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Federal water data

County water report

Lewis and Clark County Water Report

Drinking-water compliance, watershed health, monitoring records, and river conditions for Lewis and Clark County, Montana.

Water grade

F

Water score

26.1

State rank

#39

of 55

Health violations

40

EPA SDWIS, 5-year lookback

Watershed impaired

0.0%

59 water bodies assessed

Monitoring sites

97

9,897 recent measurements

Live streamflow

62%

Missouri River bl Holter Dam nr Wolf Cr MT

Water at a glance

Key Water Indicators for Lewis and Clark County

EPA SDWIS

Safety Grade

F

Score: 26.1 / 100

EPA SDWIS

Active Violations

40

5-year health-based lookback

EPA ATTAINS

Watershed Health

0% impaired

59 bodies assessed

USGS NWIS

Streamflow Snapshot

62% of mean

Missouri River bl Holter Dam nr Wolf Cr MT

EPA WQP

Monitoring Sites

97

9,897 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.

26.1/100

Health violations

40

Health-based violations

Violations per 100K served

84.7

Population-normalized SDWIS rate

Editorial analysis

Understanding Lewis and Clark County’s Water

Drinking Water Quality Overview

EPA SDWIS

Lewis and Clark County's water systems carry a failing grade, scoring 26.1 out of 100. Over the past five years, EPA SDWIS records 40 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). None of the assessed waterways are listed as impaired (0 of 59 water bodies) across Lewis and Clark County's watersheds. 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 2026-05-14T12:45:00.000-06:00) puts Missouri River bl Holter Dam at 3.3k cfs — running somewhat below its historical average at 62% of mean. 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. Lewis and Clark County has extensive coverage with 97 active monitoring sites with 9,897 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 Lewis and Clark County

Water Verdict

Lewis and Clark County receives a poor water quality assessment with a grade of F and a score of 26.1 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

Lewis and Clark County has recorded 40 health-based violations, indicating multiple instances where federal contaminant limits or treatment requirements were not met. At 84.7 violations per 100,000 people served, this rate is high and signals significant water quality management issues.

Consumer Guidance

Lewis and Clark County has a Grade F compliance record with 40 health-based violations — among the highest levels in the country. Lewis and Clark County's drinking-water compliance score is 26.1 out of 100. The violation rate for Lewis and Clark County is 84.7 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 Montana Department of Environmental Quality or Health can expedite utility compliance action. With 97 active water-quality monitoring sites in Lewis and Clark County, data coverage is strong. A pipeline streamflow snapshot from the Missouri River bl Holter Dam gauge is also available on this page.

Regional Context

Lewis and Clark County has poorer water quality than the average county in Montana. Its water score is 22.8 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 Lewis and Clark County's water environment

Official EPA Resources for Lewis and Clark County

EPA How's My Waterway

Interactive watershed health data for Lewis and Clark County

EPA ECHO: Drinking Water Systems

Search water utilities and violation records for Lewis and Clark County

Consumer Confidence Reports (MT)

Annual water quality reports from utilities in Montana

Clean Water Act §303(d)

Watershed Health

Impaired Water Bodies

0.0%

0 of 59 assessed

Mostly healthy

Top Impairment Causes

No specific impairment causes reported for the assessed water bodies in this county.

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

97

Active in the past 5 years

Measurements Recorded

9.9K

9,897 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

3,260cfs

May 14, 6:45 PM UTC

vs Long-Term Average

62%

Below typical

Primary Streamgage

Missouri River bl Holter Dam nr Wolf Cr MT

USGS site
06066500
Drainage area
16,924 sq mi
Long-term mean
5,250 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.

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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 Lewis and Clark 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 Lewis and Clark County, Montana?
Lewis and Clark County, Montana has a drinking-water quality grade of F with a score of 26.1/100, based on EPA SDWIS compliance data. The county has 40 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 Lewis and Clark County?
Lewis and Clark County has 40 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 Lewis and Clark County?
EPA ATTAINS assessments under Clean Water Act §303(d) indicate 0.0% of Lewis and Clark County's 59 assessed water bodies are classified as impaired (0 impaired). 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 Lewis and Clark County?
EPA's Water Quality Portal records 9,897 measurements from 97 monitoring sites in Lewis and Clark 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 Lewis and Clark County right now?
Lewis and Clark County's primary USGS streamgage on the Missouri River bl Holter Dam has a pipeline snapshot of 3,260 cubic feet per second — 62% of the long-term mean of 5,249.78 cfs. Flow is within typical range for this gauge. For the latest gauge feed, visit waterdata.usgs.gov.
How does Lewis and Clark County water compare to the Montana average?
Lewis and Clark County's SDWIS water quality score of 26.1/100 is lower than the Montana state average of 48.9. The average water quality grade across Montana is D, based on data from 55 counties with available SDWIS data.
Is tap water safe to drink in Lewis and Clark County?
Based on EPA SDWIS data, Lewis and Clark County has a water quality grade of F (26.1/100). 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 Lewis and Clark County have so many water violations?
Lewis and Clark County has 40 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 Lewis and Clark County rank for water quality in Montana?
Lewis and Clark County ranks #39 out of 55 counties in Montana by SDWIS water quality score (1 = best). With a score of 26.1/100, 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

Carbon County

Montana

F

26.3

Water score

Hill County

Montana

F

26.5

Water score

Toole County

Montana

F

27.2

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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