18.5: Threats to California’s Water

Managing Water Quality and Addressing Environmental Challenges

Water quality in California is a complex topic, shaped by both natural processes and human activities. Understanding the interactions between geology, weathering, human land use, and water chemistry is crucial for managing the state’s surface and groundwater resources. Both surface water and groundwater contain a variety of dissolved and solid materials, derived from rainfall, chemical weathering of rocks, and organic contributions from vegetation and animals. However, when undesirable or toxic substances are introduced, either naturally or through human activities, water quality can be compromised, as seen in the Kesterson Reservoir case. (Table 18.5.1).

The Kesterson Effect: From Rock to Duck

The Kesterson Reservoir in the San Joaquin Valley became infamous in the 1980s due to the release of toxic substances into its waters, primarily selenium, a naturally occurring element found in the Moreno Formation, which consists of shale and sedimentary rocks. When water percolated through these selenium-rich rocks, agricultural runoff exacerbated the problem by mobilizing this element. Irrigation practices in nearby farmland increased the water flow through the drainage basin, leading to greater dissolution and transportation of selenium into the reservoir. Over time, selenium built up in the ecosystem, eventually reaching toxic levels that harmed local wildlife, particularly waterfowl. This pathway of selenium contamination was described by USGS scientists as the "Kesterson effect," defining the biogeochemical process from "rock to duck." The incident highlights the complex interplay between geology, human agricultural practices, and environmental health, underscoring the need for comprehensive management strategies to mitigate such contamination risks.

Water Contamination and Pollution

While weathering can release both essential and potentially harmful materials into water systems, it’s important to recognize that not all substances derived from weathering are toxic. Organic compounds from plants and animals in a drainage basin contribute beneficial materials to water. In groundwater systems, solid particles are filtered through the sediment, which can help purify water as it moves through aquifers. However, water pollution occurs when toxic substances—whether from natural sources like volcanic gases and dissolved metals, or human-induced contaminants such as pesticides—are introduced at harmful concentrations.

Contaminants commonly found in California’s waters include arsenic, nitrate, perfluoroalkyl and polyfluoroalkyl substances (PFAS), chromium-6, pesticides, lead, copper, and microorganisms like Escherichia coli and Cryptosporidium (Table 18.5.1). PFAS, often called "forever chemicals" because of their resistance to breaking down in the environment, are a group of synthetic substances widely used in products like non-stick cookware, water-repellent fabrics, firefighting foams, and food packaging. These chemicals have been found in water supplies across the U.S., including California, and can accumulate over time in both the environment and the human body. Exposure to PFAS has been linked to a range of adverse health effects, including liver damage, developmental problems, cancer, and immune system disruption. The persistent nature of PFAS makes them a significant challenge for water management and environmental health.

Table 18.5.1: Common contaminants to California’s water supply, their occurrences, and associated risks.

Contaminant	Occurrences and Risks
Arsenic	Arsenic (As) contamination is a significant concern in many parts of California, particularly in areas where naturally occurring geological formations contain elevated levels of this toxic element. Groundwater is particularly susceptible to arsenic contamination. Long-term exposure to arsenic has been linked to various health issues, including cancer and cardiovascular disease.
Nitrates	Nitrate contamination, primarily stemming from agricultural activities and wastewater discharges, is widespread in California's groundwater and surface water sources. High levels of nitrate pose risks to human health, particularly for infants and pregnant women, as excessive nitrate intake can interfere with the blood's ability to carry oxygen, leading to methemoglobinemia or "blue baby syndrome." This has been common near Kerman and Kettleman City on the westside of the San Joaquin Valley.
Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)	Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS) compounds, commonly found in firefighting foam, industrial processes, and consumer products. These persistent chemicals can accumulate in the environment and bioaccumulate in organisms, posing potential risks to human health. California has taken proactive measures to regulate PFAS compounds in drinking water, setting MCLs that exceed federal guidelines to ensure greater protection.
Chromium-6	Chromium-6, also known as hexavalent chromium, is a highly toxic form of chromium associated with industrial processes and natural geological occurrences. Elevated levels of chromium-6 in drinking water have been linked to adverse health effects, including cancer. California has established a stringent MCL for chromium-6, reflecting the state's commitment to protecting public health from this hazardous contaminant.
Pesticides and Herbicides	Pesticides and herbicides from agricultural activities in California, known as the nation's leading agricultural producer, contribute to water contamination. Runoff from farmlands and orchards can transport these chemicals into surface water and groundwater, posing risks to aquatic ecosystems and human health. Monitoring and managing pesticide contamination are essential for protecting California's water resources and agricultural sustainability.
Lead and Copper	Lead and copper contamination, although primarily occurring in drinking water distribution systems rather than natural sources, remain significant concerns for water quality in California. Aging infrastructure, particularly in older urban areas, can contribute to the leaching of lead and copper into drinking water, posing risks to public health, especially for vulnerable populations such as children and pregnant women.
Microorganisms	Microorganisms, including bacterial and viral pathogens such as Escherichia coli (E. coli) and Cryptosporidium, can contaminate California's water sources, particularly during heavy rainfall events or sewage system overflows. These microorganisms can cause waterborne diseases and pose serious health risks if ingested through contaminated drinking water or recreational activities.

Many of these pollutants, including nitrate from agricultural runoff, pose serious risks to human health. For instance, high nitrate levels in drinking water are particularly dangerous for infants, potentially causing a condition known as methemoglobinemia, or "blue baby syndrome," which affects oxygen transport in the bloodstream.

Table 18.5.2 highlights some of the differences between federal and California Maximum Contaminant Levels (MCLs), showcasing how California often sets stricter standards to safeguard public health. California’s stricter MCLs for certain substances, including chemicals listed under Proposition 65, demonstrate the state’s precautionary approach in regulating drinking water. Proposition 65 requires California to publish a list of chemicals known to cause cancer or reproductive harm, which influences the state's MCL standards to reflect heightened protection for public health. Differences between federal and state MCLs often stem from California’s need to address unique geological conditions, such as natural arsenic leaching from rock formations, and a focus on cumulative exposure risks.

Table 18.5.2. U.S. EPA MCLs versus California MCLs.

CONTAMINANT	MAXIMUM CONTAMINANT LEVEL (US EPA) mg/L	MAXIMUM CONTAMINANT LEVEL (CALIFORNIA) mg/L	HEALTH EFFECTS
Arsenic (As)	0.010	0.010	Circulation, skin damage, cancer
Cadmium (Cd)	0.005	0.005	Kidney damage
Fluoride (F)	4.0	2	Bone disease
Lead (Pb)	0.015	0.015	Developmental delays, attention disorders, kidney problems
Mercury (Hg)	0.002	0.002	Kidney damage
Nitrate	10	45	Methemoglobinemia (blue-baby syndrome)
Nitrite	1.0	1.0	Methemoglobinemia (blue-baby syndrome)
Perchlorate	--	0.006	Thyroid damage, cancer
Selenium (Se)	0.05	0.05	Hair and/or fingernail loss, circulatory problems
Thallium	0.002	0.002	Hair loss, blood, kidney, intestine and/or liver problems

Addressing the challenges posed by these common contaminants requires a comprehensive approach that integrates monitoring, regulation, infrastructure upgrades, and pollution prevention measures. By prioritizing water quality management and adopting proactive strategies, California can continue to safeguard its water resources and protect public health.

Saltwater Intrusion: A Coastal Threat

Saltwater intrusion, the process by which saline water encroaches into freshwater aquifers, is a significant hazard in California’s coastal regions (Figure 18.5.1). This phenomenon is exacerbated by groundwater overdraft, sea-level rise, and coastal development. As freshwater is withdrawn from coastal aquifers, the reduced pressure allows seawater to move inland, contaminating freshwater supplies.

Figure \(\PageIndex{1}\): "Saltwater-freshwater interface along the coast" by Jooja is licensed under CC BY-NC 4.0. View a detailed description of this image.

California’s coastal regions are commonly affected by saltwater intrusion, where freshwater aquifers interface with the Pacific Ocean. Coastal aquifers, particularly those situated in low-lying areas or areas with high groundwater extraction rates, are particularly vulnerable to saltwater intrusion. In regions such as the Central Coast, Southern California, and the San Francisco Bay Area, over-pumping of groundwater for agricultural, industrial, and municipal purposes has led to groundwater depletion, exacerbating the risk of saltwater intrusion.

Sea-level rise, driven by climate change, exacerbates the threat of saltwater intrusion along California's coastline. As sea levels rise, the pressure of saline water against coastal aquifers increases, facilitating the intrusion of seawater into freshwater sources. Low-lying coastal areas, such as the Sacramento-San Joaquin Delta and the Los Angeles Basin, are particularly susceptible to saltwater intrusion as rising seas encroach upon freshwater aquifers and surface water bodies.

Coastal development and urbanization further exacerbate saltwater intrusion hazards by altering natural hydrological patterns and increasing groundwater extraction rates. Urban areas along the coast, including Los Angeles, San Diego, and San Francisco, face increasing challenges in managing saltwater intrusion as population growth and development intensify water demand and groundwater extraction. Figure 18.5.2 illustrates the concentrations of chloride (salt) from offshore. USGS studies show that the geology of the Los Angeles Basin is highly complex, as shown by this modeled figure of Los Angeles seawater intrusion into the Dominguez Gap area stratigraphy. Understanding this geology helps determine where and how fast water moves within the various beds of sediment both onshore and offshore to inform more effective management strategies pertaining to saltwater intrusion.

Figure \(\PageIndex{2}\): "Modeled chloride (salt water intrusion) form offshore" by the United States Geological Survey is in the public domain. View a detailed description of this image.

The Salinas Valley is one of the most severely affected regions, where decades of groundwater extraction have allowed seawater to intrude inland by several miles (Figure 18.5.3). Groundwater elevation data from the Salinas Valley in 2019 (Figure 18.5.4) shows concerning trends of decreasing groundwater levels, exacerbating saltwater intrusion risks. In response, California has employed various mitigation strategies, such as "guardian wells" in Los Angeles (Figure 18.5.5), which pump seawater away from freshwater sources or inject freshwater into the aquifer to form a protective barrier.

Figure \(\PageIndex{3}\): "Saltwater intrusion" by Brown and Caldwell, released to Monterey County Water Resources Agency is in the public domain. View a detailed description of this image.

Figure 18.5.4 exhibits groundwater elevation in the Salinas Valley shallow aquifer in 2019. The contours show the groundwater elevation below ground surface (bgs), there are large concerns around saltwater intrusion being so close to the surface and at the same time, the inward gradient, or downslope subsurface flow towards the lower (ie deeper) freshwater table.

Figure \(\PageIndex{4}\): "Salinas Valley Basin groundwater elevation" by Monterey County Water Resources Agency is in the public domain. View a detailed description of this image.

In response to the threat of saltwater intrusion, California has implemented various strategies to manage and mitigate its impacts. These include groundwater management practices, such as groundwater recharge and conjunctive use, to replenish depleted aquifers and reduce reliance on groundwater extraction. Coastal protection measures, such as seawater barriers and managed aquifer recharge projects, aim to prevent or minimize the intrusion of saline water into freshwater sources. In some cases, like in Los Angeles, “guardian wells” have been drilled to pump the saltwater out and then transport it via pipelines back out to sea, thus trying to stop saltwater from intruding before it even has a chance to progress further inland. Figure 18.5.5 shows a barrier well pumping saltwater away to be put back into the sea. Meanwhile, a freshwater well continues to operate without interference from intruding saltwater. In other cases, like in Santa Barbara, freshwater has also been pumped into guardian wells to maintain a “barrier” of freshwater that holds back the intruding saltwater.

Figure \(\PageIndex{5}\): "Conceptual diagram of saltwater pumping by a barrier well" by Shinichi Ozaki, Christel Abi Akl, Tatsuya Nagino, and Yshinari Hiroshiro is licensed under CC BY-NC 4.0. View a detailed description of this image.

Figure 18.5.6 exhibits conceptual figures of Los Angeles seawater intrusion: A. In Los Angeles in the 1800s, the potentiometric surface of groundwater (the levels to which pressure in the aquifer would make water rise in cased wells) was above sea level causing net seaward freshwater discharge from the aquifer. B. Potentiometric surface drops below sea level due to pumping as early as the 1920’s causing landward seawater intrusion into the aquifer. C. In the 1950’s sets of closely spaced wells were installed to inject high-quality fresh water into aquifers – ideally to stem the flow of seawater into coastal aquifers.

Figure \(\PageIndex{6}\): "Conceptual diagram of saltwater pumping by a barrier well" by the United States Geological Survey is in the public domain. View a detailed description of this image.

Furthermore, land-use planning and zoning regulations play a critical role in minimizing the risks associated with saltwater intrusion by guiding development away from vulnerable coastal areas and promoting sustainable water management practices. Collaboration between government agencies, water districts, stakeholders, and the public is essential to effectively address saltwater intrusion hazards and ensure the long-term sustainability of California's coastal and groundwater resources.

Saltwater intrusion poses significant hazards to California's coastal regions and groundwater resources, particularly in areas where freshwater aquifers interface with the Pacific Ocean. Addressing this challenge requires a comprehensive approach that integrates groundwater management, coastal protection measures, land-use planning, and stakeholder collaboration. By implementing proactive strategies and sustainable water management practices, California can mitigate the impacts of saltwater intrusion and safeguard its valuable water resources.

Activity - Assessing Saltwater Contamination \(\PageIndex{1}\)

Land Subsidence: Sinking Ground, Rise Problems

Land subsidence, the gradual sinking of the Earth’s surface, occurs when groundwater extraction compacts unconsolidated sediments, especially in regions like the Great Valley Province, where extensive groundwater pumping supports agriculture. As water is withdrawn from aquifers, the loss of pore pressure causes sediment layers to compress, leading to a decline in surface elevation. The effects of subsidence are most visible in the San Joaquin Valley, where parts of the valley floor have sunk by several feet over the past century (Figure 18.5.7).

This phenomenon not only damages infrastructure, such as roads and canals, but also reduces the storage capacity of aquifers, making it harder to replenish water supplies. Subsidence can also worsen saltwater intrusion by lowering the land surface and altering natural water flow patterns, particularly in coastal areas. While tectonic activity also causes subsidence in some parts of California, such as Humboldt Bay, groundwater-induced subsidence in regions like the Central Valley remains a major concern due to its scale and long-term impacts on water management and infrastructure.

One of the primary geological causes of land subsidence in California is the over-extraction of groundwater from underlying aquifers. As groundwater is pumped from aquifers to meet agricultural, municipal, and industrial water demands, the reduction in pore pressure within the aquifer leads to the compaction of overlying sedimentary layers. This compaction results in the gradual sinking of the land surface, a phenomenon known as "subsidence."

In regions with extensive groundwater pumping, such as the Central Valley and parts of Southern California, land subsidence has become a significant concern. The Central Valley, one of the most productive agricultural regions in the United States, relies heavily on groundwater for irrigation. As a result, prolonged and intensive groundwater extraction has led to significant land subsidence in parts of the Central Valley, particularly in the San Joaquin Valley, where subsidence rates have exceeded several inches per year in some areas.

The geological processes underlying land subsidence in California are also influenced by the composition and structure of underlying sedimentary deposits. In areas with unconsolidated sediments, such as alluvial plains and coastal regions, groundwater extraction can lead to rapid compaction and subsidence, most often attributed to stratigraphic intervals of fine grained sediments, such as silts and clays. Additionally, natural processes such as tectonic activity and seismic events can induce localized subsidence, particularly in regions with active faulting and seismicity. However, these often pale in comparison to induced subsidence due to groundwater extraction.

The impacts of land subsidence in California are diverse and can have far-reaching consequences for infrastructure, water management, and environmental sustainability. Subsidence-related infrastructure damage includes the cracking and deformation of roads, bridges, buildings, and other structures, posing risks to public safety and increasing maintenance costs.

Furthermore, land subsidence exacerbates challenges related to water management and sustainability by reducing the storage capacity of aquifers, altering surface water drainage patterns, and exacerbating flooding risks. Subsidence-induced changes in land elevation can also exacerbate seawater intrusion in coastal aquifers, leading to saltwater contamination of freshwater resources.

In response to the challenges posed by land subsidence, California has implemented various strategies and initiatives aimed at managing groundwater resources, reducing groundwater overdraft, and mitigating subsidence-related impacts. These include the implementation of groundwater management plans, regulation of groundwater extraction, promotion of water conservation and efficiency measures, and investment in alternative water sources such as recycled water and stormwater capture. Figure 18.5.7 shows basin prioritization based on the California Sustainable Management Act (SGMA) of 2014.

Figure \(\PageIndex{7}\): Basin Prioritization of 2019. “California Sustainable Groundwater Management Act (SGMA) Basin Prioritization” by Cole Heap from data in California Open Data Portal is licensed under CC BY-NC 4.0. Constructed in a GIS. View a detailed description of this image.

Collaborative efforts among government agencies, water districts, stakeholders, and the public are essential for effectively addressing the complex challenges of land subsidence in California. By implementing proactive measures, promoting sustainable groundwater management practices, and fostering innovation, California can mitigate the impacts of land subsidence while ensuring the long-term resilience of its infrastructure, water resources, and communities.

Surface Water Depletion and Drought: A Worsening Challenge

Surface water depletion, the reduction in the availability of surface water resources due to various factors such as over-extraction, climate change, and land-use practices, presents significant challenges to water management and sustainability in California. As one of the most populous and agriculturally productive states in the United States, California relies heavily on surface water sources for irrigation, municipal supply, and environmental needs. However, increased demand, coupled with changing environmental conditions, has led to the depletion of surface water supplies in various regions across the state.

One of the most common areas affected by surface water depletion in California is the Sacramento-San Joaquin Delta, a critical hub for water conveyance and ecosystem support. The delta serves as a primary source of water for agriculture, urban areas, and environmental conservation in California. However, decades of intensive water diversions for agricultural irrigation and municipal supply have significantly reduced freshwater flows into the delta, leading to habitat degradation, saltwater intrusion, and ecosystem decline.

In addition to the Sacramento-San Joaquin Delta, surface water depletion is also prevalent in other regions of California, particularly in the Central Valley and Southern California. The Central Valley, often referred to as the "breadbasket of the world,” relies heavily on surface water for irrigating vast agricultural lands just as well as groundwater aquifers as previously stated. However, groundwater overdraft and competing water demands have strained surface water resources as denizens have fallen back to surface water use to compensation for the lack in groundwater in some areas.

Southern California, including metropolitan areas such as Los Angeles, San Diego, and Orange County, faces challenges related to surface water depletion due to high population density and water demand. The region relies on imported water from distant sources, including the Colorado River and Northern California, to meet its water needs. However, prolonged droughts, regulatory constraints, and competing interests for water allocation have exacerbated the strain on surface water supplies in Southern California.

Climate change further exacerbates surface water depletion in California by altering precipitation patterns, reducing snowpack levels, and increasing the frequency and severity of droughts. Decreased snowpack in the Sierra Nevada Mountains, a vital source of surface water replenishment during the dry season, reduces runoff into rivers and reservoirs, leading to decreased water availability for downstream users.

To address the challenges of surface water depletion, California has implemented various water management strategies and policies aimed at enhancing water conservation, improving water efficiency, and promoting sustainable water use practices. These include investments in water recycling and reuse, implementation of water conservation measures, development of regional water management plans, and enhancement of groundwater recharge and storage projects.

Furthermore, collaborative efforts among stakeholders, including water agencies, agricultural producers, environmental organizations, and local communities, are essential for effectively managing surface water resources and ensuring the long-term sustainability of California's water supply.

Surface water depletion poses significant challenges to water management and sustainability in California. However, by implementing proactive water management strategies, investing in water infrastructure, and fostering collaboration among stakeholders, California can address the impacts of surface water depletion and ensure the resilience of its water resources in the face of changing environmental conditions.

Drought and Climate Change

Drought, a recurrent phenomenon characterized by prolonged periods of below-average precipitation and water scarcity, poses significant challenges to water resources, agriculture, ecosystems, and communities in California. The state's Mediterranean climate, characterized by wet winters and dry summers, makes it inherently susceptible to drought conditions. However, the increasing frequency, intensity, and duration of drought events in California are exacerbated by climate change, leading to heightened concerns about water availability, ecosystem health, and socioeconomic impacts (see Figure 18.5.8).

Climate change plays a pivotal role in exacerbating drought conditions in California by altering precipitation patterns, increasing temperatures, and influencing atmospheric circulation patterns. Rising temperatures contribute to increased evaporation rates, leading to greater water loss from soil and vegetation. Additionally, changes in atmospheric circulation patterns, such as the weakening of the Pacific storm track and the expansion of subtropical high-pressure systems, can result in reduced winter precipitation and snowpack levels, further exacerbating drought conditions.

Figure \(\PageIndex{8}\): "Dry lakebed in California" by National Oceanic and Atmospheric Administration is in the public domain. View a detailed description of this image.

The impacts of drought and climate change in California are multifaceted and far-reaching, affecting various sectors of the economy, environment, and society. Agriculture, which accounts for 40% of California's water consumption, is particularly vulnerable to drought-induced water shortages. Reduced water availability for irrigation can lead to crop failures, decreased agricultural productivity, and economic losses for farmers and rural communities.

Ecosystems and natural habitats in California also face severe challenges during drought events, as diminished water availability disrupts aquatic ecosystems, reduces habitat quality, and threatens the survival of native species. Wetlands, rivers, and streams experience reduced flows, leading to habitat fragmentation and alterations in water temperature and quality, which can negatively impact fish populations and other aquatic organisms. As shown in Figure 18.5.9, the difference between a wet year versus a dry year near Lake Oroville.

Figure \(\PageIndex{9}\): Satellite images-Lake Oroville by the United States Geological Survey is in the public domain. View a detailed description of this image.

Furthermore, drought conditions exacerbate wildfire risk and intensity in California, as dry vegetation becomes more susceptible to ignition and rapid fire spread. Wildfires not only pose direct threats to human life and property but also have long-term ecological consequences, including habitat destruction, soil erosion, and loss of biodiversity.

Figure 18.6.10 is an animated map of the progression of the historic drought from 2012 to 2014. These maps span December 31, 2013, to July 29, 2014. This animation highlights the rapid spread of extreme and exceptional drought conditions across over 75% of California in this timeframe. This drought has been classified as the worst drought California has experienced in 1,200 years!

Figure \(\PageIndex{10}\): "Progression of the 2012-2014 historic California drought" by the National Drought Mitigation Center is in the public domain. View a detailed description of this image.

In response to the escalating challenges posed by drought and climate change, California has implemented various strategies and initiatives aimed at enhancing drought preparedness, water conservation, and climate resilience. These include investments in water infrastructure, such as water recycling and desalination plants, implementation of water conservation measures, development of drought contingency plans, and promotion of sustainable land and water management practices.

Additionally, California has adopted ambitious climate mitigation and adaptation goals, including the reduction of greenhouse gas emissions and the promotion of renewable energy sources, to address the underlying drivers of climate change and build resilience to its impacts.

Collaborative efforts among government agencies, stakeholders, and communities are essential for effectively addressing the complex challenges of drought and climate change in California. By implementing adaptive strategies, fostering innovation, and promoting sustainable water and land management practices, California can enhance its resilience to drought and climate variability while safeguarding the long-term sustainability of its water resources, ecosystems, and communities.

Activity - Water in the Atmosphere \(\PageIndex{1}\)

Flooding: Natural Hazards and Infrastructure Challenges

Flooding, the overflow of water onto normally dry land, is a recurrent natural hazard in California, influenced by various geologic factors and exacerbated by human activities and climate change. Understanding the geological factors that influence flooding and identifying vulnerable regions are essential for effective flood risk management and resilience building efforts in the state.

Geological factors that contribute to flooding in California include topography, soil characteristics, land use patterns, and proximity to water bodies. California's diverse topography, characterized by mountains, valleys, and coastal plains, plays a significant role in modulating the distribution and intensity of rainfall-runoff processes, influencing flood patterns across the state. Areas located in low-lying coastal plains, river valleys, and floodplains are particularly susceptible to flooding due to their proximity to water bodies and their relatively flat terrain, which facilitates the accumulation of water during heavy rainfall events.

Figure 18.5.11 shows flooding in the Sacramento Valley after weeks of storms. Clear skies on Jan 17, 2023, allowed NASA's Terra satellite to capture flooding in California's Sacramento Valley using Moderate Resolution Imagery Spectroradiometer (MODIS). This satellite imagery shows flooding near Sacramento and the Sutter Buttes in dark blue. The Sutter Buttes look like an island in the north central portion of the satellite image. Flooding intensified with wider rivers and increased snowpack in the mountains. Another storm struck after a brief sunny period, worsening flooding.

Figure \(\PageIndex{11}\): "Flooding in California’s Sacramento Valley (MODIS)" by the National Aeronautics and Space Administration is in the public domain. View a detailed description of this image.

Soil characteristics also play a crucial role in determining flood susceptibility, with factors such as soil permeability, compaction, and saturation capacity influencing the rate of infiltration and surface runoff. Impermeable soils, such as clay-rich or heavily compacted soils, inhibit water infiltration and promote surface runoff, increasing the risk of flash flooding and urban flooding in developed areas with extensive impervious surfaces.

Land use patterns and human activities further exacerbate flooding risks in California. Urbanization, deforestation, and alteration of natural drainage patterns disrupt the natural hydrological cycle, increasing surface runoff and reducing infiltration rates, leading to higher flood frequencies and intensities. Additionally, the construction of levees, dams, and other flood control infrastructure can alter riverine and coastal ecosystems, exacerbating downstream flooding risks and increasing vulnerability to flood hazards.

Climate change, characterized by rising temperatures, altered precipitation patterns, and increased frequency of extreme weather events, is amplifying the impacts of flooding in California. Changing climate conditions contribute to more frequent and intense rainfall events, leading to increased runoff and higher flood risks, particularly in urbanized areas with limited natural drainage capacity.

Several regions in California are particularly prone to flooding due to a combination of geological, hydrological, and anthropogenic factors. Coastal areas, including low-lying coastal plains and estuarine environments, are vulnerable to flooding from storm surges, high tides, and sea-level rise, which can inundate coastal communities and infrastructure (see California’s Coastline).

Riverine floodplains, such as those along the Sacramento and San Joaquin Rivers in the Central Valley, are also susceptible to flooding, especially during periods of heavy rainfall and snowmelt. These floodplains serve as vital agricultural areas but are prone to inundation, posing risks to crops, livestock, and rural communities. A great example of this is in the activity below where one can use the slider to compare a dry Tulare Lake bed against a refilled Tulare Lake bed southwest of Corcoran, CA in the Central Valley. The resurgence of Tulare Lake flooded much of the farm operations outside of Corcoran.

Activity - The Return of Lake Tulare \(\PageIndex{1}\)

Urbanized areas, particularly in densely populated regions such as the San Francisco Bay Area, Los Angeles Basin, and San Diego metropolitan area, face significant flood risks due to extensive impervious surfaces, altered drainage patterns, and proximity to water bodies. Flash flooding, urban flooding, and riverine flooding are common in these urban areas during intense rainfall events.

In response to these challenges, California has implemented various flood risk management strategies, including levee systems and wetland restoration projects, such as the Napa River Flood Protection Project. One particularly innovative solution involves the use of permeable pavement, which is designed to reduce urban runoff by allowing rainwater to infiltrate the ground rather than flowing over impermeable surfaces into storm drains.

In Figure 18.6.12, the sign reads,

“You are standing on N. School Street which was paved with permeable pavement. Rain that falls on normal pavement rushes downhill through drains, carrying pollutants from the street straight into Wolf Creek where it leaves the community within hours. The permeable pavement on N. School Street allows rainfall to penetrate down to the soil. It is then filtered through the soil and slowly released into the groundwater where it can be used by people and plants downslope.”

The technical process of permeable pavement works by allowing water to permeate through minuscule gaps in the concrete, initiating a process of natural filtration. As water seeps through the pavement into the underlying soil, it undergoes purification, removing contaminants and contributing to the recharge of groundwater reservoirs. This replenishment is vital for sustaining aquifers that nourish local flora and fauna, including Wolf Creek during the dry summer and fall seasons, ensuring a consistent water supply for ecosystems and communities.

Figure \(\PageIndex{12}\): Permeable Pavement sign in Grass Valley, California by Allison Jones is licensed under CC BY-NC 4.0. View a detailed description of this image.

Furthermore, any excess water not absorbed locally flows downstream, eventually reaching larger water bodies such as the Bear River, and ultimately contributing to the Sacramento-San Joaquin Delta. This flow supports critical ecosystems, including salmon rearing habitats within the delta, before the water finally reaches the Pacific Ocean. The installation of 4,500 square feet of permeable pavement at N. School Street demonstrates the potential of this technology to capture and store up to 15,000 gallons of water during a single storm event, showcasing its practical benefits in urban water management.

Permeable pavement, therefore, represents a sustainable water management practice that not only reduces urban flooding but also enhances groundwater recharge and supports local ecosystems. By integrating permeable pavement and other flood resilience strategies, California can mitigate the risks associated with increased urban runoff while promoting sustainable development in flood-prone areas.

Protecting California's Water Resources

Maintaining the quality of California’s water is a complex and ongoing challenge, influenced by both natural geological processes and human activities. From the selenium contamination in Kesterson Reservoir to the persistent threat of PFAS, nitrate, and other pollutants, the state’s water systems are continually at risk of degradation. Addressing these issues requires a multi-faceted approach that integrates careful regulation, modern infrastructure, and sustainable water management practices. With the increasing pressure from agricultural runoff, urbanization, and climate change, California must remain vigilant in monitoring and mitigating water quality threats. By prioritizing both surface water and groundwater protection and implementing strategies like stricter Maximum Contaminant Levels (MCLs) and innovative technologies, the state can continue to safeguard public health and the environment for future generations. Collaborative efforts among scientists, policymakers, and local communities will be critical to ensuring that California’s water resources remain resilient and sustainable amidst these ongoing challenges.

References

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