Sarah M. Yarnell

Ecology and Management of the Spring Snowmelt Recession

We present a conceptual model for the ecology of the spring snowmelt recession based on the natural flow regime that relates the quantifiable components of magnitude, timing, and rate of change to abiotic and biotic factors that govern riverine processes. We find that shifts in the magnitude of the recession largely affect abiotic channel conditions, whereas shifts in the timing of the snowmelt primarily affect biotic conditions. Shifts in the rate of change affect both abiotic and biotic conditions, creating the largest observed changes to the stream ecosystem. We discuss these components with regard to the success of riverine species in Californias Mediterranean-montane environment. We then present two scenarios of change to the spring snowmelt recession—effects of flow regulation and climate warming—and discuss their potential implications for riverine ecology. Our conceptual model can help guide watershed stakeholders toward a better understanding of the impacts of changing spring recession conditions on stream ecosystems.

Water Velocity Tolerance in Tadpoles of the Foothill Yellow-legged Frog (Rana boylii): Swimming Performance, Growth, and Survival

We explored the effects of large magnitude flow fluctuations in rivers with dams, commonly referred to as pulsed flows, on tadpoles of the lotic-breeding Foothill Yellow-legged Frog, Rana boylii. We quantified the velocity conditions in habitats occupied by tadpoles and then conducted experiments to assess the tolerance to values at the upper limit of, and outside, the natural range. In laboratory flumes and field enclosures we mimicked the velocities observed during pulsed flows. In all experimental venues, the behavioral response of tadpoles was to seek refuge in the channel substrate when velocity increased. In a large laboratory flume, tadpoles moved freely at low water velocities (0–2 cm•s−1) and then sheltered among rocks when velocity increased. In a smaller scale laboratory flume, the median critical velocity was 20.1 cm•s−1. Critical velocity varied inversely with tadpole size, developmental stage, and proportion of time spent swimming. Velocities as low as 10 cm•s−1 caused tadpoles approaching metamorphosis to be displaced. In field mesocosm experiments, tadpoles exposed to repeated sub-critical velocity stress (5–10 cm•s−1) grew significantly less and experienced greater predation than tadpoles reared at ambient velocities. Responses to velocity manipulations were consistent among tadpoles from geographically distinct populations representing the three identified clades within R. boylii. The velocities associated with negative effects in these trials are less than typical velocity increases in near shore habitats when recreational flows for white water boating or peaking releases for hydroelectric power generation occur.

Management of the Spring Snowmelt Recession in Regulated Systems

In an effort to restore predictable ecologically relevant spring snowmelt recession flow patterns in rivers regulated by dams, this study defined a methodology by which spring flow regimes can be modeled in regulated systems from the quantifiable characteristics of spring snowmelt recessions in unregulated rivers. An analysis of eight unregulated rivers across the Sierra Nevada mountain range in California found that unregulated systems behaved similarly with respect to seasonal spring patterns and recession limb curvature, and thus prescribed flows could be designed in a manner that mimics those predictable characteristics. Using the methodology to quantify spring recession flows in terms of a daily percent decrease in flow, a series of flow recession scenarios were created for application in an existing hydrodynamic model for the regulated Rubicon River. The modeling results showed that flow recessions with slow ramping rates similar to those observed in unregulated rivers (less than 10% per day) were likely to be protective of native aquatic species, such as the Foothill yellow-legged frog, while flows that receded at greater rates would likely result in desiccation of egg masses and potential stranding of tadpoles and fry. Furthermore, recession rates of less than 10% per day provided the most spatially diverse hydraulic habitat in the modeled domain for an appropriate duration in spring to support all native species guilds and maximize aquatic biodiversity.

Beyond Metrics? The Role of Hydrologic Baseline Archetypes in Environmental Water Management

Balancing ecological and human water needs often requires characterizing key aspects of the natural flow regime and then predicting ecological response to flow alterations. Flow metrics are generally relied upon to characterize long-term average statistical properties of the natural flow regime (hydrologic baseline conditions). However, some key aspects of hydrologic baseline conditions may be better understood through more complete consideration of continuous patterns of daily, seasonal, and inter-annual variability than through summary metrics. Here we propose the additional use of high-resolution dimensionless archetypes of regional stream classes to improve understanding of baseline hydrologic conditions and inform regional environmental flows assessments. In an application to California, we describe the development and analysis of hydrologic baseline archetypes to characterize patterns of flow variability within and between stream classes. We then assess the utility of archetypes to provide context for common flow metrics and improve understanding of linkages between aquatic patterns and processes and their hydrologic controls. Results indicate that these archetypes may offer a distinct and complementary tool for researching mechanistic flow–ecology relationships, assessing regional patterns for streamflow management, or understanding impacts of changing climate.

Tools for Sediment Management in Rivers

Sediment management, including supply and continual redistribution of sediments for channel maintenance and channel forming, is critical for effective environmental water programs. Many tools have been developed to more effectively manage sediment in rivers for ecosystem maintenance and improvement. This chapter reviews some of these tools, including sediment mobilization theory, environmental water requirements for geomorphic maintenance, and complementary options. Sediment mobilization theory addresses entrainment, transport, and deposition of sediment particles. Environmental flows assessment approaches that support geomorphic processes include threshold-based approaches, field/laboratory methods, and holistic methods. Complementary options for sediment management include catchment-scale conceptual models and budgets for sediment, basin land management (e.g., reforestation, better agriculture practices, etc.), sediment management through dams, and sediment augmentation below dams. These complementary options often have cobenefits with other management objectives. The chapter emphasizes that sediment management is generally complex, such that continual research is needed, ranging from improved sediment mobilization models to large-scale river experiments.