An index for assessing the ecological effects of altered river flow

Canadian scientists have developed an index for assessing rivers' ecological health after dams or other human impacts have altered flow patterns. The index, presented in a recent study, is based on the sensitivity of macroinvertebrates to river flow.

Intended as a tool for improving river management and restoration efforts, the Canadian Ecological Flow Index (CEFI) is an easily calculated metric that can be applied in many places across Canada without requiring collection of new data.

Many experts in river ecology and management believe that altered flow regimes represent one of the biggest threats to river ecosystems and that maintaining a river's natural flow regime is critical for sustaining biodiversity and ecosystem health.

However, few science-based methods have been available that resource managers can use to assess the ecological effects of altered flow or evaluate efforts intended to restore natural flow conditions. 

Typically, such methods have focused on measuring physical characteristics of the flow, but these parameters often do not relate clearly to ecosystem health. A major issue is that flow regime is a deceptively simple name for a very complex set of physical processes.

The scientists who developed CEFI note that flow regime includes the “instream habitat, hydraulics and the timing, magnitude, duration, frequency and flashiness of flow stages and events.” Standard methods for characterizing flow regimes are too simplistic to be ecologically meaningful. 

As an alternative, David Armanini and scientists from Environment Canada took a new approach based on the Lotic-Invertebrate Index for Flow Evaluation, which was developed and tested in European rivers.

Instead of measuring physical parameters, this approach begins by analyzing macroinvertebrate communities in rivers and then determines how the communities reflect specific variables related to flow regime, such as flow velocity. 

For the Canadian Ecological Flow Index, the scientists tailored the approach specifically for the types of macroinvertebrates found in Canada. The index is calculated based on macroinvertebrate data, essentially using these data to reflect the complex physical conditions in an ecologically relevant way.

One goal was to create an index that was practical and could be used without requiring much new data collection.

With that in mind, the scientists built CEFI with data from the Canadian Aquatic Biomonitoring Network. From that database, they gleaned data from 77 studies conducted in all Canadian provinces and territories except Nunavut. 

By comparing impacted and reference sites with different flow conditions, the scientists created a mathematical formula that quantifies the naturalness of the flow regime, based on the presence and relative abundance of 55 macroinvertebrate species. According to the authors, 

We have developed a practical approach to evaluate relationships between hydrological regime and an important component of the river biota, permitting the development of an index which has good potential as an indicator for the effects of flow alteration.  

--by Peter Taylor

Armanini, D., Horrigan, N., Monk, W., Peters, D., & Baird, D. (2010). Development of a benthic macroinvertebrate flow sensitivity index for Canadian rivers River Research and Applications DOI: 10.1002/rra.1389