Some flow-on effects of river restoration

Posted on July 20, 2015

It’s often the case that a lack of monitoring makes it hard to assess the success or failure of river restoration projects. In an example of a project where monitoring has revealed some of the broader ecological impacts of stream restoration, in 2001-2004, a series of 25 rock weirs, each 15 m long, were installed along the upper Cache River in Illinois, U.S.A. to stabilise the channel. Later, in 2009-2010, a study was undertaken to assess the effects of weir construction on the emergence of aquatic insects and the local diversity of riparian birds. Four weirs were selected as treatment sites and another four sites, located midway between weirs, were chosen as controls. Using emergence traps, aquatic insects were sampled every four weeks on average, and birds were counted weekly during the spring migration and breeding period. A total of 60 insect families and 113 bird species were recorded. The number and diversity of insect species, the average body size of emerging insects, and total bird abundance were all significantly higher at the weir sites. Bird densities were positively related to the biomass of emerging insects. These findings support the hypothesis that, by providing a range of stable crevice microhabitats, rock weirs increase the production of emerging aquatic insects, which in turn attract predatory birds. However, nest box monitoring showed that the fledging success of a common warbler (Prothonotaria citrea) was higher at the control sites, possibly due to the negative impacts of rock weir construction on nearby vegetation. Therefore, rock weirs can boost biodiversity, but it’s important to keep riparian disturbance to a minimum when carrying out stream restoration work.

Reference: Heinrich, K.K. et al. 2014. Cascading ecological responses to an in-stream restoration project in a Midwestern river. Restoration Ecology 2(1), 72–80.