Low Flows, Instream Flow Needs and Fish Ecology in Small Streams

Abstract

Low flows in Canadian streams and rivers can occur in both summer and winter and can be stressful for fish and other aquatic biota. Low flows can cause a reduction in habitat availability, food production, and water quality and can accentuate the effects of river ice during the winter. Human demands for out-of-channel use of water during low flow periods have resulted in the development of a suite of tools for determining the instream flows needed to maintain desired ecological attributes (most often fish populations) of the stream. This paper reviews the impact of low flows on aquatic resources in small streams as well as instream flow methods and the empirical support for them and concludes that there remains substantial uncertainty in the prediction of impacts of flow reductions or diversions. Some of this uncertainty is due to a lack of understanding of the relationship between flow and fish populations, but much is probably due to site-and time-specific variation in how stream biota responds to habitat changes. A risk-based approach, that explicitly acknowledges the uncertainty in both the hydrology and biology, is needed for decision-making in water management. 166Canadian Water Resources Journal/Revue canadienne des ressources hydriques

Run-of-River hydropower and salmonids: potential effects and perspective on future research

“…As found by studies conducted in reservoir-storage systems and in unregulated rivers, increased temperatures coinciding with low flows have the potential to change the timing of spawning migrations and interspecific interactions (e.g., Freeman et al 2001;Bendall et al 2012;Malcolm et al 2012), reduce survival of smolts before and during migrations (Nislow and Armstrong 2012), and make fish more vulnerable to pathogens (Crozier et al 2008;Mantua et al 2010). Additionally, these extended periods of low flow can reduce water quality, limit movement of nutrients and sediment, and increase competition and predation (Lake 2000;Bradford and Heinonen 2008;Walters and Post 2011). In contrast, during winter months, reduced flows may decrease water temperature and increase the occurrence of frazil ice (i.e., ice anchored to the stream bottom) and freeze-thaw cycles, potentially leading to mortality of salmonid eggs by reducing oxygen concentrations and of fry by damaging gill tissues (Bradford 1994;Bradford and Heinonen 2008).…”

“…Additionally, these extended periods of low flow can reduce water quality, limit movement of nutrients and sediment, and increase competition and predation (Lake 2000;Bradford and Heinonen 2008;Walters and Post 2011). In contrast, during winter months, reduced flows may decrease water temperature and increase the occurrence of frazil ice (i.e., ice anchored to the stream bottom) and freeze-thaw cycles, potentially leading to mortality of salmonid eggs by reducing oxygen concentrations and of fry by damaging gill tissues (Bradford 1994;Bradford and Heinonen 2008). Although the consequences of low flows and changes to water temperature regimes observed in reservoir-storage systems and unregulated rivers may also manifest in bypassed reaches of rivers regulated by RoR hydropower, the magnitude of the differences in temperature experienced in bypassed reaches following flow diversion is poorly documented and so should be established more clearly and rigorously by future research and monitoring.…”

Winter Disturbances and Riverine Fish in Temperate and Cold Regions

“…Articles with latitude, altitude, and distance to the sea, which leads to pronounced regional differences in the occurrence and predictability of winter disturbances (Swanson et al 1998). For instance, rain-induced winter floods are characteristic of winter-wet coastal systems, whereas continental rivers have decreased winter flows, low water temperatures, and excessive ice formation (box 2; McMahon and Hartman 1989, Cunjak 1996, Bradford and Heinonen 2008. Icecovered temperate streams are subject to periodic thaws and floods, whereas higher-latitude streams rarely experience rain-on-snow events or warming periods long enough to trigger midwinter breakup (Cunjak et al 1998).…”

“…The two events are often interrelated, particularly in smaller streams. Flow reduction increases the surface-to-volume ratio, which boosts water cooling and ice formation (Bradford and Heinonen 2008). In return, the storage of large volumes of water in ice may lower the discharge (Prowse 2001a).…”

“…First, the wetted area and water volume contract, which causes shallower habitats to dry out (Cunjak et al 1998). Hydrologic connectivity decreases (Bradford and Heinonen 2008), which can cause complete habitat isolation (Schmidt et al 1989). Second, physicochemical conditions in the contracted habitat space deteriorate, as is demonstrated by interrupted hyporheic flows (Bradford and Heinonen 2008) or hypoxia in isolated, densely populated pools (Schmidt et al 1989).…”