Impacts of flow regulation on slackwaters in river channels

Abstract

[1] Flow regulation between irrigation storages and demand offtakes often results in prolonged elevated flows in the river channel during a period that would otherwise typically experience lower flows. This is a growing concern worldwide with negative implications for aquatic flora and fauna susceptible to changes in the hydraulic environment. Slackwaters, a hydraulic patch characterized by low velocities and shallow depths, are important habitat for zooplankton and have been associated with an order of magnitude more fish and shrimp than midchannel patches. Slackwaters are vulnerable to flow regulation and reductions in slackwater area have been shown to reduce abundances of juvenile and larval fish with poor swimming abilities; invertebrates dependent on slackwater refugia; and organic matter retention. It is therefore surprising that the relationship between flow regulation and slackwaters has not been explicitly quantified. In this study, we quantify changes to slackwaters (by surface area relative to inundated area) under the influence of flow regulation, specifically total area, patch area, and number of patches. Using twodimensional hydraulic modeling for sites on the Broken River, southeastern Australia, we demonstrate that the area of slackwaters decreases with increasing discharge until inundation of higher-elevation bars and benches. We assess changes in slackwaters against three levels of increasing flow regulation intensity and find that increasing levels of regulation reduce slackwater area: With a heavily regulated scenario resulting in half the slackwater area and patch sizes 5 times smaller, compared with natural conditions. These findings provide the conceptual model and quantifiable relationships for testing population dynamics, identifying key hydrologic drivers of ecological decline and assisting river storage operators to manage flows during critical periods for biota.

Restoring in‐stream habitat in urban catchments: Modify flow or the channel?

“…Bed shear stress, velocity, and water depth model outputs were used to evaluate each studied scenario in keeping with previous studies that have investigated their relevance to stream ecosystem functioning (Brooks et al, ; Gibbins et al, ; Sawyer, Pasternack, Moir, & Fulton, ; Shearer, Hayes, Jowett, & Olsen, ; Vietz, Sammonds, & Stewardson, ). In this study, the ecologically relevant hydraulic metrics used to characterize the hydraulic change include Shields stress, shallow slow‐water habitat (SSWH) area, and floodplain inundation area.…”

“…In this study, the ecologically relevant hydraulic metrics used to characterize the hydraulic change include Shields stress, shallow slow‐water habitat (SSWH) area, and floodplain inundation area. In most aquatic ecosystem flow investigation, the spatial and temporal variabilities of these hydraulic conditions have been closely linked to the ecological condition (Humphries, Cook, Richardson, & Serafini, ; Vietz et al, ) as well as geomorphic processes (Strom, Pasternack, & Wyrick, ). The bed shear stress is usually examined to investigate refuge concept for benthic space available as refugia where magnitude and duration of Shields stress above a specific threshold are used to assess possible disturbance of benthic biota from bed movement (Jorde & Bratrich, ; Mérigoux & Dolédec, ).…”

“…SSWH availability within the wetted channel has primarily been investigated, which is mostly essential determinant of species population dynamics (Gibbins et al, 2007;Lobera, Muñoz, López-Tarazón, Vericat, & Batalla, 2017). Extinction of this habitat has been shown to reduce fish abundance, macroinvertebrates that depend on them as refugia, and organic matter retention (Vietz et al, 2013). In addition, the duration and frequency of floodplains inundation have been linked with a flow-mediated exchange of energy, organic matter, and biota (Cienciala & Pasternack, 2017).…”

“…The relatively large availability of SSWH occurring in the natural reach, even given urban hydrology with frequent storm flows, has important habitat implications for the life stages of many biota (Mellin, Kulbicki, & Ponton, 2007;Vietz et al, 2013). In urban catchments where streams experience frequent elevated flows (Walsh et al, 2012), the decreased availability of SSWH can persist for extended periods, eliminating rearing and breeding habitat and refuge.…”

“…In urban catchments where streams experience frequent elevated flows (Walsh et al, 2012), the decreased availability of SSWH can persist for extended periods, eliminating rearing and breeding habitat and refuge. This can impact the production and survival of large numbers of biota such as fish, zooplankton, and microinvertebrate populations (Freeman, Bowen, Bovee, & Irwin, 2001;Nielsen, Gigney, & Watson, 2010;Ward & Stanford, 1995) as well as organic matter retention processes (Vietz et al, 2013). SSWH habitats are a primary habitat within natural aquatic systems (Cooper, Barmuta, Sarnelle, Kratz, & Diehl, 1997).…”