By Hana Moidu, Mariska Obedzinski, Stephanie Carlson, and Ted Grantham

You may have heard the saying from the Ancient Greek philosopher Heraclitus, “No man steps in the same river twice, for it is not the same river and he is not the same man.” If you walk along a coastal stream in California at the end of the summer, you will understand the dynamic nature of these systems. In a wet year, such as 2017, you might find a stream that is fully connected with flowing water. But in a dry year, like 2021, you might walk miles of dry stream channel before discovering an isolated pool. Many of California’s streams naturally become intermittent at some point in the dry season. However, when and where stream channels go dry is highly variable year-to-year and difficult to predict (van Meerveld et al., 2020).

Intermittent streams may seem like harsh environments, but are actually hotspots for freshwater biodiversity. Native California fishes, invertebrates, and other aquatic organisms have evolved a variety of adaptions for the natural seasonal drying of streams. Many native species, including salmon and trout, depend on intermittent streams. However, climate change is intensifying drought in California (Diffenbaugh et al., 2015), decreasing stream flows and prolonging dry seasons, enhancing the vulnerability of freshwater species.

What do these changes mean for intermittent streams and the biodiversity they support? Will intermittent streams exhibit more extensive seasonal drying in drought years? If so, what management strategies are needed to limit impacts to native species? The Russian River watershed is an ideal system for exploring these questions.

Since 2012, the California Sea Grant Russian River and Salmon Steelhead Monitoring Program has monitored tributaries to the Russian River, surveying where wetted channels persist at the end of the summer. Field crews walk the length of the stream and map the presence of surface water with a GPS unit (Fig. 1). Over many years of monitoring, they realized that some streams contract in proportion to the amount of rainfall in the previous wet season (Mill Creek in Fig. 2), while others show the same pattern of drying year after year (Dutch Bill Creek in Fig 2). To understand why different streams exhibit different drying patterns, we analyzed wetted channel survey data collected at 25 streams between 2012-2019.

In our recently-published study, a statistical model determined the influence of various physical factors (such as topography, soils, and geology) and climatic variables (such as antecedent rainfall and dry season temperatures) on the extent and interannual variability of stream drying. Not surprisingly, rainfall in the previous wet season was the most important variable predicting stream drying –more rainfall in the wet season led to less drying at the end of the dry season.

Total rainfall over the previous 5 years also influenced stream drying. This “hydrologic memory” occurs when the landscape retains the effects of a hydroclimatic event longer after it occurs (Jacobs et al., 2020). This means that in some streams, a wet year may not produce the expected higher flows if it is preceded by a multi-year drought. Conversely, an unusually dry year may not result in the same amount of stream drying after high precipitation in previous years. The degree to which streams showed “hydrologic memory” was related to the underlying geology and soil characteristics of their watersheds.

The model helped characterize stream drying patterns across the entire lower Russian River watershed over the past 10 years. We classified stream reaches (100-m segments of the stream network) as “reliably dry” and “reliably wet” – those expected to be dry and wet, respectively, regardless of antecedent rainfall – and as “variable” – those that tend to be wet or dry depending on the amount of rainfall in the wet season (Fig. 3).

These classifications predict where aquatic habitat is likely to persist, which can aid resource managers. For example, stream reaches identified as reliably wet could serve as critical drought refugia for native aquatic species, such as coho salmon and steelhead (Vander Vorste et al., 2020), and warrant increased protection. Similarly, reaches with variable drying patterns should be monitored during drought to assess if management interventions are needed. For example, streams at risk of drying might be targeted by California’s Department of Fish and Wildlife for coho salmon rescue and relocation, as has been performed in previous droughts.  Efforts to maintain streamflows during drought, such as through voluntary efforts or management actions, could also be focused on streams most vulnerable to drying. As the frequency and severity of drought continues to increase, efforts to predict stream drying can play an important role in improving management and protection of intermittent streams and the aquatic biodiversity they support.

Hana Moidu is a PhD candidate at UC Berkeley, working with Stephanie Carlson and Ted Grantham. Mariska Obedzinski is a PhD student at UC Berkeley working with Stephanie Carlson and Ted Grantham. She is also a California Sea Grant Extension Specialist and has coordinated the Russian River Salmon and Steelhead Monitoring Program for over 15 years. Stephanie Carlson is a Professor in the Environmental Science, Policy, and Management Department at UC Berkeley. Ted Grantham is an Assistant Cooperative Extension Specialist and Adjunct Professor in the Environmental Science, Policy, and Management Department at UC Berkeley.

Further Reading:

Bogan, M.T., Leidy, R.A., Neuhaus, L., Hernandez, C.J. & Carlson, S.M. 2019. Biodiversity value of remnant pools in an intermittent stream during the great California drought. Aquatic Conservation: Marine and Freshwater Ecosystems 29(6): 976-989.

Diffenbaugh, N. S., Swain, D. L. & Touma, D. 2015. Anthropogenic warming has increased drought risk in California. Proceedings of the National Academy of Sciences 112(13): 3931–3936.

Jacobs, E. M., Bertassello, L. E. & Rao, P. S. C. 2020. Drivers of regional soil water storage memory and persistence. Vadose Zone Journal 19(1): e20050.

*Moidu, H., Obedzinski, M., Carlson, S.M. & Grantham, T.E. 2021. Spatial patterns and sensitivity of intermittent stream drying to climate variability. Water Resources Research e2021WR030314.

Vander Vorste, R., Obedzinski, M., Nossaman Pierce, S., Carlson, S.M. & Grantham, T.E., 2020. Refuges and ecological traps: Extreme drought threatens persistence of an endangered fish in intermittent streams. Global Change Biology 26(7): 3834-3845.

van Meerveld, H. J. I., Sauquet, E., Gallart, F., Sefton, C., Seibert, J. & Bishop, K. 2020. Aqua temporaria incognita. Hydrological Processes 34(26): 5704–5711.

*Please email the lead author (hanamoidu@berkeley.edu) for a copy of the full article!