Can one atmospheric river end California’s drought?

By Andrew L. Rypel and Jay Lund

Given the quantity and intensity of last week’s rain, an obvious question is: ‘Is the drought over?’ Alas, the answer is a resounding no. But, the data are interesting and worth thinking about in more detail.

Fig. 1. Accumulated precipitation for northern California is high at the start of the new water year. But, there is a long way to go to reach even average conditions. Graph from the Department of WaterResources, California Data Exchange Center

As of now, the 8 station index in the northern Sierra registers 12.6 inches of cumulative precipitation. Because the water year just started (October 1), all of this came from the recent atmospheric river. It was an impressive storm, and set quite a few local one-day precipitation records. This atmospheric river storm produced half as much precipitation as graced the region in all of last year (the 3rd driest year on record, 24 inches). This is roughly one fifth of an average year’s precipitation in just one day. This huge early storm also places us on a preliminary pace for the wettest year on record. But, the event was so early, that we have a long way to go to reach even average conditions.

We also have a major water deficit in soil moisture, empty reservoirs and groundwater to repay to end the drought. The storm did reverse the direction of storage in most reservoirs, from declining to climbing. Berryessa and Millerton are now both at 59 and 80% of capacity. But many large reservoirs remain extraordinarily low. The three largest (Shasta, Oroville, and Trinity) are at only 22%, 28% and 27% capacity, respectively. So while some systems have improved, larger ones require much more precipitation to recover. Nevertheless, this storm is an excellent start to the new water year.

Fig. 2. Reservoir levels as of 10/30/2021. Graph from the Department of Water Resources, California Data Exchange Center

Watersheds have been quite dry after two years of intense drought. The recent atmospheric river has wetted soils statewide, which will increase runoff to streams from later storms. Indeed dry soils were part of the reason why so little rainfall made it to reservoirs last year – the parched soils sucked it up. This buffer of dry soils reduces flooding as well. Further, the water that did hit the river must move through the state’s many empty reservoirs, further lessening downriver flooding.

Spawning pair of Chinook salmon on the Lower Stanislaus River. Photo source: Dan Cook (USFWS), downloaded from

The rain is welcome news for both humans and ecosystems. Firefighters who have fought two intense wildfire seasons are provided some relief for this fire season. And the water and flows will overall be positive for aquatic ecosystems, fishes, and forests. Adult Chinook salmon (O. tshawytscha) are ascending river systems to spawn or have already spawned; flows will assist in upriver passage of remaining adults, and longitudinal and lateral distribution of gloriously spawned-out carcasses/nutrients. For spring-run Chinook salmon who have successfully oversummered, these fish will be capable of ascending higher into the landscape to spawn. Outmigration survival of smolts increases non-linearly with flow (Michel et al. 2021). Thus, juvenile winter-run and late fall-run Chinook salmon and even steelhead (O. mykiss) smolts will benefit from these flows en route to the Pacific Ocean. Effects of such early flows on incubating and rearing salmonids are less clear. Increased flows will reduce water temperatures which will benefit eggs, although the high flows will likely also scour and destroy some salmon redds. But taken together, the rains will help our native fishes overall.

Importantly, this atmospheric river demonstrates important lessons about droughts, floods, climate change and California water. We should pay attention and learn from these events. First, perhaps paradoxically, flood events can occur in the midst of major drought. Correlations of precipitation across months in California are weak, so it is possible that recent wet conditions will be followed by a return to little precipitation that prolongs the drought. Such variations are a natural aspect of California’s hydrology. Also, climate change scientists have long-warned how climate extremes would become increasingly commonplace over time (Hayhoe et al. 2004, Cayan et al. 2008), making California’s hydrology even more like California’s hydrology. We can expect a continued real time roller coaster of extremes, accelerated by climate change, including drought, wildfire smoke, intense heat, flooding, mudslides etc.

Second, these moments expose how our water infrastructure will need to adapt with a changing climate. Interestingly, most climate change models do not show a major shift in average rainfall over time, but rather a shift in the timing and severity of storms and snowpack dynamics. California reservoirs were mostly designed to capture spring snowmelt from the Sierra. As more precipitation comes as rain, in more intense events like the recent atmospheric river, we might consider new approaches for storing water. Additional dams will not often be economical because 1) we already have dams in the most cost-effective locations; and 2) environmental costs of dams are increasingly obvious and contributing to declining salmon, smelt and the listing of new species under the Endangered Species Act (Moyle et al. 2017, Bork et al. 2020, Rypel et al. 2020). New and more environmentally sensitive modes of water storage are needed. This includes mountain meadows (Viers et al. 2013), beaver ponds (Baldwin 2015, Rypel et al. 2021), and expanded underground storage via groundwater recharge (Yao and Lund 2021). Given the constraints and timing of SGMA mandates, recharge of groundwater supplies will be especially critical in regions like the San Joaquin basin (Dobbin 2018, Gailey and Lund 2021).

So, it’s good news and bad news on the drought front. The atmospheric river clearly helped – soils have been quenched and some reservoir levels have increased. Fishes may find better fall conditions overall. Yet the drought is not over, and recovery may take awhile.  We have quite a soil moisture, reservoir, and groundwater deficit to recover – more than this one huge storm could provide.  It’s unclear what the rest of the winter may bring.

State and local agencies should be well-practiced by now in their drought plans, and should continue to plan for drought conditions that extend into the foreseeable future. Other managers in the water and environmental sectors will need to continue planning as the season wears on. California has a long history of innovating around the socioecological pressures that drought induces. Let’s all look at these events with an eye towards innovation once again.

Lake Oroville, May 2021. Photo credit: Frank Schulenburg, downloaded from

Andrew Rypel is an Associate Professor and the Peter B. Moyle & California Trout Chair of coldwater fish ecology at the University of California, Davis. He is a faculty member in the Department of Wildlife, Fish & Conservation Biology and Co-Director at the Center for Watershed Sciences.   

Jay Lund is a Professor of Civil and Environmental Engineering Director, and Co-Director at the Center for Watershed Sciences at the University of California, Davis.

Further Reading

Baldwin, J. 2015. Potential mitigation of and adaptation to climate-driven changes in California’s highlands through increased beaver populations. California Fish and Game 101: 218-240.

Cayan, D.R., P.D. Bromirski, K. Hayhoe, M. Tyree, M.D. Dettinger, and R.E. Flick. 2008. Climate change projections of sea level extremes along the California coast. Climatic Change 87: 57-73.

Hayhoe, K., D. Cayan, C.B. Field, P.C. Frumhoff, E.P. Maurer, N.L. Miller, S.C. Moser, S.H. Schneider, K.N. Cahill, E.E. Cleland, L. Dale, R. Drapek, R.M. Hanemann, L.S. Kalkstein, J. Lenihan, C.K. Lunch, R.P. Neilson, S.C. Sheridan, and J.H. Verville. 2004. Emissions pathways, climate change, and impacts on California. Proceedings of the National Academy of Sciences 101: 12422-12427.

Michel, C.J., J.J. Notch, F. Cordoleani, A.J. Ammann, and E.M. Danner. 2021. Nonlinear survival of imperiled fish informs managed flows in a highly modified river. Ecosphere 12: e03498.

Moyle, P.B., R. Lusardi, and P. Samuel. 2017. SOS II: fish in hot water. California Trout and University of California Davis.

Viers, J.H., S. Purdy, R.A. Peek, A. Fryjoff-Hung, N.R. Santos, N.R., J.V.E. Katz, J.D. Emmons, D.V. Dolan, and S.M. Yarnell. 2013. Montane meadows in the Sierra Nevada: changing hydroclimatic conditions and concepts for vulnerability assessment. Center for Watershed Sciences Technical Report.

About Andrew Rypel

Andrew L. Rypel is a Professor and the Peter B. Moyle and California Trout Chair of coldwater fish ecology at the University of California, Davis. He is a faculty member in the Department of Wildlife, Fish & Conservation Biology and Director of the Center for Watershed Sciences.
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