The flow-on effects of lowered flows

Posted on March 27, 2012

Several field studies have shown that the regulation of rivers by dam construction can have dramatic effects on the form of the channel and downstream bankside vegetation. Observed changes attributed to damming include declines in habitat diversity, decreases in native plant species and invasions by exotics, but such changes have been difficult to anticipate with precision because the dynamic interactions between river flow, sediment transport and streamside vegetation aren’t fully understood and quantitative predictive models haven’t been available. This deficiency has been partly remedied by a simulation model that relates vegetation biomass at a given point on the river to the water level, the depth of the aquifer, the type of vegetation, and timescales of plant growth and decay. The model has been used to predict how the distribution of vegetation responds to changes in mean flow and/or flow variability that typically accompany dam construction. Previous case studies indicate that, after damming, flow rates usually decline, while the relative variability of flow can increase or decrease. In model simulations, increases in flow variability always had the effect of widening the channel, often considerably. This occurred because of the greater probability of flooding, which was associated with the uprooting and burial of vegetation, the development of anoxic conditions, and a vegetative shift away from the channel centre. Decreases in mean flow, and to a lesser extent flow variability, led to narrowing of the channel, because vegetation moved closer to the channel centre to tap water at lower levels. However, such channel narrowing tends to increase water levels and this may have negative impacts downstream. Dam-related changes predicted by the model agreed well with those observed in field studies, for example on Green River (Utah) and Dry Creek (California). The model captures the non-linear responses of channel form and vegetative cover to damming and helps to explain the very large changes in plant biomass (100-1000%) that have been observed.

Reference: Tealdi, S., Camporeale, C. & Ridolfi, L. 2011. Modeling the impact of river damming on riparian vegetation. Journal of Hydrology 396, 302–312.

Posted in: flooding, flow, plants