By David Dralle, Gabe Rossi, Phil Georgakakos, Jesse Hahm, Daniella Rempe, Monica Blanchard, Mary Power, Bill Dietrich, and Stephanie Carlson
You’ve probably noticed that some streams flow year-round while others are seasonally dry, despite receiving similar amounts of rainfall. Through a recent NSF-funded effort (“Eel River Critical Zone Observatory”), we learned several things about how landscapes filter climate to produce such diverse flow behavior–and the implications for how salmon live their lives.
Our 25-year field study revealed that belts of California’s Eel River watershed underlain by different geologies have different Critical Zones (CZs) – Earth’s permeable surface layers from the top of the vegetation canopy down to fresh bedrock, where water can be stored and exchanged. Different below-ground CZ structures have different subsurface water storage capacity–that is, the maximum seasonal volume of water stored belowground that is available for streamflow and plant transpiration (Hahm et al 2019). The deep CZ of the Coastal Belt geology in the western Eel watershed has a large subsurface water storage capacity, in contrast to the Central Belt geology’s thin CZ, which has relatively low water storage capacity (Dralle et al. 2018). The rocks underlying the Northern California Coast Range—and the hills themselves—only emerged above sea level relatively recently, geologically speaking (in the last few million years). As they were uplifted and eroded, forming the landscape we see today, subtle differences in mineralogy and structure resulted in different chemical and physical weathering near the surface, resulting in this diversity of water storage and release behaviors.
Differences in subsurface water storage capacity affect water availability for both terrestrial and aquatic ecosystems. To understand why there is water present in some but not other stream channels in the Northern Coast Range, we need to look up slope from the stream, at the hills. We’re generally familiar with aquifers in valleys—large sedimentary basins where we pump groundwater. But upland, hilly landscapes also host aquifers—instead of being buried beneath sediment in valleys, however, they are within the hillslopes themselves. Every hillslope is, in a sense, a small water tower. In addition to the saturated (aquifer or groundwater) portion of the hillslope, there is also an important overlying unsaturated zone: the upper layers of soil and weathered fractured bedrock that contain water and air in pores. Hillslopes in the Coastal Belt have relatively large volumes of water in this unsaturated zone, which supplies plants with the moisture they need to transpire. The result is a highly productive, dense conifer forest that blankets the Coastal Belt. In contrast, the Central Belt supports a less productive (yet biodiverse!) oak savanna.
Below the unsaturated zone in the saturated groundwater zone, all pores, including fractures within the weathered bedrock, are filled with water. Unlike the unsaturated zone where water is held in place under tension (as in a moist but no longer dripping sponge, see Figure 3), saturated zone flows move laterally towards stream channels that bound hillslopes, where water emerges at seeps and springs to become surface water, thus supporting stream flows (Lovill et al 2018). So, unlike in the Central Valley, where most aquifers are generally below the rivers, in the Coast Range many of the aquifers that feed streams are actually above the rivers, in the adjacent hillslopes. In the Coastal Belt, the saturated zone occurs deep in weathered bedrock, often > 10 m below the surface, yet because the hills are so steep, this saturated zone is still vertically above the adjacent channels. In the Central Belt, water tables rise all the way to the ground surface after only six inches of rainfall at the onset of the wet season. When this happens, water flows below and above ground (the above ground component is called “saturation overland flow”) toward the stream. We learned about these contrasting behaviors in two intensively monitored watersheds that straddle the lithologic boundary between the Coastal Belt and the Central Belt: Elder Creek and Dry Creek, respectively. The two watersheds receive nearly identical amounts of rainfall during the wet season, allowing us to study how subsurface structures filter climate.
How exactly does the subsurface CZ impact storage and runoff? Dralle et al. (2023) emphasize that “storage capacity in the subsurface sets the maximum volume of water that can be stored for later use by vegetation, which itself interacts with climate and subsurface storage to dictate the timing and magnitude of groundwater recharge, and thus runoff generation and flow regime features”. Let’s translate that to plain language using our favorite metaphor: the kitchen sponge. Sponge behavior is a surprisingly accurate analogue for the unsaturated root zone that can help explain plant water supply and runoff behavior. Anyone who has cleaned dishes knows a sponge can only hold so much water (a storage capacity, if you will), after which additional “wetting” from the tap (i.e. precipitation or snowmelt) results in a drippy sponge. The sponge will drip excess water temporarily until the tap inflows stop. At nearly every site where we’ve directly observed subsurface flowpaths and storage, this “drippy sponge” effect is actually how root zone water storage works. Most of the excess water dripped from the sponge drains down vertically to recharge groundwater, refilling the saturated zone that feeds streams. Once the sponge finishes dripping, it nevertheless remains wet; only evapotranspiration fully dries the sponge. The properties of the root-zone sponge therefore provide two key insights:
- The sponge size (storage capacity) determines how much moisture is available for plants to transpire (with implications for plant water stress, especially in drought).
- Once dried out, the sponge must be refilled to its storage capacity to produce groundwater recharge. Since groundwater is the primary driver of streamflow (especially in the dry season), the sponge indirectly affects all aspects of streamflow behavior.
Measurements of streamflow from Elder Creek (big sponge) and Dry Creek (little sponge) are consistent with our metaphor (Figure 5). Dry Creek has a flashy flow regime, the wet season flow is activated quickly following the first rains, and the spring flow recedes quickly, leading to stream intermittency. In Elder Creek (big sponge), wet-season flow activation is delayed relative to timing of the rains, the spring flow drops more slowly, and flows are perennial.
A parallel story emerges when we consider stream temperature and the energy budget between a Coastal Belt stream and a Central Belt stream. Low storage capacity in the Central Belt geology results in low canopy cover and little shading, and flow paths that are nearer the surface. The result is warmer water temperatures and more light delivery to streams. In contrast, Coastal Belt watersheds have deep flowpaths and cold, robust flows all summer. Water storage supports large trees that shade the stream, filtering out most light in smaller watersheds (Figure 1).
Different flow and energy characteristics driven by different water storage capacities create different opportunities and challenges for salmonid fish. Streams like Dry Creek, with early high flows, create opportunities for upriver migrating fish to access the stream early in the breeding season. The more open canopy and warmer conditions support early emergence of juveniles and the development of a rich food web that allows rapid growth of newly emerged salmon fry (Rossi et al. 2022). However, following flow recession and disconnection, such streams become inhospitable for salmonids – to be successful, fish need to leave and rear elsewhere. In contrast, in perennial, shaded, cool streams like Elder Creek, fish can rear year-round, but food is more limited early in the season, so fish productivity peaks later. Watersheds with diverse geologies would support fish that spend 1-2 years in freshwater before migrating to the ocean (so called “natal rearers”) in tributaries where CZs are deep, and where CZs are shallow, would support early migrants that leave their natal habitat in spring and rely on downstream (non-natal) habitats to rear and grow before migrating to the ocean (i.e., “non-natal rearers”).
Our central hypothesis is that we will not recover salmon abundance without recovering a diversity of paths through the watershed and through the life cycle and, moreover, that the strategies that are missing or only weakly contributing today are ones that relied on the mainstem and other non-natal habitats for rearing / as stop over sites (i.e., the non-natal rearing life histories). Systems like Dry Creek that have intermittent flows can offer great spawning habitat and a productive growth environment for newly emerged fry. Following a period of potentially rapid growth, fish that leave early can potentially take advantage of rich pastures downstream like floodplains, estuaries, or perennially flowing Coastal Belt watersheds. Our ongoing work in the Eel and elsewhere is exploring the importance of this habitat mosaic, including the importance of variation in growth potential across the landscape and through time, in supporting multiple life histories and resilient salmon complexes (Rossi et al. 2024).
David Dralle is a research hydrologist at the United States Forest Service Pacific Southwest Research Station. Gabe Rossi is a CalTrout-UC Berkeley Coastal Rivers Ecologist. Phil Georgakakos is a research scientist in the Environmental Science, Policy, and Management Department at the University of California, Berkeley. Jesse Hahm is an assistant professor in the Department of Geography at Simon Fraser University in Burnaby, British Columbia, Canada. Daniella Rempe is an assistant professor in the Department of Geological Sciences at the University of Texas-Austin. Monica Blanchard is a biologist in the U.S. Fish and Wildlife Service in Portland, Oregon. Mary Power is a professor in the Department of Integrative Biology at the University of California, Berkeley. William Dietrich is a professor in the Department of Earth and Planetary Science at the University of California, Berkeley. Stephanie Carlson is a professor in the Environmental Science, Policy, and Management Department at the University of California, Berkeley.
Further Reading
Dralle, D. N., W. Jesse Hahm, D. M. Rempe, N. J. Karst, S. E. Thompson, and W. E. Dietrich. 2018. “Quantification of the Seasonal Hillslope Water Storage That Does Not Drive Streamflow.” Hydrological Processes 32(13): 1978–1992. https://doi.org/10.1002/hyp.11627
Dralle, D. N., Rossi, G., Georgakakos, P., Hahm, W. J., Rempe, D. N., Blanchard, M., Power, M. E., Dietrich, W. E., and S. M. Carlson. 2023. “The Salmonid and the Subsurface: Hillslope Storage Capacity Determines the Quality and Distribution of Fish Habitat.” Ecosphere 14(2): e4436. https://doi.org/10.1002/ecs2.4436
Hahm, W. J., D. M. Rempe, D. N. Dralle, T. E. Dawson, S. M. Lovill, A. B. Bryk, D. L. Bish, J. Schieber, and W. E. Dietrich. 2019. “Lithologically Controlled Subsurface Critical Zone Thickness and Water Storage Capacity Determine Regional Plant Community Composition.” Water Resources Research 55(4): 3028–3055. https://doi.org/10.1029/2018WR023760
Lovill, S. M., W. J. Hahm, and W. E. Dietrich. 2018a. “Drainage from the Critical Zone: Lithologic Controls on the Persistence and Spatial Extent of Wetted Channels during the Summer Dry Season.” Water Resources Research 54(8): 5702–5726. https://doi.org/10.1029/2017wr021903
Rossi, G. J., M. E. Power, S. M. Carlson, and T. E. Grantham. 2022. “Seasonal Growth Potential of Oncorhynchus mykiss in Streams with Contrasting Prey Phenology and Streamflow.” Ecosphere 13(9): e4211. https://doi.org/10.1002/ecs2.4211
Rossi, G.J., Bellmore, J.R., Armstrong J.B., Jeffres, C., Naman, S.M., Carlson, S.M., Grantham, T.E., Kaylor, J.M., White, S., Katz, J., Power, M.E. 2024. Foodscapes for Salmon and Other Mobile Consumers in River Networks. Provisionally accepted, Bioscience, March, 2024.
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