At peak flow every year, hydrologists estimate that over 7,000 gallons of water per second can rush over Yosemite Falls to join the Merced River in its journey down to California’s Central Valley.
Thundering waterfalls are iconic signs of spring in Yosemite, usually reaching peak flow in late spring like 3,000-foot clockwork. They may also become one of the park’s most visible signs of change.
Yosemite Falls, like many streams and waterfalls in the park, is fed almost entirely by snowmelt. Waterfall flow is largely determined by how much snow the park receives and when it melts. As high elevations experience warmer temperatures, winter snow shifts to rain and spring snowmelt arrives early. Future visitors may find Yosemite’s waterfalls at their peak during winter, leaving cliffs bare and meadows dry by midsummer.
Wildfire season may also be affected, as a drier landscape is easily kindled in lightning storms (six times as many fires occur in low-snowpack as opposed to high-snowpack years). Ecosystems adapted to the gradual release of meltwater throughout the summer may be forced to recalibrate, as less drought-hardy plants and animals are replaced by those that can withstand a shorter wet season and lengthening dry spells. But the same shifts that threaten some low-elevation species may aid others in the high country; Sierra Nevada bighorn sheep and endangered Sierra Nevada yellow-legged frogs are known to thrive in years with less snow.
Human use of the park in winter is also defined by snowpack. Warming temperatures may lengthen the seasons of Tioga and Glacier Point Roads, allowing access to vast swaths of wilderness during a season when human presence has historically been minimal. On the other hand, the season may shorten for skiers at Badger Pass and in the Yosemite wilderness, where backcountry travelers are already confronting multiplying stream crossings and vanishing snow bridges.
A ranger performing a snow survey wades across a stream. Stream crossings and melted snow bridges have become more frequent obstacles to winter travel.
A Frozen Bank Account
Snowmelt is the lifeblood of Yosemite. Even after the springtime tumult of rushing rivers and roaring falls has passed, trickling meltwater continues to fill streambeds through the hot, dry summers of Yosemite’s Mediterranean climate. Ecosystems from alpine gardens to giant sequoia groves rely on this vital savings account of frozen water, refilled each winter and drawn down during the spring and summer. A winter rainstorm, on the other hand, is the equivalent of spending a paycheck right away: streams and waterfalls fill immediately, but little is saved for the dry summer ahead.
Meltwater matters far beyond park boundaries: a snowflake that falls in Tuolumne Meadows in January can flow out of a San Francisco tap in June. Snow from elsewhere in the Sierra Nevada supplies water to cities as distant as San Diego. Overall, 60% of California’s fresh water (and 75% of its agricultural water) comes directly from the Sierra Nevada snowpack.
Compounding Losses
Have you ever worn sunglasses or ski goggles to shield your eyes from the bright reflection off snow? Snow has a high albedo, meaning it is extremely reflective of both light and heat. Like the white lid of an ice chest, reflecting the sun’s rays helps snow (or your picnic) stay cool for much longer. However, middle elevations (5,000–8,000 feet) are increasingly vulnerable to a cycle called snow albedo feedback: melting snow leaves darker-colored ground exposed, which absorbs more heat and leads to even more melting. At some elevations, this feedback loop alone accounts for more than 3.6°F of extra warming. Forty percent of Yosemite (466 square miles) lies between 5,000–8,000 feet of elevation, putting it at risk for more rapid change than would be caused by atmospheric warming alone.
Snow surveys over the past 70 years in and near Yosemite National Park show declines in the heaviest snowpack years at low elevations. (Source: CDEC snowpack data.)
What changes are we seeing?
Each month during the winter, rangers ski out to conduct snow surveys deep in the Yosemite wilderness. This data record stretches back to 1930 for some survey sites—among the longest continuous snow records in the western US. Meanwhile, seven gaging stations measure streamflow continuously on the Merced and Tuolumne Rivers, with some data going back to the 1910s.
Using this information, we can measure changes already taking place in seasonal water patterns:
Decreasing snowpack at low elevations: Throughout the Sierra Nevada, April 1 snowpack is decreasing at elevations below 8,500 feet but increasing slightly at higher elevations (this may reflect natural long-term variation or global changes in atmospheric moisture). This pattern is reflected inside Yosemite, where low-elevation snow surveys show that snowpack in the heaviest years is decreasing over time, while no trend appears at higher elevations.
Dropping spring streamflow: April–July streamflow in the Merced River is decreasing relative to the rest of the year, indicating that more snow is melting before the typical peak season.
What’s next?
Without a major reduction in greenhouse gas emissions, scientists project the following by 2100:
Dramatic snowpack loss: Total annual snowpack in the Merced River basin (including the southern portion of Yosemite National Park) may decline by 49%; across the Sierra Nevada, snowpack may decline by 64%.
Earlier snowmelt: Melting in the Merced River basin may arrive around one month earlier.
More rain, less snow: The fraction of precipitation that falls as rain rather than snow may rise by 10% in the Merced River basin.
Climate models simulate past (green) and future (red) conditions in the Merced River basin, including the southern part of Yosemite. The “business-as-usual” model represents a central scenario on the spectrum of possible future greenhouse gas concentrations. (Figures from Dettinger et al, 2004.)
Lower spring streamflow: The total April–July flow of the Merced may drop by 16%, while winter flows may rise.
More winter flooding: Days when the Merced River measures over 8.5 feet at Happy Isles may be more than twice as common (flood stage is 8 feet).
Snowpack size and snowmelt timing fundamentally shape ecosystem structure, forest health, and wildfire activity in Yosemite, as well as the experience of skiing, waterfall viewing, boating and backpacking in the park. Across California, 23 million people are served by the vast frozen reservoir of the Sierra Nevada snowpack. As hydrologists continue to study the impacts of climate change in the High Sierra, they are recording the beginnings of a story that will continue hundreds of miles downstream and centuries into the future.
