By Ted Grantham, Stephanie Carlson, and Albert Ruhi

As we move into the full swing of summer, water managers are paying close attention to the remaining snowpack in the Sierra Nevada. Each year, water from melting snow flows into rivers, creating important environmental cues for native freshwater species and filling reservoirs, just as agricultural water demands peak during the growing season. But as California gets hotter with global climate change, our snowpack is shrinking and melting earlier in the year (Stewart et al. 2009), profoundly changing snow-dependent river ecosystems (Leathers et al. 2024), and leaving us with less water when people need it most. 

The loss of snow isn’t the only effect that climate change is having on our rivers. Two recent papers from the Berkeley Freshwater group investigated how flow regimes – the natural seasonal patterns of flowing water – are shifting in the state’s rivers.

We first explored if climate change is causing river drying in Mediterranean-climate regions of the world, including California, Chile, South Africa, Australia, and Spain (Carlson et al. 2024). We focused our attention on stream gages in watersheds with limited human land- and water-use. Such “reference-quality” gages are rare, but they are important for understanding how natural flow patterns in rivers respond to climate variability. Since most rivers are impacted by dams, diversions, and land use change, reference gages make it possible to isolate the effects of climate from these other factors. 

We found evidence of stream drying at reference gages in each of our study regions, including several rivers that have historically been perennial (flowing year-round) but in recent years have become seasonally intermittent (Figure 1). As noted in a previous blog, flow intermittency is a natural occurrence in many streams. And despite the seasonal periods of drying, our native species are adapted to, and even thrive, under such intermittent conditions (see also Bogan et al. 2015, Moidu et al. 2020). But when perennial streams go dry, the ecological impacts can be more severe, extirpating sensitive species and shifting the composition of the aquatic community. The drying of perennial streams can also have important social consequences, including the loss of water supplies, fishing, and recreational opportunities. With recent rulings narrowing the scope of waters regulated under the Clean Water Act (Greenhill et al. 2024), drying can also result in the loss of legal protections, leaving rivers more vulnerable to pollution and development pressures. 

Our second paper (Ayers et al. 2024) further investigated drying patterns in California’s rivers and streams. Relying on the network of about 150 reference gages in the state, we separated perennial from intermittent streams according to the number of zero-flow days recorded each year. We then developed a model to determine how the observed stream class is influenced by its watershed characteristics – such as its geology, soils, topographic features, and annual precipitation. Finally, we used the model to predict the expected, natural flow class at 350 stream gages located in both reference-quality and human-impacted watersheds.  

We found similar evidence of drying in many California streams (Figure 2). Some perennial reference streams have become intermittent and both perennial and intermittent streams are trending towards drier conditions. In impacted (non-reference) watersheds, changes in flow patterns were also widespread, but more variable. We found evidence of both increasing and decreasing low-flows within our population of gages, including some intermittent streams that have become perennial (for example, as a result of wastewater treatment facility discharges). These findings highlight how human activities and climate change interact to cause complex, but significant impacts on river flow regimes.   

Both of these studies underscore the importance of streamflow gages to assess changes in river flow regimes. Unfortunately, in California, there is a woefully inadequate number of streamflow gages in current operation. Another recent study by our group (to be discussed further in a future blog post) found that only 8% of the states’ total stream network is being monitored (Andrews and Grantham 2024). As the climate changes and human water-use pressures intensify, additional monitoring will be critical to detecting flow regime shifts and taking steps to limit their impacts to ecosystems and society. 

Ted Grantham, Stephanie Carlson, and Albert Ruhi are professors in the Environmental Science, Policy, and Management Department at the University of California and jointly lead the Berkeley Freshwater group.

Further Reading

Andrews and Grantham. 2024. Strategic stream gauging network design for sustainable water management. Nature Sustainability 7: 714-723. https://doi.org/10.1038/s41893-024-01357-z

Ayres, J.R., Yarnell, S.M., Baruch, E., Lusardi, R.A., Grantham, T.E. 2024. Perennial and non-perennial streamflow regime shifts across California, USA. Water Resources Research 035768. https://doi.org/10.1029/2023WR035768

Bogan, M.T., Hwan, J.L. and Carlson, S.M., 2015. High aquatic biodiversity in an intermittent coastal headwater stream at Golden Gate National Recreation Area, California. Northwest Science, 89(2), pp.188-197. https://doi.org/10.3955/046.089.0211

Carlson, S.M., Ruhí, A., Bogan, M.T., Hazard, C.W., Ayers, J., Grantham, T.E., Batalla, R.J. and Garcia, C. 2024. Losing flow in free‐flowing Mediterranean‐climate streams. Frontiers in Ecology and the Environment e2737. https://doi.org/10.1002/fee.2737

Greenhill, S., Druckenmiller, H., Wang, S., Keiser, D.A., Girotto, M., Moore, J.K., Yamaguchi, N., Todeschini, A. and Shapiro, J.S., 2024. Machine learning predicts which rivers, streams, and wetlands the Clean Water Act regulates. Science, 383(6681), pp.406-412. https://www.science.org/doi/10.1126/science.adi3794

Leathers, K., Herbst, D., de Mendoza, G., Doerschlag, G. and Ruhi, A., 2024. Climate change is poised to alter mountain stream ecosystem processes via organismal phenological shifts. Proceedings of the National Academy of Sciences, 121(14), p.e2310513121. https://www.pnas.org/doi/abs/10.1073/pnas.2310513121

Moidu, H., Rodríguez‐Lozano, P., Leidy, R.A., Bogan, M.T., Grantham, T.E. and Carlson, S.M., 2023. Ecological consequences of shifting habitat mosaics within and across years in an intermittent stream. Freshwater Biology, 68(7), pp.1161-1175. https://doi.org/10.1111/fwb.14094

Stewart, I.T., 2009. Changes in snowpack and snowmelt runoff for key mountain regions. Hydrological Processes: An International Journal, 23(1), pp.78-94. https://doi.org/10.1002/hyp.7128