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Federal water data

County water report

Tuolumne County Water Report

Drinking-water compliance, watershed health, monitoring records, and river conditions for Tuolumne County, California.

Water grade

F

Water score

34.0

State rank

#40

of 58

Health violations

32

EPA SDWIS, 5-year lookback

Watershed impaired

13.9%

115 water bodies assessed

Monitoring sites

17

160 recent measurements

Live streamflow

277%

TUOLUMNE R BL EARLY INTAKE NR MATHER CA

Water at a glance

Key Water Indicators for Tuolumne County

EPA SDWIS

Safety Grade

F

Score: 34.0 / 100

EPA SDWIS

Active Violations

32

5-year health-based lookback

EPA ATTAINS

Watershed Health

14% impaired

115 bodies assessed

USGS NWIS

Streamflow Snapshot

277% of mean

TUOLUMNE R BL EARLY INTAKE NR MATHER CA

EPA WQP

Monitoring Sites

17

160 recent readings

Source: EPA SDWIS · Safe Drinking Water Information System

Drinking Water Compliance

Compliance grade

F

Based on EPA SDWIS compliance history.

Water score

Higher scores indicate cleaner recent compliance records.

34.0/100

Health violations

32

Health-based violations

Violations per 100K served

50.0

Population-normalized SDWIS rate

Editorial analysis

Understanding Tuolumne County’s Water

Drinking Water Quality Overview

EPA SDWIS

Tuolumne County's water systems carry a failing grade, scoring 34.0 out of 100. Over the past five years, EPA SDWIS records 32 health-based violations — a pattern that public water utilities are required to disclose and correct.

Watershed Conditions

EPA ATTAINS

Under the Clean Water Act §303(d), EPA ATTAINS tracks whether waterways meet quality standards for drinking, recreation, and aquatic life (reporting cycle: 2022). A notable 13.9% of assessed waterways carry an impairment designation (16 of 115 water bodies) across Tuolumne County's watersheds. The leading impairment causes are pathogens and mercury. Impairment does not mean tap water is unsafe — it measures ambient waterway conditions upstream of treatment, not finished drinking water.

River & Streamflow Status

USGS NWIS

USGS NWIS gauge data (as of 2026-05-14T12:00:00.000-07:00) puts TUOLUMNE R BL EARLY INTAKE at 1.6k cfs — running significantly above its long-term average at 277% of mean flow. Streamflow is a leading indicator of drought stress, sediment load, and dilution capacity: low flows concentrate pollutants and warm water temperatures, stressing aquatic life and, in surface-water-dependent systems, the source water quality for treatment plants.

Monitoring Network

EPA WQP

EPA's Water Quality Portal (WQP) aggregates monitoring data from federal, state, and tribal agencies. Tuolumne County has moderate coverage with 17 active monitoring sites with 160 recent measurements on record. Predominant monitoring categories include physical and nutrient. More monitoring sites generally indicate greater scientific attention to local water conditions — and provide the baseline data that regulators use to set future impairment listings.

Editorial advisory

What the data suggests for Tuolumne County

Water Verdict

Tuolumne County receives a poor water quality assessment with a grade of F and a score of 34.0 out of 100. The water supply has documented quality issues. Residents are strongly encouraged to use filtered or bottled water for drinking and to stay informed about utility improvement plans.

Violation Context

Tuolumne County has recorded 32 health-based violations, indicating multiple instances where federal contaminant limits or treatment requirements were not met. At 50.0 violations per 100,000 people served, this rate is high and signals significant water quality management issues.

Consumer Guidance

Tuolumne County has a Grade F compliance record with 32 health-based violations — among the highest levels in the country. Tuolumne County's drinking-water compliance score is 34.0 out of 100. The violation rate for Tuolumne County is 50.0 per 100,000 people served. Residents are strongly advised to use a certified NSF 58 reverse-osmosis filter or bottled water for all drinking and cooking until violations are corrected. Contacting the California Department of Environmental Quality or Health can expedite utility compliance action. Pathogens is the leading impairment cause in Tuolumne County's watershed. With 17 active water-quality monitoring sites in Tuolumne County, data coverage is strong. A pipeline streamflow snapshot from the TUOLUMNE R BL EARLY INTAKE gauge is also available on this page.

Regional Context

Tuolumne County has poorer water quality than the average county in California. Its water score is 10.6 points lower than the state average, suggesting more challenges with contamination control or infrastructure than neighboring counties.

Advisory text summarizes county-level public records and is not a replacement for your utility's current Consumer Confidence Report or direct local notices.

Contaminants & Resources

Key issues flagged in Tuolumne County's water environment

Watershed Impairment Causes (EPA ATTAINS)

  • 1

    Pathogens

    Impairment cause per EPA Clean Water Act §303(d) assessment

  • 2

    Mercury

    Impairment cause per EPA Clean Water Act §303(d) assessment

  • 3

    pH imbalance

    Impairment cause per EPA Clean Water Act §303(d) assessment

Source: EPA ATTAINS · Reporting cycle 2022

Official EPA Resources for Tuolumne County

EPA How's My Waterway

Interactive watershed health data for Tuolumne County

EPA ECHO: Drinking Water Systems

Search water utilities and violation records for Tuolumne County

Consumer Confidence Reports (CA)

Annual water quality reports from utilities in California

Clean Water Act §303(d)

Watershed Health

Impaired Water Bodies

13.9%

16 of 115 assessed

Some impairment

Top Impairment Causes

  • 1

    PATHOGENS

  • 2

    MERCURY

  • 3

    PH

Source: EPA ATTAINS · Reporting cycle 2022

Impairment is determined under the Clean Water Act §303(d): a water body is impaired when it fails to meet state-defined quality standards for designated uses (drinking, recreation, aquatic life). Assessment coverage varies by state; counties without assessed water bodies are not shown.

Past 5 years

Water Quality Monitoring

Monitoring Sites

17

Active in the past 5 years

Measurements Recorded

160

160 total readings

Most Measured

  • Physical
  • Nutrient
  • Inorganics, Minor, Metals

Categories measured most frequently

Data from the EPA Water Quality Portal (WQP), aggregating monitoring records from federal, state, and tribal sources. Each measurement represents a single sample analyzed for a specific characteristic (e.g., E. coli, pH, dissolved oxygen, nitrogen).

Live USGS Streamgage

River & Stream Conditions

Current Discharge

1,620cfs

May 14, 7:00 PM UTC

vs Long-Term Average

277%

Well above typical

Primary Streamgage

TUOLUMNE R BL EARLY INTAKE NR MATHER CA

USGS site
11276900
Drainage area
487 sq mi
Long-term mean
585 cfs

One representative streamgage (the one with the largest drainage area in the county). Many counties have multiple gauges; this view summarizes the primary one. The long-term mean is the full-record annual average; the percent-of-typical value compares the latest reading against that average.

Free tool

Estimate Your Water Costs

Water Cost Estimate

3

3 people  ·  ~225 gal/day

Annual Total

$558

Monthly

$47

Water Bill

$558/yr

Filter Cost

$0/yr

Safety Grade for Tuolumne County:C — Moderate

Some violations or watershed impairment detected.

Estimates use the national average residential water rate ($0.0068/gal, EPA/AWWA 2023) and EPA WaterSense per-person consumption baseline (75 gal/person/day). Actual bills vary by utility, usage tier, and local infrastructure fees. For informational purposes only.

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Frequently Asked Questions

What is the water quality in Tuolumne County, California?
Tuolumne County, California has a drinking-water quality grade of F with a score of 34.0/100, based on EPA SDWIS compliance data. The county has 32 health-based drinking water violations over the past 5 years. Watershed health, monitoring records, and streamflow snapshots are reported separately on this page.
Are there any water violations in Tuolumne County?
Tuolumne County has 32 health-based drinking water violations recorded by the EPA over the past 5 years. Health-based violations indicate instances where contaminant levels exceeded EPA Maximum Contaminant Levels (MCLs). Violations may have been resolved — check with your local water utility for current status.
How healthy are the watersheds in Tuolumne County?
EPA ATTAINS assessments under Clean Water Act §303(d) indicate 13.9% of Tuolumne County's 115 assessed water bodies are classified as impaired (16 impaired). The top reported causes are PATHOGENS, MERCURY, PH. Impairment means the water body fails to meet state quality standards for at least one designated use — drinking water source, recreation, aquatic life, or fish consumption. Note: watershed impairment doesn't always translate to tap-water issues; treatment plants can remove most regulated contaminants.
How much water-quality monitoring happens in Tuolumne County?
EPA's Water Quality Portal records 160 measurements from 17 monitoring sites in Tuolumne County over the past five years. The most frequently measured characteristic groups are Physical, Nutrient, Inorganics, Minor, Metals. Each measurement is a single sample analyzed for one characteristic (E. coli, pH, dissolved oxygen, etc.). High monitoring density means more scientific evidence behind any reported signal — it does not by itself indicate water quality.
What's happening with rivers in Tuolumne County right now?
Tuolumne County's primary USGS streamgage on the TUOLUMNE R BL EARLY INTAKE has a pipeline snapshot of 1,620 cubic feet per second — 277% of the long-term mean of 584.66 cfs. This is well above typical — often a signal of recent precipitation or storm runoff. For the latest gauge feed, visit waterdata.usgs.gov.
How does Tuolumne County water compare to the California average?
Tuolumne County's SDWIS water quality score of 34.0/100 is lower than the California state average of 44.6. The average water quality grade across California is D, based on data from 58 counties with available SDWIS data.
Is tap water safe to drink in Tuolumne County?
Based on EPA SDWIS data, Tuolumne County has a water quality grade of F (34.0/100). This indicates below-average compliance with significant violations. Residents may want to consider home water filtration or independent testing. The grade speaks to the public water system, not the watershed — for watershed-level concerns, see the Watershed Health zone. For the most up-to-date information, contact your local water utility or review your Consumer Confidence Report (CCR).
Why does Tuolumne County have so many water violations?
Tuolumne County has 32 health-based drinking water violations on record from the EPA SDWIS database. A higher violation count can result from aging infrastructure, underfunded water utilities, agricultural runoff contamination, or industrial pollution. Counties with more water systems may also see more violations simply due to scale. Residents concerned about water quality should consider independent water testing and home filtration systems.
How does Tuolumne County rank for water quality in California?
Tuolumne County ranks #40 out of 58 counties in California by SDWIS water quality score (1 = best). With a score of 34.0/100, it falls in the bottom third of counties statewide. The ranking reflects EPA SDWIS compliance only — not watershed impairment, monitoring density, or streamflow, which are tracked separately on this page.

Counties with Similar Water Quality

Napa County

California

F

33.4

Water score

Amador County

California

F

34.8

Water score

Mendocino County

California

F

35.2

Water score

Data Sources

Drinking-water compliance data from the EPA Safe Drinking Water Information System (SDWIS) via the ECHO enforcement database. Scores reflect compliance history and health-based violation counts.

Watershed health and impaired-waterway data from the EPA ATTAINS Clean Water Act §303(d) assessments, state-reported and EPA-finalized.

Water-quality monitoring counts from the EPA Water Quality Portal (WQP), federated USGS, EPA, and state agency sampling records over a rolling 5-year window.

Live streamflow from the USGS National Water Information System (NWIS), continuous discharge measurements from the largest-drainage gauge in each county, compared against the full-record long-term annual mean.

Disclaimer: This data is informational only. It is not health, legal, or professional advice. For concerns about your specific water supply, contact your local water utility.

By Evan Brooks, Data EditorUpdated Reviewed by Evan Brooks, Data Editor

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