The salmonid and the subsurface: Hillslope storage capacity determines the quality and distribution of fish habitat

Abstract

Water in rivers is delivered via the critical zone (CZ)-the living skin of the Earth, extending from the top of the vegetation canopy through the soil and down to fresh bedrock and the bottom of significantly active groundwater.Consequently, the success of stream-rearing salmonids depends on the structure and resulting water storage and release processes of this zone. Physical processes below the land surface (the subsurface component of the CZ) ultimately determine how landscapes "filter" climate to manifest ecologically significant streamflow and temperature regimes. Subsurface water storage capacity of the CZ has emerged as a key hydrologic variable that integrates many of these subsurface processes, helping to explain flow regimes and terrestrial plant community composition. Here, we investigate how subsurface storage controls flow, temperature, and energetic regimes that matter for salmonids. We illustrate the explanatory power of broadly applicable, storage-based frameworks across a lithological gradient that spans the Eel River watershed of California. Study sites are climatically similar but differ in their geologies and consequent subsurface CZ structure that dictates water storage dynamics, leading to dramatically different hydrographs, temperature, and riparian regimes-with consequences for every aspect of salmonid life history. Lithological controls on the development of key subsurface CZ properties like storage capacity suggest a heretofore unexplored link between salmonids and geology, adding to a rich literature that highlights various fluvial and geomorphic influences on salmonid diversity and distribution. Rapidly advancing methods for estimating and observing subsurface water storage dynamics at large scales present new opportunities for more clearly identifying landscape features that constrain the distributions and abundances of organisms, including salmonids, at watershed scales.

Daily stream temperature predictions for free-flowing streams in the Pacific Northwest, USA

“…Here, we included the percent of the watershed covered by extrusive volcanic rock (V), but it is unclear how broadly this applies to other regions. Similarly, Dralle et al [69] found that underlying geology in the Eel River basin, California, influenced water storage capacity, flow dynamics, vegetation type and canopy cover, all of which subsequently impacted water temperature. The depth of groundwater aquifers can also be important in determining groundwater influence on surface stream temperature [70].…”

Forestry impacts on stream flows and temperatures: A quantitative synthesis of paired catchment studies across the Pacific salmon range

“…For example, several population model simulations have found reduced salmon spawner abundance resulting from peak flow and temperature increases within the range of our results (Battin et al., 2007; Nicol et al., 2021). Realized impacts to salmon are likely to be context‐dependent, given that flow and temperature affect salmon through complex and non‐linear pathways mediated by local geomorphology (Dralle et al., 2023; Gronsdahl et al., 2019; Sloat et al., 2017), behaviour and physiology (Railsback, 2021) and population dynamics (Ohlberger et al., 2018). While these complexities may further challenge our ability to clearly link forestry activities to salmon population trends (Peacock et al., 2023), our results still provide evidence that forestry poses a risk to salmon systems that in many cases already face an array of other pressures (Munsch et al., 2022).…”

Inferring Hillslope Groundwater Recharge Ratios From the Storage‐Discharge Relation

“…Above this boundary, groundwater moves laterally through a network of fractures, eventually reaching the stream via seeps and springs (Lovill et al., 2018). The groundwater reservoir can store upwards of 200 mm of dynamic, drainable groundwater (in addition to the catchment‐averaged 300 mm of dynamic storage in the unsaturated soils and rock) that supports year‐round cold baseflows and a thriving salmon population (Dralle et al., 2018; Dralle, Rossi, et al., 2023; Rempe & Dietrich, 2018).…”

“…The groundwater reservoir can store upwards of 200 mm of dynamic, drainable groundwater (in addition to the catchment-averaged 300 mm of dynamic storage in the unsaturated soils and rock) that supports year-round cold baseflows and a thriving salmon population (Dralle et al, 2018;Dralle, Rossi, et al, 2023;.…”