River Chemistry in Yosemite National Park
The pristine environment of Yosemite National Park provides ideal circumstances to study the natural chemistry of a snow-melt fed river and evaluate the impact of human activities on the river downstream.
We value our nation’s parks as they preserve our heritage and environment as well as provide locations where we can connect with the land. From a chemist’s perspective, these parks have an additional value: the parks provide opportunities to study relatively pristine environments that have been far less affected by human activities than most locations in the country. By studying the environment in a national park it is possible not only to evaluate the health of that ecosystem, but also to use this information to understand the impacts of agriculture and cities downstream of the park. Yosemite National Park, famous for its waterfalls and for the cold, clean Merced River, provides an excellent location for monitoring the water chemistry of an area and providing such a benchmark.
When exploring the chemical story of a river one of the most important properties to evaluate is its conductivity. Conductivity is a measure of the concentration of substances in the water, and is related to salinity, the amount of salt dissolved in the water. Snowmelt salinity is about three parts per million (ppm) whereas coastal ocean salinity is typically three parts per thousand, so as expected the Merced River has a rather low conductivity. As the Merced flows toward the Pacific Ocean, a slight increase in salinity due to natural circumstances would be expected. But scientists have measured a relatively high conductivity downstream from the park, which indicates the important influence agricultural and urban runoff has on the river.
Two additional chemical measurements tell other aspects of the Merced River’s story. Studies show that very little nitrate enters the river as it flows through Yosemite, the predominant contributor being nitrogen produced from decaying vegetation; downstream of the park, fertilizers from residential lawns and agricultural crops boost the nitrate levels substantially. Another factor that scientists measure is silica levels, which tells the story of impacts due to erosion on the river. The biggest contributor to silica levels is the weathering of Sierra Nevada granite as the river gently wears away the mountains in its downhill path.
More details of the river chemistry of Yosemite National Park are available at http://www.sierranaturenotes.com/naturenotes/NatNote_DHPet.pdf