Snowpack that collects on mountains is vital to the water resources of the United States, but also to the waterways and water supplies around the world. As mountain snowpack melts, it slowly releases water into streams, rivers and lakes. Although snowmelt supplies up to 50 percent of the world’s freshwater and around 75 percent of water to the American West, snowpacks are becoming less reliable because of climate change.
How Mountains Act As Natural Water Towers
Snowpack is vital to water resources in the United States. Mountains can be called natural “water towers” because they are vital headwaters to many rivers and other freshwater sources. This freshwater arrives from melting snow that produces streamflow that winds up in streams, rivers, lakes and eventually oceans. This meltwater can also replenish aquifers. In total, mountain meltwater and runoff account for more than 50 percent of the world’s freshwater. From the Himalayas in Asia to the Alps in Europe to the Rockies in North America, high-elevation-mountains are all important sources of water to the billions of people in the lowlands.
The Relationship Between Snowpack and Snowmelt
Even though it seems straightforward, the relationship between snowpack and snowmelt runoff is complex. Snow accumulates in the cold winter months to form the snowpack, which thaws during the spring and early summer, and the subsequent snowmelt produces the streamflow that fills rivers and lakes below. The amount of snow that accumulates depends on such factors as moisture content of the soil, air temperature, precipitation patterns, storm frequency, the number of plants in the area and their water use, and how much snow runoff feeds groundwater. Not only are these factors in constant flux, the location (geography) makes a significant difference in how these factors play out. For instance, before snowpack builds in the cold months, the amount of moisture stored in soil by early winter determines the amount of runoff available, since dry soils tend to absorb more water than moist soils.
Another major consideration regarding the amount of water runoff that comes from snow is its texture (the structure of the snow) and density (the amount of water per unit volume of snow). While “powder” or “dry snow” is prime snow for skiing, it does not contain as much water moisture as “wet snow.” Air temperature and air moisture determine the dryness or wetness of the snow. Deeper snowpack and wetter snow both increase density and compress lower layers. This compression changes the crystalline structure of snowpack. Both density and structure help regulate how quickly the snowpack melts and how much water runs off.
Mountains in regions that have more moisture and that get heavier, wetter snow can produce more water from a shallower depth of snow. For example, 1 foot of new snowpack can produce up to 1.5 inches of water in the more humid eastern mountain range of the Appalachians. By comparison, in the arid Wasatch Mountains of Utah, where the snow is drier, 1 foot of snowy powder might yield only 1 inch of water. Thus, the geography and climate of the mountains influence the amount of water that its snowpack can deliver.
Mountains, Water and Climate in the Western US
Mountains are a crucial source of water, especially in dry places like parts of the Western United States, as explained by author Robert Sauder in his chapter entitled, “Mountains and Lowlands: Human Adaptation in the Owens Valley” from the book, The Mountainous West. As described in the chapter, Western mountain ranges act as “islands of moisture in a vast sea of aridity.”
For all the spectacular climate and fertile soils that have created healthy ecosystems and driven agricultural and urban development in the American West, none of this bounty would be possible without the existence of mountain snowpack and the ability to convey the resulting water far and wide. Because mountain snowpack is central to freshwater resources, much of the Western United States’ society and infrastructure depends on collecting and transporting that runoff. That’s why owners and operators of reservoirs, canals and water pipelines hold such power in law, public policy, urban planning and agricultural decision-making in the country.
Climate Change is an Existential Threat to Mountain Snowpack
As global temperatures rise, mountain glaciers and snowpack melt and evaporate (called sublimation) exceptionally fast. In places like Montana’s Glacier National Park, mountain glaciers might disappear in the 21st century, which means that the glaciers – and the water they provide – likely aren’t coming back.
Another huge challenge that warmer temperatures present is “snow drought,” where precipitation falls as rain instead of snow. When rain falls on mountain heights, water quickly fills reservoirs earlier that anticipated and can result in excess water that does not get stored. Since rain flows more quickly than snow melt, there can also be a significant loss of groundwater recharge, which can be a large contributor to mountain springs and streams.
The Rocky Mountain Snowpack and the Colorado River
The treasured Colorado River starts with the trickles that flow from the high Rocky Mountain peaks of Colorado. Referred to as the “lifeline of the Southwest,” the Colorado River supplies water to seven US states and Mexico with nearly 40 million municipal and tribal users, and irrigates around 5.5 million acres of land in an area that covers 246,000 square miles. Because water is scarce throughout parts of the American West, cities like Los Angeles, Phoenix and Denver required Colorado River water to grow. Colorado River water makes up an estimated 17 percent of the water delivered hundreds of miles away to California’s South Coast Hydrologic District, which includes major cities like Ventura, Los Angeles and San Diego. The same is true for agriculture, where that Colorado River water ends up irrigating valuable crops like almonds and winter lettuce in Arizona and California as well as pasture and forage for beef cattle in Colorado.
These various sectors have placed heavy demands on Colorado River water, which is already over-allocated due to governing compacts, legal and contract frameworks and a particularly monumental Arizona v. California court case. This over-allocation is further compounded by a decline in the amount of water coursing through the river, which has fallen 19 percent below 20th century levels. Research has found that much of the drop in river flow is due to warmer temperatures and greater evaporation caused by climate change affecting the Upper Colorado River Basin. This puts the entire Southwest on the edge of a water shortage and forces the region to reconsider the quantity of river water available.
“A good chunk of the decline we’re seeing right now is temperature-related. And as the Earth continues to warm, we’re going to see less flow in the river. We need to prepare for a river that has significantly less water in it.”Water and climate scientist, Brad Udall
The Sierra Nevada Mountain Snowpack and California Water
Severe droughts are not new to California’s people or its long natural history. Yet there was something new about the most recent, intense drought that spanned from 2011 to 2017 (or the 2012 to 2016 “water years”). What was “new” came largely from climate change and the shift in precipitation patterns for the vital Sierra Nevada mountain range. As average temperatures in the West trend up over time – 2014 was (at the time) California’s warmest on record – a shift in the water cycle will occur. Results of one study find that up to 60 percent of the climate trends such as reduced snowpack, river flow and winter air temperature are human-induced.
California got a taste of these hotter temperatures and the impact on precipitation in this last drought as its arrival fell more as rain than snow in high altitudes. The warmer temperatures also made the what little snow that was available melt earlier in the winter season, which disrupted the timing of the water supply by filling – and draining – California reservoirs far ahead of the dry summer and fall months when water is most in demand. The bare peaks and dwindling snow in the Sierra Nevadas illustrated the major toll the drought took on state water supplies. People began to realize that snowcapped mountains aren’t merely a beautiful backdrop, but also a high-elevation water storage system that the American West relies on for up to 75 percent of its water supplies. In a normal year, California gets about one-third of its freshwater supply from snow melt runoff that feeds streams, fills state reservoirs and provides water to everyone from coastal urbanites to farmers in the Central Valley.
Another major concern is the strong connection between atmospheric warming, more frequent and destructive wildfires and threatened water resources. As higher temperatures and little precipitation dry out forests on and around mountain areas, these tinderbox forests burn more regularly. After wildfires denude those mountainsides, the charred remains contain toxic ash and debris on slopes that cannot retain as much water runoff. This, in turn, can significantly hurt mountain-derived water supplies because of pollution carried with it by the fast-moving runoff.
The variability of runoff from the Sierra Nevada and Coastal Range snowpacks creates unpredictability for water managers that administer resources, especially for southern to northern California water transfers (the majority of southern California water originates in northern California). The impact of climate change remains a leading concern according to the Climate Adaptation Strategy, which finds a potential 25 to 40 percent decline in California snowpack by 2050 from its 20th century average. The strategy document also forecasts a 12 to 35 percent decrease in total precipitation for the state.
Mountain snow is essential to water supplies in California and the rest of the western United States. As natural water towers lose the amount of water available to those who depend on it, no wonder people are concerned. Federal, state and local governments are all engaged in protecting their water supplies by maintaining the health of headwater ecosystems and by conserving water, all in order to buffer capricious mountain snowpack.
Image: Looking back at Matterhorn Peak. Credit: Oliver Dodd (Creative Commons).