The federal Clean Water Act requires that states and tribes restore and maintain the chemical, physical, and biological integrity of the nation's waters. States and tribes must adopt water quality standards necessary to protect fish, shellfish, and wildlife while providing for recreation in and on the waters whenever possible.

Section 303(d) of the Clean Water Act establishes requirements for states and tribes to identify and prioritize water bodies that are water quality limited (i.e., water bodies that do not meet water quality standards). States and tribes must periodically publish a priority list of impaired waters, currently every two years. For waters identified on this list, states and tribes must develop water quality improvement plans known as total maximum daily loads (TMDLs) that establish allowable pollutant loads set at levels to achieve water quality standards.

This document is a subbasin assessment only. It describes the physical, biological, and cultural setting; water quality status; pollutant sources; and recent pollution control actions in the lower Boise River Subbasin. The subbasin assessment is an important first step in leading to a TMDL.

The lower Boise River watershed is located in southwest 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. 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.