishment and survival along the Duncan River. Rigorous hypothesis testing will follow in 2018.
– The analyses from Year 8 along the lower Duncan River continue to indicate that the river flow regime is a primary driver of black cottonwood establishment and survival along the lower Duncan River. Deliberate hypothesis testing will also occur in 2018.

– The 2017 results showed that seedling establishment, survival, and recruitment continue to display variability. There has been a trend of continual decline in black cottonwood establishment levels since the implementation of Alt S73. The reference reach shows a similar decline.
– black cottonwood establishment and survival are linked to water inundation duration, river stage during the growth season, sediment erosion and deposition, establishment elevation, and the growth season weather. These factors, except the weather, are influenced by river regulation. The past 8 years results show strong trends indicating that river flow regime does influence establishment and survival along the Duncan River. Rigorous hypothesis testing will follow in 2018.
– The analyses from Year 8 along the lower Duncan River continue to indicate that the river flow regime is a primary driver of black cottonwood establishment and survival along the lower Duncan River. Deliberate hypothesis testing will also occur in 2018.