Draft California 2026 Integrated Report (303(d) List/305(b) Report)

Supporting Information

Regional Board 1 - North Coast Region

Water Body Name:	Eureka Plain HU, Mad River Slough
Water Body ID:	CAR1100005219990617094843
Water Body Type:	River & Stream


DECISION ID	161532	Region 1
Eureka Plain HU, Mad River Slough

Pollutant:	Indicator Bacteria
Final Listing Decision:	Do Not List on 303(d) list (TMDL required list)
Last Listing Cycle's Final Listing Decision:	Do Not List on 303(d) list (TMDL required list)(2018)
Revision Status	Revised
Impairment from Pollutant or Pollution:	Pollutant

Regional Board Conclusion:	The State Water Board’s Water Quality Control Plan for Inland Surface Waters, Enclosed Bays, and Estuaries of California (the “ISWEBE Plan”) was adopted on August 7, 2018, and contains bacteria water quality objectives applicable to the REC-1 beneficial use. This has resulted in a change from assessing fecal coliform to assessing E. coli in fresh water and Enterococcus in saline water. Fecal coliform lines of evidence used in previous REC-1 beneficial use indicator bacteria decisions in the North Coast Region are not included in the 2026 Integrated Report assessment. In accordance with section 6.1.5.3 of the Listing Policy, data should be representative of the critical timing that the pollutant is expected to impact the waterbody. Lacking constant inputs, indicator bacteria do not persist in the environment for a long period and effects are of relatively short duration. As a result, the historic levels of E. coli in the waterbody may be a poor indicator of current risks to human health, particularly when more recent data are available to sufficiently assess the water quality standard. Historic lines of evidence for data collected prior to October 21, 2012 were evaluated pursuant to these considerations and were not used to assess water quality standards attainment because they do not meet the temporal representation requirements of section 6.1.5.3 of the Listing Policy. This pollutant is being considered for placement on the CWA section 303(d) List under section 3.2 of the Listing Policy. Under section 3.2 a single line of evidence is necessary to assess listing status. Four lines of evidence are available in the administrative record to assess this pollutant. No geometric means are available for this assessment, so statistical threshold values will be assessed. LISCOM SLOUGH has one LOE (321856) and 3 of 4 samples exceed the E. coli STV objective. ROADSIDE DITCH AT FOSTER ROAD has one LOE (21869) and 3 of 3 samples exceed the E. coli STV objective. ROADSIDE DITCH AT JACKSON RANCH ROAD has one LOE (321891) and 3 of 4 samples exceed the E. coli STV objective. UNNAMED SLOUGH AT LANPHERE ROAD NEAR SEIDEL ROAD has one LOE (321895) and 3 of 4 samples exceed the E. coli STV objective. Based on the readily available data and information, the weight of evidence indicates that there is sufficient justification in favor of placing this water segment-pollutant combination on the CWA section 303(d) List. This conclusion is based on the staff findings that: 1. The data used satisfies the data quality requirements of section 6.1.4 of the Policy. 2. The data used satisfies the data quantity requirements of section 6.1.5 of the Policy. 3. LISCOM SLOUGH has one LOE (321856) and 3 of 4 samples exceed the E. coli STV objective; ROADSIDE DITCH AT FOSTER ROAD has one LOE (21869) and 3 of 3 samples exceed the E. coli STV objective; ROADSIDE DITCH AT JACKSON RANCH ROAD has one LOE (321891) and 3 of 4 samples exceed the E. coli STV objective; UNNAMED SLOUGH AT LANPHERE ROAD NEAR SEIDEL ROAD has one LOE (321895) and 3 of 4 samples exceed the E. coli STV objective; and this is insufficient information to apply Table 3.2 of the Listing Policy. 4. Pursuant to section 3.11 of the Listing Policy, no additional data and information are available indicating that standards are not met.

Regional Board Decision Recommendation:	After review of the available data and information, RWQCB staff concludes that the water body-pollutant combination should be placed on the section 303(d) list because applicable water quality standards are exceeded and a pollutant contributes to or causes the problem.

State Board Review of Regional Board Conclusion and Recommendation:

State Board Decision Recommendation:

Line of Evidence (LOE) for Decision ID 161532, Indicator Bacteria	Region 1
Eureka Plain HU, Mad River Slough

LOE ID:

321891

Pollutant:

Escherichia coli (E. coli)

LOE Subgroup:

Pollutant-Water

Matrix:

Water

Fraction:

STV

Beneficial Use:

Water Contact Recreation

Number of Samples:

Number of Exceedances:

Data and Information Type:

PATHOGEN MONITORING

Data Used to Assess Water Quality:

Water Board staff assessed Coastal Pathogen TMDL data for Eureka Plain HU, Mad River Slough to determine beneficial use support and the results are as follows: 3 of the 4 samples exceeded the Statistical Threshold Value (STV) water quality threshold for E. coli. The STV is based on a 10% exceedance rate that is calculated monthly.

Data Reference:

WQ from the RWQCB1 TMDL for the 2026 Integrated Report in Region 1

SWAMP Data:

Non-SWAMP

Water Quality Objective/Criterion:

The bacteria water quality objective to protect the REC-1 beneficial use where the salinity is less than 1 parts per thousand 95 percent or more of the time applies a statistical threshold value to E. coli of 320 cfu/100 mL, not to be exceeded by more than 10 percent of samples within a calendar month (ISWEBE 2018).

Objective/Criterion Reference:

Inland Surface Waters, Enclosed Bays, and Estuaries Plan. Part 1: Trash Provisions; Part 2: Tribal Subsistence Beneficial Uses and Mercury Provisions; Part 3: Bacteria Provisions and Variance Policy

Evaluation Guideline:

Guideline Reference:

Spatial Representation:

The samples were collected at 1 monitoring site(s), station(s): 110DJXNRD (Roadside Ditch at Jackson Ranch Road).

Temporal Representation:

The samples were collected between the days of 2016-12-13 and 2018-01-09 .

Environmental Conditions:

QAPP Information:

QAPP for RWQCB1_RussianRiver

QAPP Information Reference(s):

North Coast Regional Water Quality Control Board - QA Doc (2015) - RWQCB1 Russian River TMDL - 402682

QAPP Information Reference(s):

North Coast Regional Water Quality Control Board - QA Doc (2015) - RWQCB1 Russian River TMDL - 402683

Line of Evidence (LOE) for Decision ID 161532, Indicator Bacteria	Region 1
Eureka Plain HU, Mad River Slough

LOE ID:

321869

Pollutant:

Escherichia coli (E. coli)

LOE Subgroup:

Pollutant-Water

Matrix:

Water

Fraction:

STV

Beneficial Use:

Water Contact Recreation

Number of Samples:

Number of Exceedances:

Data and Information Type:

PATHOGEN MONITORING

Data Used to Assess Water Quality:

Water Board staff assessed Coastal Pathogen TMDL data for Eureka Plain HU, Mad River Slough to determine beneficial use support and the results are as follows: 3 of the 3 samples exceeded the Statistical Threshold Value (STV) water quality threshold for E. coli. The STV is based on a 10% exceedance rate that is calculated monthly.

Data Reference:

WQ from the RWQCB1 TMDL for the 2026 Integrated Report in Region 1

SWAMP Data:

Non-SWAMP

Water Quality Objective/Criterion:

Objective/Criterion Reference:

Inland Surface Waters, Enclosed Bays, and Estuaries Plan. Part 1: Trash Provisions; Part 2: Tribal Subsistence Beneficial Uses and Mercury Provisions; Part 3: Bacteria Provisions and Variance Policy

Evaluation Guideline:

Guideline Reference:

Spatial Representation:

The samples were collected at 1 monitoring site(s), station(s): 110DSEIDL (Roadside Ditch at Foster Road and Seidel Road).

Temporal Representation:

The samples were collected between the days of 2016-12-13 and 2018-01-09 .

Environmental Conditions:

QAPP Information:

QAPP for RWQCB1_RussianRiver

QAPP Information Reference(s):

North Coast Regional Water Quality Control Board - QA Doc (2015) - RWQCB1 Russian River TMDL - 402682

QAPP Information Reference(s):

North Coast Regional Water Quality Control Board - QA Doc (2015) - RWQCB1 Russian River TMDL - 402683

Line of Evidence (LOE) for Decision ID 161532, Indicator Bacteria	Region 1
Eureka Plain HU, Mad River Slough

LOE ID:

321895

Pollutant:

Escherichia coli (E. coli)

LOE Subgroup:

Pollutant-Water

Matrix:

Water

Fraction:

STV

Beneficial Use:

Water Contact Recreation

Number of Samples:

Number of Exceedances:

Data and Information Type:

PATHOGEN MONITORING

Data Used to Assess Water Quality:

Data Reference:

WQ from the RWQCB1 TMDL for the 2026 Integrated Report in Region 1

SWAMP Data:

Non-SWAMP

Water Quality Objective/Criterion:

Objective/Criterion Reference:

Inland Surface Waters, Enclosed Bays, and Estuaries Plan. Part 1: Trash Provisions; Part 2: Tribal Subsistence Beneficial Uses and Mercury Provisions; Part 3: Bacteria Provisions and Variance Policy

Evaluation Guideline:

Guideline Reference:

Spatial Representation:

The samples were collected at 1 monitoring site(s), station(s): 110UNSLPHR (Unnamed Slough at Lanphere Road near Seidel Road).

Temporal Representation:

The samples were collected between the days of 2016-12-13 and 2018-01-09 .

Environmental Conditions:

QAPP Information:

QAPP for RWQCB1_RussianRiver

QAPP Information Reference(s):

North Coast Regional Water Quality Control Board - QA Doc (2015) - RWQCB1 Russian River TMDL - 402682

QAPP Information Reference(s):

North Coast Regional Water Quality Control Board - QA Doc (2015) - RWQCB1 Russian River TMDL - 402683

Line of Evidence (LOE) for Decision ID 161532, Indicator Bacteria	Region 1
Eureka Plain HU, Mad River Slough

LOE ID:

31816

Pollutant:

Fecal Coliform

LOE Subgroup:

Pollutant-Water

Matrix:

Water

Fraction:

None

Beneficial Use:

Shellfish Harvesting

Number of Samples:

203

Number of Exceedances:

Data and Information Type:

PHYSICAL/CHEMICAL MONITORING

Data Used to Assess Water Quality:

Five of the 203 samples collected in the Mad River Slough exceeded the 43MPN/100ml objective.

Data Reference:

Data for bacteria in Humboldt Bay, Jan. 2004-Aug. 2010

SWAMP Data:

Non-SWAMP

Water Quality Objective/Criterion:

Per the Basin Plan (NCRWQCB 2011): At all areas where shellfish may be harvested for human consumption (SHELL), the fecal coliform concentration throughout the water column shall not exceed 43MPN/100ml for a 5-tube decimal dilution test or 49MPN/100ml when a three-tube decimal dilution test is used.

Objective/Criterion Reference:

Water Quality Control Plan (Basin Plan) for the North Coast Region

Evaluation Guideline:

Guideline Reference:

Spatial Representation:

The samples were collected from two sites in Mad River Slough. Those sites were WQ # 27 and T2a.

Temporal Representation:

The samples were collected once a month from 2000 to 2010.

Environmental Conditions:

QAPP Information:

The samples were collected under the Management Plan For Commercial Shellfishing In Humboldt Bay, California. California Department of Public Health.

QAPP Information Reference(s):

Line of Evidence (LOE) for Decision ID 161532, Indicator Bacteria	Region 1
Eureka Plain HU, Mad River Slough

LOE ID:

321856

Pollutant:

Escherichia coli (E. coli)

LOE Subgroup:

Pollutant-Water

Matrix:

Water

Fraction:

STV

Beneficial Use:

Water Contact Recreation

Number of Samples:

Number of Exceedances:

Data and Information Type:

PATHOGEN MONITORING

Data Used to Assess Water Quality:

Data Reference:

WQ from the RWQCB1 TMDL for the 2026 Integrated Report in Region 1

SWAMP Data:

Non-SWAMP

Water Quality Objective/Criterion:

Objective/Criterion Reference:

Inland Surface Waters, Enclosed Bays, and Estuaries Plan. Part 1: Trash Provisions; Part 2: Tribal Subsistence Beneficial Uses and Mercury Provisions; Part 3: Bacteria Provisions and Variance Policy

Evaluation Guideline:

Guideline Reference:

Spatial Representation:

The samples were collected at 1 monitoring site(s), station(s): 110UNSJXN (Liscom Slough at Jackson Ranch Road).

Temporal Representation:

The samples were collected between the days of 2016-12-13 and 2018-01-09 .

Environmental Conditions:

QAPP Information:

QAPP for RWQCB1_RussianRiver

QAPP Information Reference(s):

North Coast Regional Water Quality Control Board - QA Doc (2015) - RWQCB1 Russian River TMDL - 402682

QAPP Information Reference(s):

North Coast Regional Water Quality Control Board - QA Doc (2015) - RWQCB1 Russian River TMDL - 402683


DECISION ID	104327	Region 1
Eureka Plain HU, Mad River Slough

Pollutant:	Oxygen, Dissolved
Final Listing Decision:	Do Not List on 303(d) list (TMDL required list)
Last Listing Cycle's Final Listing Decision:	New Decision
Revision Status	Revised
Impairment from Pollutant or Pollution:	Pollutant

Regional Board Conclusion:	This pollutant is being considered for placement on the CWA section 303(d) List under section 3.2 of the Listing Policy. Under section 3.2 a single line of evidence is necessary to assess listing status. One line of evidence is available in the administrative record to assess this pollutant. Zero of 44 samples exceed the objective for COLD. Based on the readily available data and information, the weight of evidence indicates that there is sufficient justification against placing this water segment-pollutant combination on the CWA section 303(d) List. This conclusion is based on the staff findings that: 1. The data used satisfies the data quality requirements of section 6.1.4 of the Policy. 2. The data used satisfies the data quantity requirements of section 6.1.5 of the Policy. 3. Zero of 44 samples exceed the objective for COLD and this does not exceed the allowable frequency listed in Table 3.2 of the Listing Policy. 4. Pursuant to section 3.11 of the Listing Policy, no additional data and information are available indicating that standards are not met.

Regional Board Decision Recommendation:	After review of the available data and information, RWQCB staff concludes that the water body-pollutant combination should not be placed on the section 303(d) list because applicable water quality standards are not being exceeded.

State Board Review of Regional Board Conclusion and Recommendation:

State Board Decision Recommendation:	After review of this Regional Board decision, SWRCB staff recommend the decision be approved by the State Board.

Line of Evidence (LOE) for Decision ID 104327, Oxygen, Dissolved	Region 1
Eureka Plain HU, Mad River Slough

LOE ID:

120374

Pollutant:

Oxygen, Dissolved

LOE Subgroup:

Pollutant-Water

Matrix:

Water

Fraction:

None

Beneficial Use:

Cold Freshwater Habitat

Number of Samples:

Number of Exceedances:

Data and Information Type:

PHYSICAL/CHEMICAL MONITORING

Data Used to Assess Water Quality:

Water Board staff assessed STORET data for Eureka Plain HU, Mad River Slough to determine beneficial use support and results are as follows: 0 of 44 samples exceeded the water quality standard for Oxygen, Dissolved.

Data Reference:

Water quality data for tribal lands submitted through the US EPA STorage and RETrival (STORET) database for the 2018 listing cycle.

SWAMP Data:

Non-SWAMP

Water Quality Objective/Criterion:

Water Quality Control Plan, North Coast Basin, Chapter III, General Objectives for all Inland Surface Waters, Enclosed Bays and Estuaries states: Dissolved oxygen concentrations shall conform to those limits listed in Section 3.3.5. Section 3.3.5 lists dissolved oxygen concentration limits for COLD beneficial use waters to be a daily minimum of 6.0 mg/L and a 7-day moving average of 8.0 mg/L.

Objective/Criterion Reference:

Water Quality Control Plan (Basin Plan) for the North Coast Region

Evaluation Guideline:

Guideline Reference:

Spatial Representation:

The samples were collected at 1 monitoring site(s) (WIYOT_WQX-MRS-1)

Temporal Representation:

The samples were collected between the dates of 2010-09-02 and 2012-02-23

Environmental Conditions:

QAPP Information:

QAPP not required for federal data

QAPP Information Reference(s):

This is a placeholder reference for data that was collected after QAPP requirements were developed, but exempt from the requirements.


DECISION ID	163017	Region 1
Eureka Plain HU, Mad River Slough

Pollutant:	Specific Conductivity
Final Listing Decision:	Do Not List on 303(d) list (TMDL required list)
Last Listing Cycle's Final Listing Decision:	New Decision
Revision Status	Revised
Impairment from Pollutant or Pollution:	Pollutant

Regional Board Conclusion:	Insufficient information is available to determine beneficial use support for this waterbody-pollutant combination with the statistical power and confidence required by the Listing Policy. Beneficial use support will be reassessed in a future cycle, if more data are available

Regional Board Decision Recommendation:	After review of the available data and information, RWQCB staff concludes that the water body-pollutant combination should not be placed on the section 303(d) list. The readily available data and information is insufficient to determine, with the power and confidence of the Listing Policy, the applicable beneficial use support rating.

State Board Review of Regional Board Conclusion and Recommendation:

State Board Decision Recommendation:	After review of the available data and information, SWRCB staff concludes that the water body-pollutant combination should not be placed on the section 303(d) list because it cannot be determined if applicable water quality standards are not being exceeded.

Line of Evidence (LOE) for Decision ID 163017, Specific Conductivity	Region 1
Eureka Plain HU, Mad River Slough

LOE ID:

329141

Pollutant:

Specific Conductivity

LOE Subgroup:

Pollutant-Water

Matrix:

Water

Fraction:

None

Beneficial Use:

Municipal & Domestic Supply

Number of Samples:

Number of Exceedances:

Data and Information Type:

PHYSICAL/CHEMICAL MONITORING

Data Used to Assess Water Quality:

Water Board staff assessed USGS California Water Science Center data for Eureka Plain HU, Mad River Slough to determine beneficial use support and results are as follows: 2 of 5 samples exceeded the water quality threshold for SpecificConductivity.

Data Reference:

WQX from the NWQMP for the 2026 Integrated Report in Region 1

SWAMP Data:

Non-SWAMP

Water Quality Objective/Criterion:

Waters designated for use as domestic or municipal supply (MUN) shall not contain concentrations of chemical constituents in excess of the limits specified in California Code of Regulations, Title 22, Chapter 15, Division 4, Article 4, Section 64435 (Tables 2 and 3), and Section 64444.5 (Table 5), and listed in Table 3-2 of this Plan (Water Quality Control Plan, North Coast Basin).

Objective/Criterion Reference:

Water Quality Control Plan (Basin Plan) for the North Coast Region

Evaluation Guideline:

California Maximum Contaminant Levels (MCLs) apply to ambient waters under the Basin Plan's 'Chemical Constituents' objective. The California Secondary MCL for specific conductivity is 900 uS/cm.

Guideline Reference:

California Code of Regulations, Title 22, Division 4, Chapter 15. Domestic Water Quality and Monitoring

Spatial Representation:

The samples were collected at 1 monitoring site: USGS-405219124085601 (MAD R SLOUGH NR ARCATA CA).

Temporal Representation:

The samples were collected between the dates of 2016-03-16 and 2018-03-03

Environmental Conditions:

QAPP Information:

QAPP not required for USGS projects

QAPP Information Reference(s):

QAPP for USGS Data Collected during the 2026 Integrated Report Cycle


DECISION ID	104328	Region 1
Eureka Plain HU, Mad River Slough

Pollutant:	pH
Final Listing Decision:	Do Not List on 303(d) list (TMDL required list)
Last Listing Cycle's Final Listing Decision:	New Decision
Revision Status	Revised
Impairment from Pollutant or Pollution:	Pollutant

Regional Board Conclusion:	This pollutant is being considered for placement on the CWA section 303(d) List under section 3.2 of the Listing Policy. Under section 3.2 a single line of evidence is necessary to assess listing status. One line of evidence are available in the administrative record to assess this pollutant. None of the samples exceed the objective for the COLD beneficial use(s). Based on the readily available data and information, the weight of evidence indicates that there is sufficient justification against placing this water segment-pollutant combination on the CWA section 303(d) List. This conclusion is based on the staff findings that: 1. The data used satisfies the data quality requirements of section 6.1.4 of the Policy. 2. The data used satisfies the data quantity requirements of section 6.1.5 of the Policy. 3. None of the samples exceeded the objective and this does not exceed the allowable frequency listed in Table 3.2 of the Listing Policy. 4. Pursuant to section 3.11 of the Listing Policy, no additional data and information are available indicating that standards are not met.

Regional Board Decision Recommendation:	After review of the available data and information, RWQCB staff concludes that the water body-pollutant combination should not be placed on the section 303(d) list because applicable water quality standards are not being exceeded.

State Board Review of Regional Board Conclusion and Recommendation:

State Board Decision Recommendation:	After review of this Regional Board decision, SWRCB staff recommend the decision be approved by the State Board.

Line of Evidence (LOE) for Decision ID 104328, pH	Region 1
Eureka Plain HU, Mad River Slough

LOE ID:

120456

Pollutant:

LOE Subgroup:

Pollutant-Water

Matrix:

Water

Fraction:

None

Beneficial Use:

Cold Freshwater Habitat

Number of Samples:

Number of Exceedances:

Data and Information Type:

PHYSICAL/CHEMICAL MONITORING

Data Used to Assess Water Quality:

Water Board staff assessed STORET data for Eureka Plain HU, Mad River Slough to determine beneficial use support and results are as follows: 0 of 43 samples exceeded the water quality standard for pH.

Data Reference:

Water quality data for tribal lands submitted through the US EPA STorage and RETrival (STORET) database for the 2018 listing cycle.

SWAMP Data:

Non-SWAMP

Water Quality Objective/Criterion:

Water Quality Control Plan, North Coast Basin, Chapter III, General Objectives for all Inland Surface Waters, Enclosed Bays and Estuaries states the following: For waters not mentioned by a specific beneficial use, the pH value shall not be depressed below 6.5 or raised above 8.5.

Objective/Criterion Reference:

Water Quality Control Plan (Basin Plan) for the North Coast Region

Evaluation Guideline:

Guideline Reference:

Spatial Representation:

The samples were collected at 1 monitoring site(s) (WIYOT_WQX-MRS-1)

Temporal Representation:

The samples were collected between the dates of 2010-09-02 and 2012-02-23

Environmental Conditions:

QAPP Information:

QAPP not required for federal data

QAPP Information Reference(s):

This is a placeholder reference for data that was collected after QAPP requirements were developed, but exempt from the requirements.