Brook trout vulnerability to drought: eastern component of USGS national integrated ecohydrological research and monitoring plans

Science Center Objects

There is a growing and urgent need to develop and implement innovative strategies to research, monitor, and manage freshwater resources as societal demands escalate simultaneously with climate-driven changes in water availability. Over the past several years, many regions have experienced extreme droughts, fueled by prolonged periods of reduced precipitation and exceptionally warm temperatures. As temperatures warm, the frequency, severity, extent, and duration of droughts are expected to increase across North America, affecting both humans and natural ecosystems. To better manage and improve the resilience of our Nation’s communities and ecosystems in the face of drought, an ecohydrological research and monitoring network that advances the understanding of drought effects and climatic resilience in headwater streams is needed. Cold water basins that support salmonid populations were prioritized based upon their perceived vulnerability to changing climate, economic and conservation value, and their requirements for headwater habitats to complete their life cycles.

Study description, information and objectives

Measuring and modeling ecologically relevant measures of low-flow conditions in headwaters is complex.  Hydrologic responses to seasonal variation in precipitation or snowmelt are strongly tied to local processes affecting water storage and release. Evapotranspiration (ET) associated with riparian vegetation and interactions of groundwater with surface water are particularly important processes controlling streamflow at local scales. Moreover, the physical drivers of these processes (e.g., topography, soil, geology) often vary across steep gradients or exhibit spatial discontinuities, particularly in mountainous regions. The result of this spatial complexity is often a mosaic of hydrologically losing and gaining reaches distributed longitudinally throughout the drainage network. Ecologically, these reaches can be thought of as habitat patches defined by flow permanence and hydrologic and thermal suitability. It is the spatial topology of these patches and how they vary over time that ultimately control habitat suitability, connectivity, and drought and thermal refugia for stream fishes.

The objectives and methods described in this study plan will be replicated in the other three basins throughout the US in order to provide an integrated assessment of drought effects on salmonid populations in various landscape settings. Specific objectives are to:

1. Assess flow variation and derive drought relevant flow statistics within selected basins

2. Link flow variability to spatial and temporal trends in brook trout population abundance

We hypothesize that (i) watersheds will vary in terms of the spatial distribution of ecologically relevant streamflow statistics, (ii) spatial variation in streamflow will be a function of the interaction between flow history and site geomorphology that control storage and ET, and (iii) drought vulnerability of brook trout populations will be strongly linked to site variation in streamflow statistics.

Study Area

The upper Shenandoah River basin represents a hydrologic landscape typical of brook trout habitat in the mid-Atlantic U.S. where baseflow hydrology in forested watersheds is largely driven by seasonal precipitation patterns and shallow groundwater sources. In contrast, the upper Connecticut River basin is representative of brook trout habitats in the northeastern U.S. where snowmelt and deeper groundwater sources represents a larger influence on baseflow stream hydrology. 

Three small (<3rd order) watersheds in Shenandoah National Park (SNP), Virginia were selected to represent headwater streams in the upper Shenandoah. They include Piney River, Paine Run, and the Staunton River. These three watersheds were selected because i) they stratify the three dominant bedrock geologic types in the Park, ii) contain active streamflow gages located at the bottom of each watershed, and iii) have long-term (>25-years) records of brook trout population densities and age structure (Wofford and Demarest 2011). In the upper Connecticut, the West Brook (WB) watershed in Massachusetts was selected for study. Similar to the Shenandoah watersheds, the West Brook watershed supports a well-studied regionally important brook trout population. Population models based on about 15 years of PIT tag data relate body growth and survival to flow conditions, allowing a detailed assessment of observed flow conditions of trout populations.

 

Brook trout image

Brook trout sampling and measuring.

(Credit: Craig Snyder, USGS Leetown Science Center. Public domain.)