Measuring the benefits of resized rivers

Posted on March 27, 2012

The ability of dam construction to disrupt water flows, sediment transport and aquatic populations is well known, and there is continuing interest in defining relationships between flow rates and the ecological integrity of stream systems. Given social pressures for water extraction and the difficulties of fully restoring pre-existing flows to dry river reaches downstream of dams, it’s worth considering the extent to which radically downscaled flows can deliver ecological benefits. To gather information on this topic, a controlled flow experiment was carried out on a four-km stretch of river immediately below a diversion dam on the Bridge River in British Columbia, Canada. The study reach had been dry for most of the previous 50 years. Following a four-year baseline period of no flow, a pattern of discharge with a peak in spring, to resemble the seasonality of snowmelt, was maintained for the next seven years. The average flow rate (3 m3/sec) was much lower than the pre-dam rate (100 m3/sec) . The study organism was black cottonwood, Populus trichocarpa, which is a pioneering, dominant tree species that creates significant wildlife habitat in the riparian zone. Cottonwood responses to restored flows were recorded by measuring branch elongation and the size of growth rings on trunk cross-sections and cores. Although mature trees changed little between the pre- and post-flow periods, the radial growth rates of juvenile trees, which were closer to the river, roughly doubled, and there was a strong surge in seedling establishment along the river edge in the post-flow years. The results supported the findings of previous studies pointing to the key role of seasonal flows in cottonwood recruitment. They also showed that even very low flows can produce a measurable response with a surprisingly short time lag. Aquatic sampling revealed that the Bridge River experiment also created instream benefits to periphyton, invertebrates and fish. The study was therefore able to demonstrate that river resizing can provide measurable ecological benefits.

Reference:  Hall, A.A., Rood, S.B. & Higgins, P.S. 2011. Resizing a river: a downscaled, seasonal flow regime promotes riparian restoration. Restoration Ecology 19(3), 351–359.

Posted in: flow, plants, restoration