Boise River (Lower) Subbasin

Data	Details
Size	1,290 square miles (825,600 acres)
Water Bodies with EPA-Approved TMDLs (Category 4a)	Boise River, Lake Lowell
Beneficial Uses Affected	Cold water aquatic life, salmonid spawning, domestic and agricultural water supply, primary and secondary contact recreation
Major Land Uses	Range, grazing, forest, agriculture, urban
Date Approved by EPA	January 2000 Approval Letter
Date Approved by EPA	Subbasin assessment only; not subject to EPA approval
Date Sediment and Bacteria Addendum Approved by EPA	June 2008 Approval Letter
Date Lake Lowell TMDL Addendum Approved by EPA	December 2010 Approval Letter
Date 2015 Sediment and Bacteria Addendum Approved by EPA	September 2015 Approval Letter
Date 2015 Total Phosphorus Addendum Approved by EPA	December 2015 Approval Letter

Subbasin Characteristics

The lower Boise River subbasin is located in southwestern Idaho. The lower Boise River itself is a 64-mile stretch that flows in a northwesterly direction through Ada and Canyon Counties and the cities of Boise and Caldwell. It originates at Lucky Peak Dam
and flows into the Snake River near Parma. Major tributaries include Fifteen Mile Creek, Mill Slough, Mason Creek, Indian Creek, Conway Gulch, and Dixie Drain

1999 Subbasin Assessment and TMDL

Of the seven listed pollutants, only sediment and bacteria require TMDLs. Pollutant targets are based on existing water quality criteria for bacteria and on a numeric interpretation of the state narrative standard for sediment. Because the lower Boise River is a major tributary to the lower Snake River, phosphorus (total and dissolved) will be examined for possible load and wasteload allocations after completion and approval of The Snake River-Hells Canyon TMDL . Load allocations for temperature are not recommended for the lower Boise River segments listed for temperature since atmospheric conditions preclude compliance with cold water biota temperature criteria during June, July, and August. The US Environmental Protection Agency does not believe that flow (or lack of flow) is a pollutant as defined by the Clean Water Act. Since TMDLs are not required for water bodies impaired by pollution but not pollutants, a TMDL was not developed for flow alteration.

1999 TMDL: Streams and Pollutants for Which TMDLs Were Developed

Stream	Pollutants
Lower Boise River	Sediment, bacteria

2001 Subbasin Assessment

Two segments of the lower Boise River are listed for nutrients on Idaho’s 1998 § 303(d) list and eight tributaries are listed for a variety of pollutants. The Snake River-Hells Canyon TMDL is scheduled to be completed in 2001. Nutrients and sediment are
listed as pollutants of concern in that TMDL and will be addressed by assigning load allocations to the major tributaries to the Snake River, including the lower Boise River. When The Snake River-Hells Canyon TMDL allocates a nutrient load to the lower Boise River, load reductions from the tributaries to the lower Boise River will be necessary to meet the Snake River-Hells Canyon
load allocation.

2001 Summary of Assessment Outcomes

Water Body	Listed Pollutants	Recommendations
Lower Boise River	Nutrients	Delist for nutrients
Blacks Creek	Dissolved oxygen, sediment, nutrients	Delist for dissolved oxygen, sediment, nutrients
Fivemile Creek	Dissolved oxygen, sediment, nutrients	Delist for dissolved oxygen, sediment, nutrients; list for bacteria
Tenmile Creek	Dissolved oxygen, sediment, nutrients	Delist for dissolved oxygen, sediment, nutrients; list for bacteria
Mason Creek	Dissolved oxygen, sediment, nutrients	Delist for dissolved oxygen, sediment, nutrients; list for bacteria
Indian Creek	Dissolved oxygen, oil and grease, sediment, nutrients, temperature	Delist for nutrients, oil, and grease; no TMDL for dissolved oxygen or sediment but leave on § 303(d) list; temperature will be addressed in the future; list for bacteria
Lower Boise River	Dissolved oxygen, sediment, nutrients	Delist for nutrients, oil and grease; no TMDL for dissolved oxygen or sediment but leave on § 303(d) list; temperature will be addressed in the future; list for bacteria

2008 Addendum

This is an addendum to the Lower Boise River TMDL for sediment and bacteria approved by the US Environmental Protection Agency on January 25, 2000. The addendum provided the Avimor Development and the City of Kuna, Idaho, with wasteload allocations for
total suspended solids and E. coli bacteria for a discharge to Dry Creek and Indian Creek, respectively. DEQ also outlines in this addendum the manner in which the sediment reserved for growth shall be allocated to new and existing sources in the
future. The bolded changes in the addendum are made on pages 61, 62, 64, and 72 of the original document. Table 15 was revised in June 2012.

2010 Addendum

This addendum addresses Lake Lowell in the lower Boise River subbasin. The lake is impaired by dissolved oxygen and nutrients, and a TMDL was developed for total phosphorus to address these impairments. Identified sources of nutrients include phosphorus from agricultural runoff via canal and drain tributaries and waterfowl. Numeric water quality targets were set for Lake Lowell based on Idaho’s water quality standards and established targets for similar water bodies.