Christine White Loberg
Yosemite National Park’s hydrologic resources include the headwaters of two magnificent rivers—the Merced and the Tuolumne. All of the creeks, streams, and lakes in Yosemite will eventually join with one of these two rivers, which have both been declared as “Wild and Scenic Rivers” by Congress in the 1980s.
What is hydrology? Simply put, hydrology is the study of water and its interactions with the landscape. The source of water in lakes, rivers, and groundwater is precipitation that falls as rain or snow. It can be released immediately to flow over Yosemite’s spectacular waterfalls and through magnificent river canyons, be stored in snowpack or groundwater, be used by plants and trees, or be evaporated back into the atmosphere only to fall somewhere else. The science of hydrology seeks to understand the processes that control how much water flows into our streams, how much is stored underground, where it comes from, how it moves through the landscape, the quality of that water, and the ways that water is recycled in the natural environment.
Yosemite's hydrologic resources are fascinating to examine because the park lies at the heart of one of the most extreme Mediterranean climates on earth. Such climates are characterized by cool, wet winters and long, dry summers, and they generate some of the most diverse ecosystems on earth adapted to survive these extremes. In the case of Yosemite, most precipitation falls in the form of snow that accumulates above 6,000 feet (1,830 meters) during the winter, making a natural water tower that slowly releases melt-water through the spring and early summer. This slow release of water nourishes lower regions well into the hot dry season.
Yosemite National Park’s hydrology program in cooperation with the USGS, Merced Irrigation District, and Hetch Hetchy Water and Power monitors water quantity and quality in order to assure maintenance of clean mountain water as well as provide information for drought management, long-term hydrologic trends and possible ecological impacts, and park planning. River gages within Yosemite that have produced valuable long-term records. (View real-time flow data at Happy Isles gage station.) Yosemite assesses water quality along the Merced and Tuolumne Wild and Scenic Rivers. The Wild and Scenic Rivers Act awards special protection to the quality of the water and the free-flowing condition of these rivers, and the park is responsible for upholding these high standards.
Large storms bring tremendous amounts of moisture from the Pacific Ocean into California each winter. As this warm moist air encounters the Sierra Nevada Mountain range, it is forced up and over a crest with peaks as high as 14,000 feet (4,270 meters). As the air moves up and over the mountains, it cools adiabatically (without heat transfer) at an average rate of 6.5°C per 1,000 meters, causing water to condense and often precipitate as rain and/or snow. This result, called the orographic effect, is the primary reason why 50% of California’s water supply originates in the Sierra Nevada.
Sierra Nevada ecosystems have adapted to take advantage of the snowmelt driven hydrologic system. As spring temperatures rise, snow begins to melt, saturating soil and filling streams and rivers. This annual rise in water levels is referred to as the "spring pulse." Rising groundwater levels and streams inundate meadows and wetland areas, bringing nutrients and sediments that sustain these biologically rich areas. Plants and animals from the foothills to alpine meadows take advantage of the spring pulse to carry out important aspects of their life cycles.
A dramatic warming of the climate will profoundly affect this delicate cycle. Northern California is predicted to warm by 3-6°C (5-11°F) by the year 2100, an increase that will decrease the annual snowpack volume, melt it earlier, and increase the potential for torrential winter rainstorms that may cause flooding. This shift in the cycle will diminish the strength of the spring pulse result in longer, drier summers with less water in rivers, streams, and groundwater storage. Warmer temperatures will result in rising snowline elevation and an increase in park area receiving rain instead of snow. The trio of maps above conceptualizes this idea by showing approximate snowline positions for a 3°C (snowline ~ 7,250 feet) and a 6°C (snowline ~ 9,020 feet) rises in temperature.
Webcam: View from Happy Isles Gaging Station (provided by the USGS) of the Merced River. This Yosemite webcam is believed to be the first webcam view of a river in the National Park Service. When using the cam, you can zoom in or out.
Did You Know?
At the east end of El Portal, just west of Yosemite National Park’s boundary, changing river gradients, glacial history, and powerful floods have created a boulder bar with boulders much larger than typically found in such deposits. This is no ordinary boulder bar, however, for it contains massive boulders over a meter in diameter and weighing many tons.