Hydropower operations in groundwater‐influenced rivers: implications for Atlantic salmon, Salmo salar, early life stage development and survival

Abstract

During their early-life stages (egg maturation, hatching, alevin development), between late autumn and 4 early spring, young Atlantic salmon are exposed to surface-groundwater interactions in the hyporheic 5 zone and may depend on influx of sub-surface water during periods of regulated low discharge for 6 survival. Two recent studies, one in a seasonally regulated river and one in a river exposed to 7 hydropeaking, displayed unexpectedly high survival of eggs in surface de-watered areas because of the 8 influx of oxygen-rich sub-surface water. Field observations of newly-hatched alevins in these two 9 rivers showed them to be more sensitive (i.e. suffered higher mortality from) to surface de-watering 10 than were eggs. Exposure to dry conditions in drawdown areas was highlighted as the main cause for 11 alevin mortality. Therefore, shorter periods of surface de-watering in the river with hydropeaking 12 resulted in higher alevin survival compared to the seasonally-regulated river when still permanently 13 drained after egg hatching. Greater consideration should be given to all early life-history stages when 14 implementing discharge release strategies, and the extent of groundwater influence and the potential 15 for flexible hydropower operations should be taken into account. 16 17 Keywords: hydropower regulation, hydropeaking, sub-surface water influx, egg survival, hatching 18 success, alevin survival 19 20

Life Stage-Specific Hydropeaking Flow Rules

“…During the egg incubation, limited redd dewatering through the hydropower plant operation might not entail complete losses in some cases [16,32], such as in the presence of local groundwater upwellings and at temperatures above freezing [43]. However, considering that spawning can occur over an extended period and that multiple species can be present in the same river, allowing limited redd dewatering is not recommended since alevins, which require continuous inundation [7], may be present throughout the entire time [16].…”

“…Fish communities, in particular, are severely threatened by hydropeaking [4]. Fish can be affected by changes in various components of the hydrograph, whereby the most common responses-stranding, drift, and dewatering of spawning grounds-are mostly related to up-and downramping rates [5,6], peak flow magnitude [5], and baseflow duration [7].Considering the large capacity of existing storage hydropower plants [8], as well as new ones that are currently being planned and installed [9], it is imperative to develop appropriate and transferable management measures to mitigate these ecological impacts. Many structural (e.g., constructing retention basins) and operational (e.g., reducing flow fluctuation rates) mitigation measures have been proposed [10,11], but implementation remains difficult, among other issues, because of significant reductions in the energy yield when setting ecological thresholds [2,12].…”

“…Regarding the latter, multiple studies point out the need to identify critical flows, which include seasonal and diel considerations when determining operational mitigation strategies in rivers affected by hydropeaking [5,[13][14][15][16]. To maintain self-sustaining fish populations in regulated water bodies, river management must take all life-history stages into account, especially during ecologically sensitive periods, when designing and implementing customized flow regimes [7]. This study aims to establish a framework for hydrological mitigation in rivers affected by hydropeaking, based on life cycle stages of fish and their relationship to key parameters of the hydrograph.…”

“…Fish communities, in particular, are severely threatened by hydropeaking [4]. Fish can be affected by changes in various components of the hydrograph, whereby the most common responses-stranding, drift, and dewatering of spawning grounds-are mostly related to up-and downramping rates [5,6], peak flow magnitude [5], and baseflow duration [7].…”

Drought Conditions Maximize the Impact of High-Frequency Flow Variations on Thermal Regimes and Biogeochemical Function in the Hyporheic Zone

“…Frequent river stage fluctuations can significantly alter the hydraulic gradient between groundwater and surface water (Arntzen et al, 2006;Fritz & Arntzen, 2007), resulting in variations to the thermal and biogeochemical dynamics in the HZ by creating more frequent changes in gaining/losing conditions for streams or aquifers compared to steady-state flow conditions (Sawyer et al, 2009;Slater et al, 2010;Francis et al, 2010;Gerecht et al, 2011;Ye et al, 2012;Gu et al, 2012;Graham et al, 2015;Yellen and Boutt, 2015;Briody et al, 2016;Trauth and Fleckenstein, 2017). Both the thermal and biogeochemical processes in the HZ play important roles in fluvial ecology, such as benthic food production, salmonid growth, and fish spawning (Hanrahan, 2007;Casas-Mulet et al, 2016;Mejia et al, 2016). The upwelling of HZ water is especially important to temperature-sensitive organisms (Geist et al, 2002;Ebersole et al, 2003;Galbraith et al, 2012;Mejia et al, 2016).…”