Designing flows to enhance ecosystem functioning in heavily altered rivers

Abstract

More than a century of dam construction and water development in the western United States has led to extensive ecological alteration of rivers. Growing interest in improving river function is compelling practitioners to consider ecological restoration when managing dams and water extraction. We developed an Ecological Response Model (ERM) for the Cache la Poudre River, northern Colorado, USA, to illuminate effects of current and possible future water management and climate change. We used empirical data and modeled interactions among multiple ecosystem components to capture system‐wide insights not possible with the unintegrated models commonly used in environmental assessments. The ERM results showed additional flow regime modification would further alter the structure and function of Poudre River aquatic and riparian ecosystems due to multiple and interacting stressors. Model predictions illustrated that specific peak flow magnitudes in spring and early summer are critical for substrate mobilization, dynamic channel morphology, and overbank flows, with strong subsequent effects on instream and riparian biota that varied seasonally and spatially, allowing exploration of nuanced management scenarios. Instream biological indicators benefitted from higher and more stable base flows and high peak flows, but stable base flows with low peak flows were only half as effective to increase indicators. Improving base flows while reducing peak flows, as currently proposed for the Cache la Poudre River, would further reduce ecosystem function. Modeling showed that even presently depleted annual flow volumes can achieve substantially different ecological outcomes in designed flow scenarios, while still supporting social demands. Model predictions demonstrated that implementing designed flows in a natural pattern, with attention to base and peak flows, may be needed to preserve or improve ecosystem function of the Poudre River. Improved regulatory policies would include preservation of ecosystem‐level, flow‐related processes and adaptive management when water development projects are considered.

Life and death in a dynamic environment: Invasive trout, floods, and intraspecific drivers of translocated populations

“…Flooding prior to spawning is also important for recruitment and persistence of stream fishes (Budy et al, 2015; Healy, Schelly, et al, 2020). Floods maintain channel complexity and create aerated substrates for lithophilic spawners including salmonids (Bestgen et al, 2020) and humpback chub (Gorman & Stone, 1999; van Haverbeke et al, 2013).…”

Low summer river flows associated with low productivity of Chinook salmon in a watershed with shifting hydrology

“…Coincident changes in climate, land cover and water use are altering the natural flow regimes of the world's rivers (Palmer & Ruhi, 2019). Natural flow regimes are the patterns of ‘flow quantity, timing, and variability’ (Poff et al., 1997) that maintain diverse, productive river ecosystems (Bestgen et al., 2020; Poff, 2018). Yet climate modelling predicts large changes in flow regimes due to shifts in global precipitation (Gerten et al., 2008; van Vliet et al., 2013).…”

Multispecies Fish Passage Evaluation at a Rock‐Ramp Fishway in a Colorado Transition Zone Stream

“…Elevation in the basin ranges from 1,490 to 4,145 m, and the watershed is approximately 50% forested, 21% grassland, and 3.5% impervious (USGS 2016). The hydrology is snowmelt dominated, but it is highly altered by agricultural and urban water uses (Bestgen et al 2019). The average annual precipitation is 0.52 m, and the average annual discharge is 4.4 m 3 /s.…”