Disturbance is one of the main factors affecting benthic invertebrate diversity. Much is known about the ecological impacts of disturbances related to flow modification, channel engineering and fine sediment inputs to rivers, but as these often occur simultaneously disentangling their individual effects can be difficult. In the River Cinca periodic in-channel gravel mining takes place to aid channel maintenance. Mining causes major bed disturbance, with consequences for channel morphology and fine sediment dynamics, all in the absence of marked flow changes. Thus, mining provides an opportunity to isolate the effects of bed disturbance and increases in suspended sediment concentration (SSC) on benthic invertebrates.

We used a Before-After-Control-Impact type design to assess changes in invertebrate assemblages in the Cinca in relation to mining that took place in summer 2014. Sites in the Cinca located up and downstream of the mining, as well as others in an adjacent un-mined river were sampled on 8 occasions. Sampling took place prior to and then during two consecutive phases of mining. At each site, benthic and drifting invertebrates were collected, along with information on bed grain size distributions, hydraulic conditions, bedload transport rates and SSC. We describe the extent of changes in benthic assemblages in response to the mining, along with associated physical habitat changes. In particular, we describe changes to drift in response to marked increases in SSC (maximum 6 g/l). Results are of value to help develop guidelines for assessing gravel mining impacts in Pyrenean piedmont rivers, where channel maintenance works are frequent.