Material and methods
Study sites and hydropower facilities
The study was conducted in three HP impacted river-reaches at the rivers
Sitter (hydropower plant Kubel, KUB; canton of St. Gallen), Hasliaare
(hydropower plant Innertkirchen, KWO; canton of Bern) and Linth
(hydropower plant Linth-Limmern, KLL; canton of Glarus; Figure 1).
Additionally, RF reaches located closely upstream of the hydropower
plants were used as hydrological comparisons with relatively constant
base-flow conditions.
All HP and RF reaches are of comparable size (Strahler order 5 – 6;
natural mean annual discharge 11 – 32 m3/s) and
located at similar elevations (590 – 680 m a.s.l.).
Dominant grain size (cover
> 50%) in all HP reaches is micro- to mesolithal (grain
size 2 – 20 cm), whereas it is coarser (> 20 cm) in the RF
reaches of the Sitter and Linth.
The KUB hydropower plant has the
lowest maximum turbinable peak flow (18 m3/s), flow
amplitude (16 m3/s) and ramping rates (0.1 and 0.05
m3/s min). The
investigated HP reach was located < 0.5 km downstream of the
plant release and is characterized by the highest bankfull width (45 m),
a near-natural morphology with a right-sided gravel bar and comparably
fine substrate (Dm = 28.1 mm). Since 2016, the
hydropower facilities KWO and KLL both include a HP retention volume of
80,000 m3 and 220,000 m3,
respectively. The maximum turbinable peak flow, amplitude and ramping
rates of the corresponding hydropower plant are higher than at KUB. The
investigated HP and RF reaches of the Hasliaare and Linth are
characterized by a strongly constrained morphology.
The HP reach of the Linth has the
lowest bankfull width (15 m), no extended gravel bars, and the coarser
sediment size (Dm = 59.9 mm). Moreover, four small
run-off power plants are situated between the water release and the HP
reach. The riverbed of all HP
reaches showed no or insignificant clogging, whereas it was moderate in
the RF reaches of the Sitter and Linth.
The HP and RF reaches of these two
rivers were also characterized by higher cover of algae and mosses than
the Hasliaare. Possible relevance of these aspects on drift, stranding
and the benthic community composition, will be picked up in the
discussion. Key characteristics of the hydropower plants, as well as
associated catchments, HP and RF reaches are available in Data S1
(Appendix A).
Experimental setup
To investigate the short- and
long-term effects of HP on MIV, we defined three HP scenarios for each
HP reach and requested respective experimental flow releases from the
hydropower plant operators. We sampled benthic, drifted and stranded MIV
and recorded several environmental variables (see below) over a
four-week period between March 15th and April
8th 2018. In each HP and RF reach we defined three
sampling sites spaced by 10 – 50 m (Figure 2a).