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

Supporting Information

Regional Board 1 - North Coast Region

Water Body Name:	Cape Mendocino HU, Mattole River HA, Mattole River Watershed, Mattole River
Water Body ID:	CAR1123004020180103042949
Water Body Type:	River & Stream


DECISION ID	156788	Region 1
Cape Mendocino HU, Mattole River HA, Mattole River Watershed, Mattole River

Pollutant:	Temperature, water
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 156788, Temperature, water	Region 1
Cape Mendocino HU, Mattole River HA, Mattole River Watershed, Mattole River

LOE ID:

318904

Pollutant:

Temperature, water

LOE Subgroup:

Pollutant-Water

Matrix:

Water

Fraction:

None

Beneficial Use:

Cold Freshwater Habitat

Aquatic Life Use:

Fish Spawning

Number of Samples:

Number of Exceedances:

Data and Information Type:

PHYSICAL/CHEMICAL MONITORING

Data Used to Assess Water Quality:

Water Board staff assessed temperature data for USGS California Water Science Center and determined that 1 of 6 sample is greater than the June-October maximum of 24 degrees Celcius and 0 of 2 samples are greater than the October-May maximum of 20 degrees Celcius, so that 1 of 8 samples exceed the water quality objective.

Data Reference:

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

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: Natural receiving water temperature of intrastate waters shall not be altered unless it can be demonstrated to the satisfaction of the Regional Water Board that such alteration in temperature does not adversely affect beneficial uses. At no time or place shall the temperature of any COLD water be increased more than 5 degrees Fahrenheit (F) above natural receiving water temperature.

Objective/Criterion Reference:

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

Evaluation Guideline:

According to Carter (2008) the lethal temperature for spawning, incubation, and emergence life stages are 20 degrees C. The lethal temperatures for adult migration and holding, and juvenile migration and rearing is 24 degrees C.

Guideline Reference:

Effects of Temperature, Dissolved Oxygen/Total Dissolved Gas, Ammonia, and pH on Salmonids. Implications for California's North Coast TMDLs. California Regional Water Quality Control Board, North Coast Region

Spatial Representation:

These samples were collected at sample station name: MATTOLE R NR ETTERSBURG CA; station code: USGS-11468900.

Temporal Representation:

These samples were collected from 5/30/2014-9/26/2017.

Environmental Conditions:

QAPP Information:

QAPP for USGS California Water Science Center

QAPP Information Reference(s):

QAPP for USGS Data Collected during the 2026 Integrated Report Cycle