Lower Boise River Tributaries

Subbasin Assessments

Eight tributaries to the river are listed for multiple pollutants, including Blacks Creek, Fivemile Creek, Tenmile Creek, Mason Creek, Indian Creek and Sand Hollow Creek. These water bodies, including the river, were scheduled for assessment and possible TMDL development in 2001. Three additional tributaries were added to the 1998 303(d) in January 2001. Dixie Drain and Willow Creek were added for temperature and Cottonwood Creek was added for unknown pollutants. The TMDLs for Dixie Drain, Willow Creek and Cottonwood Creek are anticipated.

This subbasin assessment document describes the water quality conditions and beneficial use support status for the lower Boise River as it pertains to nutrients and Blacks Creek, Fivemile Creek, Tenmile Creek, Mason Creek, Indian Creek and Sand Hollow Creek as they pertain to their respective 303(d) listed pollutant. Cottonwood Creek, Willow Creek and Dixie Drain are not addressed in this document.

The assessments are provided in Adobe Acrobat PDF files. To view the files, you must have Adobe Acrobat Reader version 4.0 or higher. This program is available for free from Adobe Acrobat.

1. Introduction (PDF, 256 KB)
2. Fivemile, Tenmile, and Indian Creek Subbasin Assessment (PDF, 1.46 MB)
3. Mason and Sand Hollow Subbasin Assessment (PDF, 1.28 MB)
4. Blacks Creek Subbasin Assessment (PDF, 1.68 MB)
5. Tributary Technical Appendices (PDF, 1.55 MB)
6. Response to Comments (PDF, 85 KB)

The subbasin assessment was prepared by the DEQ Boise Regional Office, with assistance from the Lower Boise Watershed Council. For questions about these documents, contact Craig Shepard at (208) 373-0550.

DEQ Boise Regional Office
1445 N. Orchard
Boise, ID 83706-2239

Use Attainability Analysis

As noted above, DEQ began the TMDL process for these tributaries by initiating the sub-basin assessments. In parallel, the Lower Boise Watershed Council and Nampa-Meridian Irrigation District initiated a review of beneficial uses of a select few tributaries so that TMDL goals could be determined consistent with existing and attainable beneficial uses. The Use Attainability Analysis (UAA) is a process to review and potentially modify a waterbody’s designated uses. A UAA is considered when a designated use assigned to a waterbody is not an “existing use” (40 CFR 131.3(e)) and attaining the designated use is not feasible because of one or more natural or human-caused conditions (40 CFR 131.10(g)). At the time in Region 10, no guidance had been developed and only a handful of UAAs had been approved by EPA.

In May 2001, the results of the use refinement evaluation (PDF; 2.4 MB) indicated that existing beneficial uses should be revised for some of the tributary segments; these recommendations were presented to DEQ. In November 2001, the DEQ Board approved the recommended beneficial use changes. DEQ finalized the tributary sub-basin assessments based on the revised beneficial uses in December 2001. During the 2002 legislative session, the revised beneficial uses were adopted into IDAPA 58, which was subsequently forwarded to EPA Region 10 for standards approval.