Most of California’s water supply originates in mountainous headwater regions.

Headwaters—the landscapes where the state’s streams and rivers begin—benefit all Californians. Approximately two-thirds of the state’s surface water supply originates in the Sierra-Cascades and North Coast headwaters, while headwaters in other regions bolster local supplies.
Headwaters collect precipitation during wet periods, store water in snowpack and meadows during dry periods, and filter water before it moves downstream to communities and farms.
These landscapes provide high-quality wildlife habitat, store carbon in plants and soil, support outdoor recreation, and underpin livelihoods in rural communities, among other benefits.

Headwaters are managed by a patchwork of owners.

Federal agencies own about half of the land in California’s headwater regions; the other half is almost entirely privately owned. Other ownership types, including state and tribal, account for about 5% of California’s headwater regions. In the Sierra-Cascades, ownership is about 70% federal and 30% private.
California’s headwaters encompass ecosystems including conifer forest, hardwood forest and woodland, and shrubland. The range of different ecosystems poses challenges for headwaters management and requires different solutions, depending on ecosystem type.

California’s headwaters are made up of different types of ecosystems

SOURCE: Developed using 2022 California Vegetation by Wildlife Habitat Relationship Type data from Cal Fire’s Fire and Resource Assessment Program and Level III Ecoregion data from the US Environmental Protection Agency.

NOTES: Shrubland ecosystems are defined by high proportions of chaparral, coastal sage scrub, and other shrub species. Woodland ecosystems are defined by more open areas between trees than forest ecosystems. Conifer forest and conifer woodland were aggregated to “Conifer forest.” Hardwood forest and hardwood woodland were aggregated to “Hardwood forest & woodland.” “Sierra-Cascades mountains” region is composed of the Sierra Nevada, Cascades, and East Cascades slopes and foothills ecoregions.

Development in wildland areas has put people in harm’s way.

Statewide, approximately 1 million acres of residential neighborhoods were built in or near wildland areas between 1990 and 2020, roughly one-third of them in Southern California.
Wildfire damages are increasing. Average annual wildfire damages grew about five-fold between 1990–99 and 2010–18, when annual losses averaged more than $1 billion (see figure).
Most wildfire structure losses do not occur in conifer forests. Almost half of all structures destroyed in wildfires between 2000–18 were in hardwood forest, woodland, or shrubland; just 14% were in conifer forests.
Humans cause most fires in California. High-wind events in coastal California result in particularly large, damaging fires; Public Safety Power Shutoffs are an increasingly common tool used to prevent ignitions.

Multiple stressors are making some headwater regions more vulnerable to wildfire.

Historically, cultural burns managed by tribes and fires started by lightning kept forests and woodlands healthy. But more than a century of fire suppression has made forests dense and homogenous, with small trees and brush packed closely together. The harvest of large trees has also contributed to this problem.
Dense, homogenous forests and woodlands are vulnerable to increasingly intense droughts, pests, and wildfire. This vulnerability has made wildfires more severe when they occur. High-severity wildfire often converts lands to different ecosystem types, diminishing the benefits Californians receive from headwaters.
High-severity wildfire damages water infrastructure and increases erosion from hillsides into streams when it rains, leading to a heightened risk of floods and mudslides. This drives up water treatment and infrastructure maintenance costs for water utilities.

Wildfire losses and the area burned at high severity are increasing

SOURCES: High-severity burned area data are from Williams et al. 2023. Wildfire damages data are from Buechi et al. 2021.

NOTES: Values shown are annual averages within each range of years. Area burned at high severity is for the Sierra-Cascades region. Wildfire damages are for State Responsibility Areas.

The changing climate adds urgency to headwater management challenges.

As a result of warming temperatures and changing precipitation patterns, dry periods are growing longer and more intense, which makes vegetation more vulnerable to wildfire. The delayed onset of the rainy season means that dry conditions are increasingly overlapping with seasonal high winds in coastal California.
Forests and woodlands benefit from the removal of small-diameter trees (thinning), controlled and cultural burns to clear vegetation (prescribed fire), and allowing remote, lightning-ignited fires to burn in planned zones (managed wildfire).
Federal agencies, state agencies, tribes, and private companies should collaborate across jurisdictions to scale up thinning, prescribed fire, and managed wildfire.
Thinning, prescribed fire, and managed wildfire are less effective in shrubland. Instead, agencies and private parties should work to reduce ignitions (particularly powerline ignitions); carefully plan future development; create gaps in vegetation to control fires (fuel breaks); and harden and create defensible space around homes. These strategies are important in the wildland-urban interface across all ecosystem types.