Slow drawdown, fast recovery: Stream macroinvertebrate communities improve quickly after large dam decommissioning

Dam removal is increasingly considered as a river restoration tool for impoundments that harm the environment or have exceeded their lifespan. However, few studies report the ecological consequences of large dam removal. We performed a multiple Before-After/Control-Impact (mBACI) study to investigate the consequences of the decommissioning of a large dam (42 m high) on instream habitat and invertebrate communities in a temperate, forested catchment of northern Spain. Before decommissioning, a lack of fine sediments and high concentrations of manganese and iron occurred below the dam but decreased downstream. Invertebrate taxa richness and diversity were reduced, and pollution-sensitive taxa were missing just below the dam. The drawdown of the reservoir, the first step towards its decommissioning, mobilized stored sediments causing frequent turbidity peaks downstream, which nevertheless, caused no detrimental effects on macroinvertebrate communities. One year after the drawdown, the comm..., Experimental design: Our study followed a multiple before-after / control-impact (mBACI) design (Underwood, 1994). We defined four control sites, one (C1) upstream from the dam and three (C2 to C4) in free-flowing tributaries, as well as four impact sites (I1 to I4) at increasing distances downstream from the dam. All eight reaches were sampled before (before the period), during (drawdown period), and after (after the period) the drawdown. Additionally, three sites located within the reservoir (R1 to R3) were also sampled in the after period (Fig. 1). These sites could not be sampled either before the period when the reservoir was full or during the drawdown, when safe wading in the newly carved stream channel was still impossible.
Habitat characteristics: We characterized benthic substrate composition following Wolman (1954), with 100 substrate particles collected per site and period while zig-zagging along 100 m-long reaches.
Benthic invertebrates: Invertebrates were sampled in Novemb..., , # Slow drawdown, fast recovery: Stream macroinvertebrate communities improve quickly after large dam decommissioning

[https://doi.org/10.5061/dryad.w3r2280ws](https://doi.org/10.5061/dryad.w3r2280ws)

## Description of the data and file structure

We followed a before-after/control-impact design to investigate the consequences of the decommissioning of a large dam (Enobieta Reservoir, North Spain) on invertebrate communities. We sampled substrate grain size and invertebrate communities before (Before, B), during (Drawdown, D), and after (After, A) the drawdown of the reservoir at 4 sites located below the dam (Impact reaches, I) and in 4 nearby undammed reaches (Control reaches, C). Additionally, three sites located within the reservoir (Reservoir reaches, R) were also sampled in the after period. On each sampling date and site, we characterized benthic substrate composition following Wolman (1954) with 100 substrate particles collected per site and period. Substrate class assessment w...